Method of determining noise level, lidar and method of ranging

Abstract

The application provides a method for determining a noise level, comprising: S101: obtaining intensity information-time information curves of an optical signal; S102: clamping the intensity information-time information curves by using a pre-estimated noise threshold; and S103: determining the noise level by using the clamped intensity information-time information curves. The application also provides a method for ranging by using a laser radar and a laser radar.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of photoelectric detection, and in particular to a method for determining a noise level, a laser radar, and a method for ranging using a laser radar. BACKGROUND

[0002] A laser radar is a radar system that detects the position, speed, and other characteristic quantities of a target by emitting a laser beam. It is an advanced detection method that combines laser technology and photoelectric detection technology. Laser radars are widely used in automatic driving, traffic communication, unmanned aerial vehicles, intelligent robots, resource exploration, and other fields due to their high resolution, good concealment, strong anti-active interference capability, good low-altitude detection performance, small size, and light weight. The environment in which a laser radar is located is usually filled with various noises or background lights, such as sunlight, other vehicles' headlights, building lights, and laser beams emitted by other vehicles' laser radars. In order to accurately calculate the distance of a target object, a laser radar needs to accurately obtain the noise level in the surrounding environment to distinguish the echo generated by its own laser beam from the surrounding noise. In a high-precision time-to-digital converter (TDC) measurement system in a laser radar, the detection signal and the noise are superimposed on each other, and accurate noise information cannot be obtained on the basis of ensuring ranging accuracy and efficiency.

[0003] A single photon avalanche diode (SPAD) can be used as a detector in a laser radar. The SPAD can be triggered by a single photon avalanche, and a TDC can provide a measurement of the time stamp of each trigger with a picosecond level of accuracy. In some applications, the output terminals of multiple SPADs are connected to the same TDC as a macro pixel. The TDC provides the time stamp of the trigger and the number of SPADs in the macro pixel that are triggered at the same time.

[0004] In order to obtain current noise information, a period of time is often reserved without actively emitting light, and the noise is measured using SPAD trigger information, which wastes a lot of measurement time. Other methods for obtaining noise information include using a fixed noise threshold for detection in a number of periods or even a period of time, which cannot obtain real-time noise information. Alternatively, data other than the arrival time of the echo signal is used to calculate the noise while detection is being performed, but the noise and the signal will affect each other, making it difficult to obtain accurate noise information.

[0005] The content of the background section merely represents the knowledge of the inventor and does not necessarily represent the prior art in the field. SUMMARY

[0006] In view of at least one defect of the prior art, the present application provides a method for determining noise level, comprising:

[0007] S101: obtaining intensity information-time information curve of light signal;

[0008] S102: clamping the intensity information-time information curve by using estimated noise threshold; and

[0009] S103: determining noise level by using the clamped intensity information-time information curve.

[0010] According to an aspect of the present application, the intensity information-time information curve is a photon number histogram.

[0011] According to an aspect of the present application, the step S101 comprises: performing multiple scans on a field of view range, and superimposing curves of intensity information-time information of the multiple scans as the intensity information-time information curve.

[0012] According to an aspect of the present application, the method further comprises: obtaining total intensity of light signal, calculating estimated noise intensity by using the total intensity, comparing the estimated noise intensity with preset noise intensity, obtaining smaller value, and taking the smaller value as estimated noise threshold.

[0013] According to an aspect of the present application, the step of calculating estimated noise intensity comprises: multiplying total photon number S by preset proportion K to obtain estimated noise photon number as the estimated noise intensity, wherein the preset proportion K is between 0 and 1.

[0014] According to an aspect of the present application, the step S101 further comprises: receiving light signal by a detection unit, wherein the detection unit comprises a plurality of single photon avalanche diodes, and the preset noise intensity is determined based on one or more of the number of single photon avalanche diodes in the detection unit and the dead time of single photon avalanche diodes.

[0015] According to an aspect of the present application, the step S103 comprises: obtaining total intensity of the clamped intensity information-time information curve, and taking average value of the total intensity with respect to time span as the noise level.

[0016] According to an aspect of the present application, in the step S102, for intensity information corresponding to any time information, taking smaller value between the intensity information and estimated noise threshold as clamped intensity information.

[0017] The present application further provides a method for ranging by laser radar, comprising:

[0018] S201: calculating noise level by the method as described above;

[0019] S202: filtering the intensity information-time information curve based on the noise level; and

[0020] S203: calculating the distance of the target object based on the filtered intensity information-time information curve.

[0021] According to an aspect of the present application, the step S202 comprises: judging whether a peak value of the intensity information-time information curve is higher than the noise level; and filtering the intensity information-time information curve based on the noise level when the peak value is higher than the noise level.

[0022] According to an aspect of the present application, the step S203 comprises: calculating a gravity center of intensity information with respect to time information according to the filtered intensity information-time information curve, and taking the time information corresponding to the gravity center as the time of flight.

[0023] According to an aspect of the present application, the step S203 comprises: calculating a front edge time of the echo pulse according to the filtered intensity information-time information curve, and taking the front edge time as the time of flight, wherein the front edge time is the time information corresponding to the intensity information equal to a preset threshold on the front edge of the echo pulse.

[0024] The present application also provides a laser radar, comprising:

[0025] a light emitting module configured to emit a probe laser beam for probing a target object;

[0026] a light detecting module configured to receive an echo of the probe laser beam reflected on the target object and convert the echo into an electrical signal; and

[0027] a control module coupled to the light emitting module and the light detecting module, and configured to execute the method as described above to calculate the distance of the target object.

[0028] According to an aspect of the present application, the light detecting module comprises a plurality of detecting units, each of which comprises a plurality of single photon avalanche diodes to receive the echo. BRIEF DESCRIPTION OF DRAWINGS

[0029] The accompanying drawings, which form a part of the disclosure, are intended to provide further understanding of the disclosure, and the illustrative embodiments of the disclosure and their descriptions serve the purpose of explaining the disclosure. In the drawings:

[0030] Figure 1 a method for determining a noise level according to an embodiment of the present application is shown;

[0031] Figure 2Fig. 4 shows an intensity-time curve according to an embodiment of the present application;

[0032] Figure 3 Fig. 5 shows an intensity-time curve after clamping;

[0033] Figure 4 Fig. 6 shows a schematic diagram of obtaining the number of echo photons by multiple scans in one measurement;

[0034] Figure 5 Fig. 7 shows a schematic diagram of obtaining a histogram after accumulating the number of echo photons of multiple scans;

[0035] Figure 6 Fig. 8 shows a schematic diagram of a detection unit according to an embodiment of the present application;

[0036] Figure 7 Fig. 9 shows a curve waveform obtained after noise filtering of the intensity-time curve;

[0037] Figure 8 Fig. 10 shows a method of distance measurement by laser radar according to a preferred embodiment of the present application;

[0038] Figure 9 Fig. 11 shows a data storage method according to the prior art;

[0039] Figure 10 and Figure 11 Fig. 12 shows a specific schematic diagram of the storage method according to a preferred embodiment of the present application;

[0040] Figure 12 Fig. 13 shows a block diagram of a laser radar according to an embodiment of the present application. DETAILED DESCRIPTION

[0041] In the following, only certain exemplary embodiments are described in brief. As the person skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the present application. The drawings and the description are therefore to be considered as being essentially exemplary and not restrictive.

[0042] In the description of the application, it is to be understood that the orientations or positional relationships indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", and the like are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the application and simplifying the description, and do not indicate or imply that the devices or elements indicated thereby must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the application. In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated thereby. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the application, the meaning of "a plurality of" is two or more, unless otherwise explicitly specified and limited.

[0043] In the description of the application, it should be noted that, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connection" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integral connection; it can be mechanical connection, or electrical connection or can communicate with each other; it can be directly connected, or indirectly connected through intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the application can be understood according to the specific circumstances.

[0044] In the present application, unless otherwise explicitly specified and limited, the "upper" or "lower" of the first feature to the second feature can include that the first and second features are in direct contact, or that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, the "upper", "above" and "on" of the first feature to the second feature includes that the first feature is directly above and obliquely above the second feature, or only indicates that the horizontal height of the first feature is higher than that of the second feature. The "under", "below" and "under" of the first feature to the second feature includes that the first feature is directly above and obliquely above the second feature, or only indicates that the horizontal height of the first feature is less than that of the second feature.

[0045] The following disclosure provides many different embodiments, or examples, for implementing different structures of the present application. For purposes of explanation and ease of understanding, specific examples of components and arrangements are described herein. Of course, they are merely examples and are not intended to limit the application. Also, the application can be implemented in different examples with variations of the components and / or arrangements described herein, which are within the scope of the application. Moreover, the application provides examples of various specific processes and materials, but one skilled in the art will appreciate that other processes and / or materials can be used.

[0046] The preferred embodiments of the present application will be described herein with reference to the drawings, in which like numerals designate like elements in all of the figures, and the preferred embodiments of the present application are merely illustrative and do not limit the application as recited in the claims.

[0047] First aspect

[0048] Figure 1 A method 100 for determining a noise level according to an embodiment of the present application is shown, which can be used to determine a noise level or threshold in a laser radar, and will be described below with reference to Figure 1 Detailed description.

[0049] In step S101, intensity information-time information curves of light signals are obtained.

[0050] Figure 2 An intensity information-time information curve according to an embodiment of the present application is shown in FIG. 1, in which the abscissa is time information, and the ordinate is a parameter value representing intensity information of a light signal. It is easily understood that, Figure 2 The abscissa in FIG. 1 is not an absolute time coordinate, but a time difference between a time when a laser radar transmits a light pulse and a time when the light signal is received. Figure 2 The intensity information-time information curve shown in FIG. 1 can be an intensity information-time information curve obtained in one scan of a laser radar on a certain field of view range, or a curve formed by superimposing intensity information of light signals obtained in multiple scans on a certain field of view range with respect to time information. These are within the protection scope of the present application. According to a preferred embodiment of the present application, Figure 2 The ordinate in FIG. 1 can use photon number to represent intensity of a light signal.

[0051] In step S102, the intensity information-time information curve is clamped by a pre-estimated noise threshold.

[0052] Figure 2 A pre-estimated noise threshold (such as Figure 2(As shown by the straight line representing the "noise threshold" in the diagram). For any given time information, the smaller of the intensity information and the estimated noise threshold is taken as the clamped intensity information, resulting in a clamped intensity information-time information curve. In this embodiment, clamping refers to comparing the intensity information and the estimated noise threshold for any given time scale, and taking the smaller value as the clamped intensity information. That is, for the portion of the intensity information in the intensity information-time information curve where the intensity information is higher than the estimated noise threshold, the estimated noise threshold is used; for the portion of the intensity information in the intensity information-time information curve where the intensity information is lower than the estimated noise threshold, the actual intensity information is used. The intensity information corresponding to each time scale within the total time span is compared sequentially to obtain the clamped intensity information-time information curve. For Figure 2 The intensity-time information curve shown is obtained by using... Figure 2 The estimated noise threshold set in the middle is used for clamping to obtain Figure 3 The diagram illustrates the strength-time information curve after clamping.

[0053] In step S103: The noise level is determined using the intensity-time information curve after clamping.

[0054] exist Figure 3 Based on the clamped intensity-time information curve shown, preferably, the average value of the clamped intensity information relative to the time information can be obtained as the noise level. For example, the area covered by the clamped intensity-time information curve can be obtained, and then divided by the total time span, i.e., the time span of the clamped intensity-time information curve on the horizontal axis, to obtain the average value of the clamped intensity information relative to the time span, which can be used as the noise level.

[0055] As a preferred option Figure 3 The intensity information after clamping is represented by the number of photons. The total number of photons over the total time span is calculated and divided by the total time span to obtain the noise level represented by the number of photons.

[0056] Using the above method, the noise value can be directly calculated using the intensity-time information curve obtained from lidar detection, without the need to reserve detection time specifically for noise measurement. Furthermore, after obtaining an intensity-time information curve, the noise level corresponding to that detection data can be calculated based on the real-time detection data. This means that real-time noise can be acquired simultaneously with ranging, improving the accuracy of the noise level. Using the real-time calculated noise level as the corresponding ranging noise threshold can further enhance ranging accuracy.

[0057] In the detection process of lidar, taking a detector composed of single-photon avalanche diodes (SPADs) as an example, because SPADs can be triggered by a single photon to induce an avalanche effect, they are easily affected by environmental noise. Furthermore, SPADs have low photon detection efficiency (PDE) for the commonly used detection light bands in lidar, resulting in very weak signal strength obtained from a single detection. Figure 4 As shown, a single detection may only involve a few triggers within the detection time window (SPAD is only in Geiger mode within a preset time window, i.e., a state where it can be triggered by photons to induce an avalanche effect; this time window is called the "detection time window"), making it impossible to distinguish whether the trigger is caused by the echo signal reflected from the target object or by environmental noise. According to a preferred embodiment of the present invention, in order to improve the rangefinding performance of the lidar and reduce the impact of noise, such as... Figure 4 As shown, the same field of view can be scanned multiple times. For each scan, the light source at the transmitting end emits a light pulse for detection. The controller of the lidar records the emission time t1 of the light pulse. When the light pulse encounters an external obstacle, it is reflected by the obstacle and returns to the lidar, where it is received by the photodetector at the receiving end. When the photodetector is a SPAD(s) array, ambient light may also cause the SPADs to be triggered by avalanches. Once the SPAD receives a photon, it generates an avalanche electrical signal, which is transmitted to the time-to-digital converter (TDC). The TDC outputs the SPAD trigger time signal and the number of SPADs triggered at the same time. The subsequent memory stores the timestamp (i.e., the SPAD trigger time minus the emission time t1) of the SPAD trigger time. Figure 2 and 3 The horizontal axis represents the time information and the trigger count (hereinafter referred to as cnt) signal for that timestamp.

[0058] The trigger quantity (cnt) obtained from each measurement is stored in the corresponding memory location according to the timestamp. When a new trigger quantity (cnt) arrives at the location corresponding to a certain timestamp, the previously stored value is added to the new trigger quantity (cnt), and then the value is updated to that location. After multiple measurements are superimposed, a histogram is saved in the memory, such as... Figure 5 As shown, the histogram reflects the total number of triggers (cnt) corresponding to different timestamps on the time axis. By using the histogram to calculate the centroid and other operations, the flight time corresponding to the echo pulse can be obtained, and then the ranging result can be obtained.

[0059] Therefore, according to the above embodiment, in one time-of-flight measurement of a laser radar for a field-of-view range, the laser radar actually performs multiple scans (multiple emission-reception cycles), the number of scans can be from tens to hundreds, and multiple scans are performed on a field-of-view range (or approximately a target point) in a time period, and the curves of the intensity information-time information of the detector received in multiple scans are superimposed as the intensity information-time information curve. For example, as shown in Figure 4 In the first, second, …, n-th scans, only a very limited number of echoes or photons are received each time, but after superimposing the detection results of the n scans, the photon number histogram of the time-of-flight measurement of the field-of-view range is obtained, as shown in Figure 5 The scale of the abscissa is the time information, and the scale width on the time axis is usually equal to the resolution of the time digital converter in the laser radar, that is, the resolution of the detection time of the laser radar. The number of photons corresponding to each scale on the photon number histogram is accumulated, and the total number of photons S is obtained, which is used to represent the total intensity information.

[0060] In the context of the present application, the terms "measurement" and "scan" are distinguished. Specifically, one "measurement" corresponds to a time-of-flight measurement of a field-of-view range in one detection period of a laser radar (i.e., a period for generating one frame of point cloud), thereby generating one or more (one column or more columns or one block) "points" in one frame of point cloud, and after the measurement of all field-of-view ranges is completed, a complete frame of point cloud is obtained; and "scan" refers to the process of completing one emission by a laser in one detection channel and completing corresponding reception by a detector in one measurement process. One "measurement" can include one "scan", or can include multiple "scans" (e.g., hundreds of times) of the same target point.

[0061] According to one embodiment of the present application, the method 100 further comprises: obtaining the total intensity of the optical signal, calculating the estimated noise intensity (e.g., represented by the number of photons) according to the total intensity, and setting a preset noise intensity according to the parameters of the receiving end of the laser radar.

[0062] The total intensity of the optical signal can be represented by the total number of photons S received in the total time span.

[0063] The total time span corresponds to Figure 2 The curve records the total time of the intensity information, that is, the difference between the maximum and minimum values of the time information. The range of time corresponding to the maximum and minimum detection distances of the laser radar can be used as the total time span. Alternatively, as shown in Figure 2As shown, only the intensity information corresponding to partial time information is recorded. As a specific embodiment, the time period in which the echo reflected by the target object is expected to be obtained is taken as the total time span of the intensity information-time information curve. The total intensity of the light signal can be represented by the area covered by the intensity information-time information curve in the time span. Figure 2 According to a preferred embodiment of the present application, the total intensity can be represented by the total number of photons S received in the total time span.

[0064] To calculate the estimated noise intensity, the total number of photons S in the total time span can be multiplied by a preset proportion K to obtain the estimated number of noise photons, which is taken as the estimated noise intensity, the preset proportion K being between 0 and 1. The preset proportion K ∈ (0, 1) can be obtained by simulation or measurement to obtain an empirical value of the proportion of noise photons in the total number of photons as K.

[0065] In a specific embodiment of the present application, the receiving end parameters of the laser radar include the total number of detection units (pixels), the number of SPADs in each detection unit, and the dead time of the SPADs, according to which the average maximum number of noise trigger photons S per unit time of the SPADs is calculated max , and the preset noise intensity is taken as S max . Specific embodiments will be given below.

[0066] Further, the estimated noise intensity calculated according to the intensity information-time information curve and the preset noise intensity S max are compared to obtain the smaller value therebetween, which is taken as the preset noise threshold.

[0067] According to a preferred embodiment of the present application, in step S102, for the intensity information corresponding to any time information, the smaller value between the intensity information and the estimated noise threshold is taken as the clamped intensity information. The step S101 further includes receiving the echo by a detection unit, the detection unit including a plurality of single-photon avalanche diodes. The preset noise intensity is determined based on one or more of the number of single-photon avalanche diodes in each detection unit and the dead time of the single-photon avalanche diodes. Figure 6 An embodiment of the detection unit is shown. As shown, the detection module includes a plurality of detection units, and the detection units P1, P2 and P3 are shown in the figure, each of which includes nine single-photon avalanche diodes, and the nine single-photon avalanche diodes are connected to a time-to-digital converter TDC, so that the TDC can obtain the time signal of the SPAD trigger in the detection unit and the number signal of the SPADs triggered at the same time, and store them into a memory.

[0068] For Figure 6The detection unit shown, a detection unit occurs 9 times at most in the SPAD dead time. Set Figure 6 The detection unit shown, the dead time of the SPAD is 20ns, the same SPAD is triggered at most once in 20ns, assuming that one measurement includes N scans, a detection unit is triggered at most (9*N* detection time / 20ns) times in the time of one measurement, S max = 9*N / 20, that is, the maximum number of noise trigger photons per unit time caused by noise in the total flight time, as the preset noise intensity.

[0069] In the above embodiment, the intensity information-time information curve (such as a histogram) obtained by detection can be directly used to calculate the noise level, without reserving special detection time for measuring noise; and real-time noise can be obtained at the same time of ranging, which can be used as a ranging noise threshold. In the data processing process of the laser radar, the noise threshold can be used to judge whether the echo is effective, that is, whether the echo pulse signal is from the target reflection or noise signal. If the measured echo intensity is lower than the noise threshold, it will be judged as noise and filtered out. If the set noise threshold is higher than the actual noise level, the actual detection echo will be misjudged as noise, causing data loss; if the set noise threshold is lower than the actual noise level, some noise signals will be judged as target reflection echoes, resulting in noise points in the point cloud. In addition, the environmental noise around the laser radar is also constantly changing, and the noise cannot be obtained in real time, which will also cause point loss or noise points. The above embodiment of the present application can obtain the noise level in real time, improve the signal-to-noise ratio of the laser radar, and thus improve the ranging accuracy.

[0070] The present application also provides a method 200 for ranging by a laser radar, comprising:

[0071] S201: calculating the noise level by the method 100 as described above.

[0072] S202: performing noise filtering processing on the intensity information-time information curve based on the noise level.

[0073] As a preferred embodiment of the present application, a peak value is obtained on the intensity information-time information curve, that is, the maximum value of the intensity information in the total flight time span, and it is judged whether the peak value is higher than the noise level, and if it is higher than the noise level, the intensity information-time information curve is subjected to noise filtering processing.

[0074] The noise filtering processing, for example, compares the intensity information corresponding to each time scale with the noise level, retains the intensity information data higher than the noise level, and subtracts the noise level from the intensity information data to obtain the intensity information-time information curve corresponding to the actual detection signal.

[0075] S203: Calculate the distance to the target object based on the intensity information-time information curve after noise filtering.

[0076] According to a preferred embodiment of the present invention, step S203 includes: calculating the distance of the target object based on the time information corresponding to the centroid of the intensity information-time information curve after noise filtering on the time axis, as the flight time.

[0077] like Figure 7 As shown, the curve waveform obtained after noise filtering is displayed. Based on this, the centroid position of the curve waveform is obtained, and the flight time Tof1 corresponding to the centroid position is used as the flight time in this measurement process to calculate the distance of the target object and generate a data point in the point cloud.

[0078] On the other hand, the intensity information-time information curve after noise filtering is used to obtain the maximum value of the intensity information as the echo intensity. The intensity of the detection light emitted by the lidar is then obtained, and the reflectivity of the target object can be calculated based on the echo intensity and the detection light intensity.

[0079] Figure 8 A method for ranging using a lidar according to a preferred embodiment of the present invention is shown, wherein it incorporates... Figure 4-7 The detection method is shown below. (Refer to the following...) Figure 8 Detailed description.

[0080] exist Figure 8 In one embodiment, the lidar performs multiple scans in a single measurement, for example, 500 scans.

[0081] In step S301, the i-th scan is performed, with i initially set to 1. The laser of the lidar emits a detection light pulse, and the lidar controller records the emission time t1 of the emitted light pulse. When this light pulse encounters an external obstacle, it is reflected by the obstacle and returns to the lidar, where it is received by the photodetector at the receiving end. When the photodetector is a SPAD(s) array, ambient light may also cause the SPADs to be triggered by avalanches. Once a SPAD receives a photon, it generates an avalanche electrical signal, which is transmitted to the time-to-digital converter (TDC). The TDC outputs the SPAD trigger time signal and the number of SPADs triggered at the same time, and stores the timestamp (i.e., time information) of the SPAD trigger time minus the emission time t1 and the number of triggers at that timestamp (hereinafter referred to as cnt).

[0082] At step S302, the detection result of the ith scan is accumulated with the detection result of the previous i-1 scans. When a new trigger number cnt arrives at a position corresponding to a timestamp timestamp, the originally stored value is accumulated with the new trigger number cnt and then updated to the position. Meanwhile, all the cnt values are accumulated (without distinguishing the timestamps) to obtain the total trigger number of the SPAD, which is used to obtain the total intensity of the optical signal of one measurement.

[0083] At step S303, it is determined whether i has reached a preset scan number N, for example, 500. If not, i is accumulated and the process returns to step S301 to continue the scan detection. If the preset scan number N has been reached, the process proceeds to step S304.

[0084] At step S304, a histogram is generated. Since N scans have been completed, the trigger number cnt obtained in each scan is stored in the corresponding memory position according to the timestamp timestamp. After multiple measurements are superimposed, a histogram is stored in the memory, as shown in FIG. 4. The histogram reflects the sum of the trigger numbers cnt corresponding to different timestamps timestamp on the time axis. The data in the memory can be read out as a histogram. Meanwhile, all the cnt values are accumulated to obtain the total intensity of the optical signal of one measurement, which is represented by the number of photons. Figure 5

[0085] At step S305, the data of the histogram is filtered. Then steps S306 and S307 are performed respectively.

[0086] At step S306, the level of noise is obtained, for example, the noise threshold around the lidar is obtained according to the method 100 described above.

[0087] At step S307, the peak value of the signal is found according to the filtered histogram, that is, the point with the largest ordinate in the histogram is found.

[0088] At step S308, it is determined whether the signal peak value found in step S307 is valid according to the noise level obtained in step S306, that is, whether the signal peak value is greater than the level of noise. When the signal peak value is greater than the level of noise, the signal peak value is valid, and then step 309 is performed. When the signal peak value is lower than the noise level, the signal peak value is invalid and is not processed.

[0089] At step S309, the ranging information is obtained.

[0090] As an embodiment of the present application, the center of gravity of the intensity information over the total time span is calculated, and the time information corresponding to the center of gravity is taken as the time of flight, which is used to calculate the distance of the target object.

[0091] ​As another embodiment of the present application, the distance of the target object is calculated by using the echo pulse front edge time. Specifically, the intensity information of the echo pulse front edge is compared with a preset threshold value, and the time information corresponding to the intensity information with the intensity equal to the preset threshold value is taken as the flight time.

[0092] As an embodiment, the preset threshold value is a noise threshold value.

[0093] As an embodiment, the preset threshold value is the average of the signal peak value and the noise threshold value.

[0094] In step 309, the intensity information-time information curve can be filtered based on the noise level by using the method of S202-S203, and then the distance and / or reflectivity of the target object can be calculated based on the filtered intensity information-time information curve.

[0095] In the above embodiment of the present application, the detection results of N scans are superimposed and stored during one measurement of the laser radar, and after all scans are completed, the data in the memory can be read out as a histogram. The histogram data is filtered to find the signal peak value. The real-time noise value of the histogram can be obtained by using the noise calculation method of the present application as the current noise threshold value. When the signal peak value of the histogram is greater than the noise threshold value, the peak value is an effective peak value, and the echo arrival time is calculated. Therefore, the noise level and the ranging information of the current measurement can be obtained almost simultaneously after one measurement. Compared with the scheme of setting a unified noise threshold value for the laser radar, the scheme of dynamically calculating the noise level in the embodiment of the present application is more accurate.

[0096] The noise level is calculated by using the scheme of the embodiment of the present application, and the intensity information is filtered from the noise before the distance and / or reflectivity of the target object is calculated. The filtered intensity information can reflect the more real echo pulse signal, which is beneficial to improve the measurement accuracy.

[0097] The second aspect

[0098] The second aspect of the present application relates to a storage method of the detection data of the laser radar, which can be combined with the method 100 of determining the noise level of the first aspect of the present application.

[0099] In some time-to-digital converters of the laser radar, a corresponding storage location is needed at each time scale of the time resolution thereof, and all the triggered SPAD number information cnt obtained by multiple measurements is stored in the storage location corresponding to the time. The time resolution of the time-to-digital converter TDC can reach the order of picoseconds ps, and therefore a very large space register is needed. The specific explanation is as follows.

[0100] A SPAD can be triggered by a single photon avalanche effect, and thus is susceptible to ambient light noise; on the other hand, the SPAD has a low photon detection efficiency (PDE) for the light waveband commonly used in lidar detection, and the signal strength obtained by single detection is weak, as shown in FIG. 1. In a single detection scan, there may be only a few triggers (two triggers in FIG. 1) in the detection time window, and it is difficult to distinguish whether it is a return signal or ambient light noise. Figure 4 In order to improve the long-range performance of the lidar and reduce the influence of noise, as shown in FIG. 2, the lidar can perform multiple repeated measurements (one measurement is referred to as one sweep, and the number of repetitions can reach 400-500 times, or more or fewer times) in a single detection process of the same field of view range. The results of multiple measurements or sweeps are accumulated to obtain a histogram, and the distance is measured, and then a point on the lidar point cloud is obtained. Figure 4

[0101] For a single sweep, the controller of the lidar selects a part (a row or a column or any shape of interest) of the macro-pixel by supplying high voltage to the SPAD, and then sends a synchronization signal to inform the laser of the transmitting end that it can emit light. At time t a (a represents the a-th sweep), the laser of the transmitting end emits a light pulse for detection, which encounters an external obstacle, is reflected by the obstacle and returns to the lidar, and can be received by the photodetector of the receiving end. When the photodetector is an SPAD array, once the SPAD receives a photon, an avalanche electrical signal is generated and transmitted to a time-to-digital converter, and the time signal t 1a of the SPAD trigger and the number signal cnt 1a of the SPAD triggered at the same time (1a here represents the 1st trigger of the a-th sweep) are output by the time-to-digital converter. 1a The time stamp timestamp a of t 1a (t 1a hereinafter is abbreviated as tp 1a ) is calculated through a subtraction program, and tp 1a and the number of triggers cnt 2a of the time stamp are transmitted and stored in a memory. A macro-pixel includes a plurality of SPADs, and the SPAD can detect again after a dead time, so in a single sweep, SPAD triggering may occur again at another time, and the memory stores tp 2a and cnt b of this trigger (2a represents the 2nd trigger of the a-th detection). The multiple triggers in a single sweep all need to be stored according to time information.

[0102] ​At the next scan b, the controller of the lidar sends a signal to control the emitter to emit a pulse of probe light at time t b Once a SPAD receives a photon, an avalanche electrical signal is transmitted to a time-to-digital converter TDC, which outputs a time signal t 1b of the SPAD trigger and a number signal cnt 1b of the SPADs triggered at the same time (1st trigger of the 1st scan of the bth scan), the subsequent memory stores the SPAD trigger time t 1b -t b timestamp of the time t 1b (hereinafter referred to as tp 1b ) and the number cnt 1b of the triggers of the timestamp. A macro-pixel includes multiple SPADs, and the SPADs can detect again after the dead time, so in one scan, SPAD triggers may occur again at another time, and the memory stores the tp 2b and cnt 2b of this trigger.

[0103] In several hundred measurements, the number cnt of triggers obtained in each measurement is stored in the corresponding memory location according to the timestamp timestamp, and when a new number cnt of triggers arrives at the same timestamp timestamp, the original stored value is added to the new number cnt of triggers and then updated to the location. After multiple scans are superimposed, the memory saves a histogram, as shown in Figure 5 The histogram reflects the sum of the number cnt of triggers corresponding to different timestamps timestamp on the time axis, so that the histogram is used to calculate the center of gravity or the front time to obtain the time information of the echo, which is used as the time of flight for distance calculation to generate a point on the point cloud.

[0104] A data storage method is shown in Figure 9 , the horizontal coordinate is time t, and the scale interval of the horizontal coordinate is the resolution of the TDC. Each time scale corresponds to a storage location R (register). For example, in a certain detection scan a, SPAD triggers occur at time scale 0, and the timestamp tp1 (trigger time - this time of emission) and the number cnt 1a of triggers are calculated according to the emission time and the trigger time transmitted by the TDC, and cnt 1a is stored in the storage location R1 corresponding to the time tp1; if SPAD triggers occur at time scale 4, the time information tp5 and cnt 5a are obtained, and cn 5aThe storage location R5 corresponding to tp5 is stored. In another detection scan b, SPAD trigger also occurs at time scale 4, obtaining time information tp5 and cnt 5b , cnt 5b also corresponds to the storage location R5, at this time cnt 5a is read out, and the value of cnt 5b added to cnt 5a is updated to R5. (Combining Figure 9 , a represents the a-th detection, b represents the b-th detection, and the numbers represent the corresponding time scales and the corresponding storage locations; the storage location R corresponds to the time scale one by one, and the memory only stores the trigger quantity cnt, and the data processing circuit can obtain the time corresponding to the trigger quantity cnt according to the storage location when reading data.)

[0105] As can be seen from Figure 9 , a histogram is obtained by accumulating data of many detection scans (400-500 times), and the detection results of several hundred scans are superimposed to form a histogram. In the process of obtaining a point in the point cloud, the storage location corresponding to a time scale stores the sum of all trigger quantities cnt occurring at the time. Although SPAD trigger does not occur at each time scale in one scan, as shown in Figure 9 , histogram data is superimposed by many detection results, and SPAD trigger can occur at each time scale in a scan process, so that the memory receives the corresponding data. Therefore, for a TDC, each time scale needs to have a corresponding storage location, and all trigger quantities cnt obtained by multiple measurements are stored in the storage location corresponding to the time. The time interval of tp, that is, the resolution of the TDC, reaches the ps level, and a very large space register is required.

[0106] Using such a storage and ranging method, since the precision unit of the timestamp is in the ps level, a large memory is required to store a complete histogram when a long tof detection is required, and a large storage space is consumed. In particular, in order to improve the ranging capability, the measurement time and the number of repeated measurements need to be increased, and the requirement for storage space is also increasing.

[0107] The inventors of the present application conceived that it is not necessary to set a corresponding storage location for each time scale of the TDC time resolution, and when storing detection data, the intensity information is stored according to the weight of the time information instead of the time resolution. The present application adopts a weighted accumulation data storage method to compress the original signal while preserving the ranging accuracy, thereby greatly reducing the storage space required for storing the histogram. Specifically, the weighted accumulation data storage method can reduce the total storage space to 1 / 10 of the original range.

[0108] Specifically, the time precision for storing intensity information in this invention is a first time precision, which can be n times the time resolution of a time-to-digital converter (TDC). The intensity information refers to the optical signal intensity information corresponding to the time information. For different photodetectors, different parameters can be used to characterize the optical signal intensity: for example, if the detector is a SPAD array, the number of SPADs simultaneously triggered according to the time information can be used as the intensity information; if the detector is a SiPM, the output level / current intensity corresponding to the time information can be used to represent the optical signal intensity information.

[0109] The following is a detailed description with reference to the accompanying drawings.

[0110] First, radar detection data includes time information and intensity information corresponding to the time information.

[0111] by Figure 6 Taking the detection unit shown as an example, the time information is the time when one or more single-photon avalanche diodes (SPADs) in the detection unit (P1, P2, P3...) are triggered, and the intensity information is the number of single-photon avalanche diodes (SPADs) triggered at that trigger time. That is, the intensity of the optical signal is characterized by the number of triggered single-photon avalanche diodes (SPADs). According to a preferred embodiment of the present invention, the time information is the timestamp of the single-photon avalanche diode (SPAD), that is, the time t from laser emission. a The time t when the single-photon avalanche diode SPAD is triggered 1a The time difference t between 1a -t a .

[0112] Figure 6 In the embodiments described, a single-photon avalanche diode (SPAD) is used as an example. It is easy for those skilled in the art to understand that the present invention is not limited to this, and other types of photodetectors can also be used, including but not limited to avalanche photodiodes (APDs), silicon photomultiplier tubes (SiPMs), etc.

[0113] The data storage method of the present invention specifically comprises: storing the intensity information with a first time precision and according to the weight of the time information; the first time precision is the time interval between any two adjacent first time scales, and is n times the time resolution of the radar detection data, where n>1; the weight is associated with the time information and the time interval of at least one first time scale.

[0114] Figure 10 and Figure 11 A detailed schematic diagram of a storage method according to a preferred embodiment of the present invention is shown below, with reference to... Figure 10 andFigure 11 Detailed description of the implementation of the data storage method of the present application.

[0115] Figure 10 In the figure, the abscissa is the time of flight, the interval of the time scale of the abscissa is for example the time resolution of the lidar, for example the time resolution of the time-to-digital converter TDC, which can reach the order of picoseconds ps. As shown in the figure, on the basis of the time resolution of the lidar, a first time scale is set, as shown in A and A+1 in the figure, 16 intervals of the time resolution of the lidar are crossed between two adjacent first time scales. When a photon is detected at time x (for example, one or more single-photon avalanche diodes SPADs in one detection unit are triggered as shown in the figure), the detected intensity value is stored according to the weight of the time x. The time x represents that the time interval between the time x and the adjacent first time scale A on the left side is x times the radar detection data time resolution. Figure 10 Figure 10 Figure 6

[0116] As is easily understood by those skilled in the art, since the time resolution of the lidar is small and the interval of the first time scale is large, the time scale corresponding to the time resolution of the lidar can also be called “fine scale”, and the first time scale can also be called “coarse scale”.

[0117] As shown in the figure, the weight of the time x includes a first weight and a second weight, the first weight is associated with the time interval between the time x and one of the adjacent first time scales, and the second weight is associated with the time interval between the time x and the other adjacent first time scale. After the first weight and the second weight are determined, the intensity information is stored according to the first weight and the second weight, respectively, with the first time precision. Figure 10

[0118] According to a preferred embodiment of the present application, the first weight is associated with the time interval between the time x and the adjacent first time scale A on the left side, and the first weight is for example (16-x), and the second weight is associated with the time interval between the time x and the adjacent first time scale A+1 on the right side, and the second weight is for example x. Therefore, the time x is replaced by its weight at the adjacent two coarse scales (A, A+1), where the weight of x at the coarse scale A is (16-x), and the weight at the coarse scale A+1 is x (x represents the distance of this time from A), to equivalently represent the fine scale of the time x. In other words, by taking x as the weight, the data at the fine scale is stored to the corresponding addresses of the adjacent two coarse scales, to represent the value of the scale x, instead of storing the scale x itself. This process is expressed by the following equation:

[0119] ​​​​A*(16-x)+(A+1)*x=A*16+x

[0120] In the formula, the left side of the equal sign is the sum of the storage according to the coarse scale, the weight of the start value and the end value of the coarse scale, and the right side of the equal sign is the specific value of the time stamp. It can be seen that the storage method of the coarse scale + weight can accurately represent the specific value of the time stamp.

[0121] Similarly, when the trigger signal obtained includes the trigger number cnt representing the number or intensity of the trigger in addition to the time stamp, the intensity information newly added on the coarse scale A is cnt*(16-x), and the intensity information newly added on the coarse scale A+1 is cnt*x, which are respectively accumulated in multiple scans. Reference Figure 11 will be described in detail. The fine scale represents the time resolution of the time-to-digital converter TDC. For a certain time stamp timestamp, the start value of the coarse scale thereof is A, and the fine scale thereof corresponds to the 0-15 x scale of the coarse scale thereof.

[0122] Reference Figure 11 A register is allocated for each coarse scale, and the coarse scale interval of the abscissa is 16 times the TDC resolution, and each coarse scale corresponds to a register. In a certain scan a, the SPAD trigger occurs at the time scale 0, and the time information tp1(corresponding to x1 1a =0) and the trigger number information cnt 1a are obtained, which are respectively stored in the register A corresponding to the coarse scale A as cnt 1a *(16-x 1a ), and the register A+1 corresponding to the coarse scale A+1 is stored as cnt 1a *x 1a ; at another time scale 5, the time information tp6(corresponding to x6 a =5) and the trigger number information cnt 6a are obtained, the data stored in the register A corresponding to the coarse scale A is read out, added with cnt 6a *(16-x 6a ), and then stored in the register A; the data of the register A+1 corresponding to the coarse scale A+1 is read out, added with cnt 6a *x 6a , and then stored in the register A+1. In a coarse scale time (fine scale 0-15), all the trigger number information cnt are weighted, and are stored in the registers corresponding to the storage positions A and A+1 after being summed with the original data. The trigger number information cnt in the next coarse scale time is weighted and stored in the registers corresponding to the coarse scales A+1 and A+2, for example, the SPAD trigger occurs at the time 2', and the time information tp3' and cnt 3aIf the value of x is equal to 0, the data stored in the register A+1 corresponding to the coarse scale A+1 is added with cnt 3a If the value of x is equal to 16, the data stored in the register A+2 corresponding to the coarse scale A+2 is added with cnt 3a If the value of x is equal to 16, the data stored in the register A+2 corresponding to the coarse scale A+2 is added with cnt 3a If the value of x is equal to 16, the data stored in the register A+2 corresponding to the coarse scale A+2 is added with cnt 3a If the value of x is equal to 16, the data stored in the register A+2 corresponding to the coarse scale A+2 is added with cnt

[0123] In the next scanning process of b, the received signal tp2 and cnt 2b are respectively allocated weights cnt 2b *(16-x 2b ) and cnt 2b *x 2b , and are stored in the registers corresponding to the coarse scales A and A+1 after being summed with the original stored data. A histogram is obtained by accumulating the data of many times of scanning, and the number of all triggers occurring at the time points 0-15 in several times of scanning is stored in the registers corresponding to the coarse scales A and A+1.

[0124] Compared with the scheme in which a register is needed for data storage at each fine scale, the application adopts the weighted accumulation storage method, and only needs to set registers corresponding to the coarse scales of 0-n+1, so that the number of required registers is reduced to 1 / 16 of the original number. Although the bit width of each register is increased and the occupied space is larger, the required storage positions are greatly reduced, and the total storage space is reduced to 1 / 10 of the original space.

[0125] Figure 10-11 In the embodiment, the time interval of the adjacent first time scale (coarse scale) is 16 times of the radar detection data time resolution (fine scale), that is, the data compression is performed by using 16 as the weight. Those skilled in the art can understand that the application is not limited to this, and the weight can be any larger positive integer. As a preferred, the time interval of the coarse scale is 2 m times of the fine scale, where m is a positive integer, so as to facilitate the implementation in the FPGA or ASIC.

[0126] In the above embodiment, the first weight is (16-x) and the second weight is x, but the application is not limited to this. The first weight can be x and the second weight can be (16-x), or the first weight can be 1-(x / n) and the second preset weight can be x / n, as long as the first weight is associated with the time interval between the time point x and one of the adjacent first time scales, and the second weight is associated with the time interval between the time point x and the other adjacent first time scale.

[0127] The storage method of the second aspect of the present application can be applied to the method 100 of determining the noise level of the first aspect of the present application. For example, in step S101 of the method 100, the intensity information-time information curve of the optical signal is obtained, for example, the photon number histogram, which is obtained by superimposing the intensity information-time information curve of multiple scans on a field of view range, and the data obtained in each scan can be stored according to the storage method of the second aspect of the present application, and finally the photon number histogram is obtained.

[0128] In addition, in the method 100, when calculating the total intensity S of the optical signal, it can be calculated according to different ways. According to an embodiment of the present application, the total intensity S of the optical signal is the sum of the original trigger quantity cnt data accumulated before weighting, and the histogram is two parallel steps. Alternatively, the total intensity S of the optical signal can be calculated according to the histogram, which is within the scope of the present application.

[0129] The photon number histogram obtained by the storage method of the second aspect of the present application, when calculating the noise level, the time accuracy of the histogram data storage is n times the time resolution of the radar detection data, that is, the number of histogram horizontal coordinate scales is reduced by n times, and the required calculation amount of the intensity information corresponding to each time scale in steps S102, S103 and the like is also greatly reduced, which improves the data processing and calculation efficiency while maintaining the calculation accuracy unchanged.

[0130] Third aspect

[0131] The present application also relates to a laser radar 300, as shown in Figure 12 including a light emitting module 301, a light detecting module 302 and a control module 303, wherein the light emitting module 301 refers to the part of the laser radar for laser emission, which can include circuits, devices, structures, etc., and is configured to emit a detection laser beam for detecting a target object. The light detecting module 302 refers to the part of the laser radar for detecting the echo signal of the detection laser beam, which can include circuits, devices, structures, etc., and is configured to receive the echo of the detection laser beam reflected on the target object and convert it into an electrical signal. The control module 303 is coupled with the light emitting module 301 and the light detecting module 302, and is configured to execute the method 100, 200 or 300 as described above to calculate the distance of the target object. According to a preferred embodiment of the present application, the light detecting module 302 includes a plurality of detection units as shown in Figure 6 each detection unit includes a plurality of single photon avalanche diodes for receiving the echo.

[0132] The light emission module 301 comprises a light emission array 3011, for example, a vertical-cavity surface-emitting laser (VCSEL) array implementation. The light emission array 3011 comprises a plurality of rows and a plurality of columns. Each row and each column is respectively provided with a plurality of light emission units 30111, each of which comprises at least one light emitter; the light emission array 3011 is also correspondingly configured with an emission array driving circuit coupled to each light emitter for driving the light emitter to work.

[0133] The light detection module 302 comprises a photodetection array 3021, which can adopt, for example, Figure 6 The photodetection array 3021 is used to receive the light echo signal of the probe light beam after reaching the target object OB. In addition, the light detection module 302 can also comprise a signal readout circuit 3022 for reading out and transmitting the signal generated by the photodetection array 3021 to the control module 303.

[0134] In the laser radar 300, a transmission lens group 304 can also be provided on the light emission array 3011 exit light path; in the laser radar, a receiving lens group 305 can also be provided, and the photodetection array 3021 can be located on the focal plane of the receiving lens group 305.

[0135] The control module 303 is coupled to the light emission array 3011 and the photodetection array 3021; the light emission array is controlled to emit probe light beams in a certain order and power, and the corresponding photodetection array receives the echo signal.

[0136] Finally, it should be noted that: the above only describes the preferred embodiments of the present application and is not used to limit the present application, although the present application has been described in detail with reference to the foregoing embodiments, for those skilled in the art, the technical solutions recorded in the foregoing embodiments can still be modified, or some technical features can be replaced. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for determining a noise level, comprising: obtaining an intensity-time curve of a light signal, the intensity-time curve being a photon number histogram; clipping the intensity-time curve with a pre-estimated noise threshold; and determining the noise level using the clipped intensity-time curve; wherein the determining the noise level using the clipped intensity-time curve comprises obtaining a total intensity of the clipped intensity-time curve, and taking an average of the total intensity with respect to a time span as the noise level.

2. The method of claim 1, wherein said obtaining intensity information-time information curves of the optical signal comprises: The intensity-time curve is obtained by superimposing intensity information with respect to time information of multiple scans of a field of view.

3. The method of claim 1, further comprising: A total intensity of the light signal is obtained, a pre-estimated noise intensity is calculated using the total intensity, the pre-estimated noise intensity is compared with a pre-set noise intensity, and a smaller value is obtained as the pre-estimated noise threshold.

4. The method of claim 3, wherein the step of calculating an estimated noise intensity comprises: A pre-estimated noise photon number is obtained by multiplying a total photon number S with a pre-set ratio K, the pre-set ratio K being between 0 and 1, as the pre-estimated noise intensity.

5. The method of claim 3 or 4, wherein said obtaining intensity information - time information curves of the optical signal further comprises: The light signal is received by a detection unit, the detection unit comprising a plurality of single photon avalanche diodes, the pre-set noise intensity being determined based on one or more of a number of single photon avalanche diodes in the detection unit, and a dead time of the single photon avalanche diodes.

6. The method of any one of claims 1-3, wherein said clamping the intensity information-time information curve with an estimated noise threshold comprises: For intensity information corresponding to any time information, a smaller value between the intensity information and the pre-estimated noise threshold is taken as the clipped intensity information. 7.A method for ranging by a laser radar, comprising: calculating a noise level by the method of any one of claims 1-6; filtering noise of the intensity-time curve based on the noise level; and calculating a distance of a target object based on the filtered intensity-time curve.

8. The method of claim 7, wherein the filtering the intensity information-time information curve based on the noise level comprises: determining whether a peak value of the intensity-time curve is higher than the noise level; when the peak value is higher than the noise level, filtering noise of the intensity-time curve based on the noise level.

9. The method of any one of claims 7 or 8, wherein the calculating the distance of the target object based on the intensity information-time information curve after the noise filtering processing comprises: calculating a center of intensity information with respect to time information from the filtered intensity-time curve, the center of intensity information corresponding to time information as a time of flight.

10. The method of any one of claims 7 or 8, wherein the calculating the distance of the target object based on the intensity information-time information curve after the noise filtering processing comprises: calculating a leading edge time of a return pulse from the filtered intensity-time curve, the leading edge time being taken as a time of flight, wherein the leading edge time is time information corresponding to intensity information equal to a pre-set threshold on a leading edge of the return pulse. 11.A laser radar, comprising: a light emitting module configured to emit a probe laser beam for probing a target object; a light detecting module configured to receive a return of the probe laser beam reflected on the target object and convert the return into an electrical signal; and a control module coupled to the light emitting module and the light detecting module, and configured to execute the method of any one of claims 7-10 to calculate a distance of the target object. 12.The laser radar of claim 11, wherein the light detecting module comprises a plurality of detection units, each detection unit comprising a plurality of single photon avalanche diodes to receive the return.

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