CN111679290A - Photon counting correction method, laser radar, and computer-readable medium - Google Patents
Photon counting correction method, laser radar, and computer-readable medium Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/93—Lidar systems specially adapted for specific applications for anti-collision purposes
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
- G01J1/44—Electric circuits
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- G—PHYSICS
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- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/4802—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
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- G—PHYSICS
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- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/483—Details of pulse systems
- G01S7/486—Receivers
- G01S7/4861—Circuits for detection, sampling, integration or read-out
- G01S7/4863—Detector arrays, e.g. charge-transfer gates
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
- G01S7/497—Means for monitoring or calibrating
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
- G01J1/44—Electric circuits
- G01J2001/4413—Type
- G01J2001/442—Single-photon detection or photon counting
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
- G01J1/44—Electric circuits
- G01J2001/4446—Type of detector
- G01J2001/446—Photodiode
- G01J2001/4466—Avalanche
Abstract
The invention discloses a photon counting correction method for a single photon avalanche diode array, which comprises the following steps: receiving incident light pulses by the single photon avalanche diode array; obtaining photon counts of the incident light pulses according to the output of the single photon avalanche diode array; and correcting the photon count of the incident light pulse according to a preset condition. By the photon counting correction method, the due shape of the echo signal can be well measured and calculated under the conditions of strong noise and strong signal, so that more accurate distance and reflectivity results are obtained, and the detection capability of the laser radar is improved. The photon counting correction method is simple to realize on hardware, small in calculation amount, low in cost and low in requirements on power consumption and area of a chip. The actual incident light pulse signal can be restored more accurately, and more accurate ranging information can be obtained.
Description
Technical Field
The invention relates to the technical field of photoelectric detection, in particular to a photon counting correction method for a single photon avalanche diode array, a laser radar and a computer readable medium.
Background
The laser radar is widely applied to the field of unmanned driving at present, wherein an SPAD (single photon avalanche diode) array is used as a laser radar receiving end, and a digital signal can be directly output without ADC (analog to digital converter) processing, so that the laser radar has attractive prospect. But SPAD's are greatly affected when used to measure time of flight (TOF) and reflectivity due to their device characteristics and their dynamic range, which is determined by the number of all SPADs included on the array, requiring special processing to overcome their drawbacks. SPAD is generally a structure of an avalanche photodiode in series with a resistor. For the photodiode, when photons are absorbed, electrons of formed electron-hole pairs are accelerated in a space charge region to obtain enough kinetic energy, secondary electron-hole pairs are formed through impact ionization, and finally, a self-sustained ionization cascade is triggered, so that silicon conducts electricity and generates current. After the photodiode generates current, the series resistor at the tail of the SPAD obtains higher partial voltage so that the voltage at two ends of the diode is reduced to be lower than breakdown voltage, so that avalanche is prevented, and then the diode is recharged to be in a state before breakdown, so that subsequent photon detection can be carried out. This process determines that a SPAD will have an extinction recovery time after the detection of a photon.
Such a characteristic may cause that, when an incident light signal is strong or background noise is extremely high, because the number of photons incident on the SPAD array in a short time is large, when one SPAD is triggered by a previous photon, the remaining photons incident on the SPAD position cannot be triggered to the SPAD temporarily, so that the number of triggered single photon avalanche diodes of the SPAD is smaller than the actual incident photon count. The controller is used for collecting and measuring signals of the SPAD array, specifically, the controller receives photon detection signals of each SPAD on the SPAD array and counts the number of the SPADs on the incident photon trigger array, and when echo signals received by the SPAD array are deformed to a certain degree, certain errors can occur if distance and reflectivity are measured by using shape information of the signals. And when the received echo signal is stronger or in a stronger noise environment, most of the single photon avalanche diodes in the SPAD array are in a recovery state, so that the deformation of the obtained signal is extremely serious, and the normal use of the function of the obtained signal in the laser radar is seriously influenced.
In the disclosed technology, one is to utilize the fact that under the condition that the saturation is not very serious, the pulse leading edge is less influenced by deformation, and the leading edge arrival time is obtained by leading edge interpolation in combination with a certain threshold value. It is also possible here to impose some constraint or compensation on the leading edge time based on the pulse width or the signal integral. One is to design a corresponding filter according to the deformation condition of the signals received by the SPAD to process and classify the signals according to the waveform shape characteristics after deformation, so as to distinguish the condition that the deformation degree of the three waveforms of strong saturation (serious deformation), weak saturation (weak deformation) or non-saturation (unobvious deformation) of the received signals is from large to small. Then, the information is recovered by respectively processing the filter with the corresponding parameter. And finally, obtaining the accurate arrival time of the signal by interpolation filtering.
If the SPAD signal is used directly: a significant deviation occurs for the case where the noise is strong and the signal is strong. Similarly, when the SPAD sampling frequency is not high, the accuracy improvement is limited. The method has the advantages that the extremely high filter overhead is used, the requirements on power consumption and chip area are high, meanwhile, details of original signals cannot be restored accurately, and due to the fact that some information of the original signals can be lost by using the multistage filter, the finally obtained result accuracy cannot achieve the due effect. In addition, the configuration of each filter parameter depends on the measurement environment, and more filter parameters may need to be configured for the case of large environmental noise variation, so that the whole system is very complex.
Disclosure of Invention
The invention designs a method for correcting photon counting based on the luminous characteristics of an SPAD array, and recovers the optical pulse signal actually incident to the SPAD array according to the correction method. The method eliminates the serious waveform deformation caused by strong noise and signals, reduces the errors of distance and reflectivity, and simultaneously improves the saturation threshold of the SPAD receiving signal.
The invention provides a photon counting correction method for a single photon avalanche diode array, which comprises the following steps:
a: receiving incident light pulses by the single photon avalanche diode array;
b: obtaining photon counts of the incident light pulses according to the output of the single photon avalanche diode array; and
c: and correcting the photon count of the incident light pulse according to a preset condition.
According to an aspect of the invention, the step c comprises: and acquiring the corrected photon count of the incident light pulse based on the photon count acquired by the output of the single photon avalanche diode array.
According to one aspect of the invention, a look-up table is further included, and a corrected photon count of the incident light pulse is obtained based on the photon count obtained from the single photon avalanche diode array and the look-up table.
According to an aspect of the invention, the step c comprises:
photon counting total sum X in the first k corrected time periods corresponding to the current time periodiPhoton count sum X in corrected first k time periods corresponding to the previous time periodi-1And acquiring the corrected photon count of the incident light pulse, thereby correcting the photon number of the incident light pulse.
According to one aspect of the invention, said k is equal to the recovery time of said single photon avalanche diode divided by the length of said time period rounded.
The present invention also provides a laser radar comprising:
a transmitting unit including a laser array configured to emit a probe beam to detect a target object;
the receiving unit comprises a single photon avalanche diode array and is configured to receive the echo of the detection light beam reflected on the target object and convert the echo into an electric signal;
a processing unit coupled to the transmitting unit and the receiving unit and configured to perform the photon count correction method as described above to correct the photon count of the incident light pulse to the single photon avalanche diode array and calculate the distance to the target and/or the reflectivity of the target according to the corrected photon count of the incident light pulse.
According to one aspect of the invention, the processing unit comprises a statistic module and a control module, wherein the statistic module counts the number A of newly triggered single photon avalanche diodes in the current time periodiAnd Sum Sum of newly-increased triggered single photon avalanche diode number in the first k time periods of the current time periodiThe k is equal to the recovery time of the single photon avalanche diode divided by the time period length and then is rounded, and the control module obtains the AiAnd SumiAnd according to said AiAnd SumiObtaining a corrected photon count of incident light pulses, wherein the statistical module comprises a detection module and a SUM module, and the detection module is used for detecting the number A of newly triggered single photon avalanche diodes in the current time periodiThe SUM module is used for calculating the SUM Sum of the number of newly-increased triggered single-photon avalanche diodes in the first k time periods of the current time periodi。
According to one aspect of the invention, the control module comprises an arithmetic module and a query module, wherein based on said AiAnd SumiThe calculation module is configured to calculate a photon count obtained from the single photon avalanche diode array output, the lookup module obtains a corrected photon count of the incident light pulse based on the photon count obtained from the single photon avalanche diode array output and a lookup table, and/or the calculation module calculates the corrected photon count of the incident light pulse based on the photon count.
According to an aspect of the invention, the processing unit is configured to calculate the echo arrival time by a leading edge method, a centroid method or an interpolation filtering method based on the corrected photon counts.
The invention also provides a detection method of the laser radar, wherein the detection method is implemented by the laser radar.
The present invention also provides a computer readable storage medium having stored thereon computer program code which, when executed by a processor, may cause the processor to perform a photon count correction method as described above.
By the photon counting correction method, the shape of the signal can be well restored under the conditions of stronger noise and stronger signal, so that the restored information can be fully utilized by a subsequent ranging and reflectivity algorithm, more accurate distance and reflectivity results can be obtained, and the detection capability of the laser radar based on the distance and reflectivity results can be improved. The photon counting correction method is simple to realize on hardware, small in calculation amount, low in cost and low in requirements on power consumption and area of a chip. The incident optical pulse signals can be restored more accurately, and therefore more accurate ranging information can be obtained. If the device characteristics are changed, for example, a single photon avalanche diode on the SPAD is damaged or some special effects occur, the parameters of the lookup table can be modified through actual tests to realize corresponding matching adjustment; the compensation can be started after the optical pulse signal is found, so that the power consumption of the circuit can be reduced.
Drawings
The accompanying drawings, which are included to provide a further understanding of the disclosure, illustrate embodiments of the disclosure and together with the description serve to explain the disclosure and are not to limit the disclosure. In the drawings:
figure 1 illustrates a photon count correction method for a single photon avalanche diode array according to one embodiment of the present invention;
figure 2 shows the relationship between the number of photons actually arriving and the number of triggered single photon avalanche diodes in an array of single photon avalanche diodes;
3A, 3B, and 3C illustrate a process of performing a correction according to an embodiment of the present invention;
FIG. 4 shows a schematic diagram of a photon count correction method according to an embodiment of the invention;
FIG. 5 shows a schematic diagram of a lidar in accordance with one embodiment of the invention;
FIG. 6 illustrates a logical structure of a processing unit in accordance with a preferred embodiment of the present invention; and
fig. 7 illustrates a detection method of a lidar according to one embodiment of the present invention.
Detailed Description
In the following, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.
In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", and the like, indicate orientations and positional relationships based on those shown in the drawings, and are used only for convenience of description and simplicity of description, and do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be considered as limiting the present invention. Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, features defined as "first", "second", may explicitly or implicitly include one or more of the described features. In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.
In the description of the present invention, it should be noted that unless otherwise explicitly stated or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection, either mechanically, electrically, or in communication with each other; either directly or indirectly through intervening media, either internally or in any other relationship. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.
In the present invention, unless otherwise expressly stated or limited, "above" or "below" a first feature means 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 with each other via another feature therebetween. Also, the first feature being "on," "above" and "over" the second feature includes the first feature being directly on and obliquely above the second feature, or merely indicating that the first feature is at a higher level than the second feature. A first feature being "under," "below," and "beneath" a second feature includes the first feature being directly above and obliquely above the second feature, or simply meaning that the first feature is at a lesser level than the second feature.
The following disclosure provides many different embodiments or examples for implementing different features of the invention. To simplify the disclosure of the present invention, the components and arrangements of specific examples are described below. Of course, they are merely examples and are not intended to limit the present invention. Furthermore, the present invention may repeat reference numerals and/or letters in the various examples, such repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. In addition, the present invention provides examples of various specific processes and materials, but one of ordinary skill in the art may recognize applications of other processes and/or uses of other materials.
The preferred embodiments of the present invention will be described in conjunction with the accompanying drawings, and it will be understood that they are described herein for the purpose of illustration and explanation and not limitation.
Figure 1 illustrates a photon count correction method 10 for a single photon avalanche diode array according to one embodiment of the present invention, described in detail below with reference to the accompanying drawings.
Step S11: incident light pulses are received by the single photon avalanche diode array. The single photon avalanche diode array comprises a plurality of single photon avalanche diodes, each of which is commonly used in connection with a quenching resistor; the single photon avalanche diode array can be square, wherein the single photon avalanche diode array can be formed by uniformly arranging 10 by 10 single photon avalanche diodes into a square, and can also be formed by uniformly arranging 20 by 20 single photon avalanche diodes into a square. When the single photon avalanche diode receives incident photons, the single photon avalanche diode is triggered to generate current and can convert the current into voltage through the quenching resistor, the single photon avalanche diode array can be coupled to the controller through a single data line, the controller collects and measures signals of the single photon avalanche diode array, specifically, the controller receives photon detection signals of each SPAD of the SPAD array and counts the number of triggered SPADs of the incident photons, and the controller periodically counts the number of triggered SPADs of the single photon avalanche diode array at preset time intervals (usually 1ns), namely, the voltage level of the SPAD array is converted into the number of SPADs at each preset time interval. Counting the number of triggered SPADs on an array of single photon avalanche diodes can count the number of single photon avalanche diodes in the array of single photon avalanche diodes that detected a photon within a predetermined time interval. The number of single photon avalanche diodes of the array of single photon avalanche diodes that do not detect photons within a predetermined time interval may also be counted and subtracted from the total number of single photon avalanche diodes included in the array of single photon avalanche diodes to derive the number of single photon avalanche diodes of the array of single photon avalanche diodes that detect photons within the predetermined time interval.
Step S12: and obtaining the photon count of the incident light pulse according to the output of the single photon avalanche diode array. According to one embodiment of the invention, each single photon avalanche diode of the array of single photon avalanche diodes is individually addressable for counting from the output of the array of single photon avalanche diodes for obtaining a photon count of an incident light pulse.
Step S13: and correcting the photon count of the incident light pulse according to a preset condition. As described above, when a single photon avalanche diode receives a triggered photon, there is a recovery time during which the single photon avalanche diode cannot respond to the incident photon. The photon count of the incident light pulse obtained at step S12 may be smaller than the number of actually incident photons. Especially, when a received signal is strong or in a strong noise environment, most of the single photon avalanche diodes in the single photon avalanche diode array are in a recovery state, so that the obtained pulse signal is seriously deformed, and the normal use of the laser radar is influenced. According to the invention, the photon counting of the incident light pulse can be corrected through a certain preset condition so as to obtain more accurate photon counting, so that the distance and the reflectivity of the laser radar to a target object can be more accurately calculated, wherein the time of flight (TOF) information can be calculated through a gravity center method and an interpolation filtering method according to the distribution condition of the shape of an echo signal in sampling time, the reflectivity can also be calculated through an integral quantity, and when the shape of the echo signal is deformed to a certain degree, errors can be caused if the distance and the reflectivity are measured by using the shape information of the signal.
The following describes a correction method according to a preferred embodiment of the present invention.
The physical characteristics of a single photon avalanche diode determine that it needs a recovery time after being triggered by a photon before it can recover the photon detection capability. This determines that when the noise signal and the detection signal are strong, more single photon avalanche diodes of the single photon avalanche diode array will be triggered and then in a recovery state, so that the newly incident photons cannot trigger the single photon avalanche diodes of the single photon avalanche diode array and generate current. For the incident photons in a short time, the probability that the single photon avalanche diode cannot be triggered is proportional to the number of the currently triggered single photon avalanche diodes, so that the photon count of the corrected incident light pulse, namely the number x of photons actually arriving in a short time and the number of triggered single photon avalanche diodes on the single photon avalanche diode array, can be obtained based on the photon count obtained by outputting the single photon avalanche diode arrayThe relationship y (which in a short time can be considered to be the number of single photon avalanche diodes at recovery time after a photon has been received at the previous time) is for example: the photon count obtained from the output of the single photon avalanche diode array has a function relationship inversely proportional to the exponential function of the photon count of the corrected incident light pulse, which is specifically, for example:
(where N istotalThe total number of single photon avalanche diodes on the single photon avalanche diode array). The graph of the above relationship is shown in fig. 2, where the abscissa is the number x of photons actually reached and the ordinate is the number y of triggered single photon avalanche diodes in the single photon avalanche diode array.
By using the curve relation in fig. 2, the number of photons actually incident on the single photon avalanche diode array can be obtained according to the number of single photon avalanche diodes which are triggered currently. When the method is applied to the laser radar, a corresponding lookup table can be stored in a signal processing device of the laser radar, so that the corresponding actually-reached photon number x can be queried according to the number y of the triggered single photon avalanche diodes, wherein the lookup table is obtained by actually testing and counting devices, and the parameters of the lookup table can be modified in real time through the actual test. Or alternatively, a calculation program or a corresponding hardware circuit may be built in the signal processing device, and is used for calculating the corresponding actually-reached photon number x according to the number y of the triggered single photon avalanche diodes.
Fig. 3A, 3B and 3C show the correction process according to the above embodiment, which is the result of inputting the original light pulse into a single photon avalanche diode array model composed of 1000 single photon avalanche diodes, where the abscissa is the time axis and the ordinate is the photon count. Fig. 3A is a schematic diagram of a light source actually emitting photons. Figure 3B is the incident photon count from the output of the single photon avalanche diode array without correction by the present invention, where the incident photon count is determined by the controller counting the number of SPADs that the single photon avalanche diode array is triggered. Figure 3C is the incident photon count after correction using the present invention. As shown in fig. 3B, the back end portion of the resulting signal actually triggered by the incident photons is severely suppressed, and thus the image of fig. 3B is very severely distorted relative to fig. 3A. According to the distribution situation of the echo signal shape in the sampling time, the time of flight (TOF) information can be calculated through a gravity center method and an interpolation filtering method, and the reflectivity can also be calculated through an integral quantity, when the echo signal shape is deformed to a certain degree, errors can be caused if the distance and reflectivity measurement is carried out by using the shape information of the signal, as shown in FIG. 3C, after the photon counting correction method is carried out according to the embodiment of the invention, the actual incident photon number can be recovered according to the curve relation of FIG. 2, and it can be observed that the result obtained through the photon counting correction method is basically consistent with the original pulse shape, so the time of flight (TOF) information and the reflectivity information can be calculated more accurately according to the corrected echo signal shape.
Performing the photon counting correction method according to the above-described embodiment requires knowing the total number S of single photon avalanche diodes that have been photon triggered and are in a recovered state at the end of each time periodiAnd the number A of the newly added triggered single photon avalanche diodes in the time periodi. By means of which the photon counts arriving at the single photon avalanche diode array during this time period can be obtained using a photon count correction method, i.e. an estimate F (S) of y as mentioned in the above equationi，Ai)＝Si+Ai。
But S cannot be directly measured by a single photon avalanche diode arrayiThe number A of the newly triggered single photon avalanche diodes in each time period can be obtained through counting by the controller according to the photon detection signals output by the SPAD arrayi. So that the corrected photon count sum X in the first k time periods is based on the current time periodiPhoton count sum X in corrected first k time periods corresponding to the previous time periodi-1To obtain a corrected photon count of the incident light pulse and thereby correct the photon count of the incident light pulse, according to one embodiment of the present invention, k times before the i-th time period may be usedAdding Sum Sum of newly increased photon counting in time intervaliTo replace Si. Wherein A isiAnd Ai-1Is one sample time, e.g., 1 ns; the whole interval AiTo Ai-kThe length is the recovery time of the single photon avalanche diode. The recovery time can be set in advance, for example 10ns, for a particular single photon avalanche diode. The flow is shown in fig. 4. According to one embodiment of the invention, k is equal to the recovery time of the single photon avalanche diode divided by the sampling time length.
As shown in fig. 4, for the ith time period, the number a of newly triggered single photon avalanche diodes in the time period is obtainediSimultaneously calculating the number A of single photon avalanche diodes triggered in the previous k time periodsi-1、Ai-2、…、Ai-kAdding them to obtain SumiThen using AiAnd SumiThe photon count of the incident light pulse is corrected by the formula of the above-described photon count correction method. A specific example of the calculation is given below.
At the end of the (i-1) th time period (before the (i) th time period), the total number S of single photon avalanche diodes which have been photon triggered and are in a recovery stateiCalculated by the following formula:
at the end of the ith time period, the total number of single photon avalanche diodes which have been photon triggered and are in a recovery state is Si+AiCalculated by the following formula:
wherein Xi-1+x＝Xi。
According to said XiAnd Xi-1The number of photons of the incident light pulse is corrected.
Where A isiRepresenting the total number of newly triggered single photon avalanche diodes in the current time period; xiIndicating the corrected first k time segments corresponding to the current timeSum of number of internal incident photons, Xi-1The sum of the incident photon number in the first k corrected time periods corresponding to the previous time can be used as X because the signal intensity is much greater than the noise and the pulse width is less than the recovery timei-Xi-1Considered as the incident photon count x for the current time period.
The recovery can be performed using the inverse function of the above formula, as follows:
Xi-1＝Ntotal*ln(Ntotal/(Ntotal-Si))
Xi＝Ntotal*ln(Ntotal/(Ntotal-(Si+Ai)))
x＝Xi-Xi-1＝Ntotal*((Ntotal-Si)/(Ntotal-(Si+Ai)))
where x is the count of photons actually incident on the single photon avalanche diode array during the current time period after correction.
Fig. 5 shows a lidar 20 according to an embodiment of the invention. As shown in fig. 5, the laser radar 20 includes a transmitting unit 21, a receiving unit 22, and a processing unit 23. Wherein the emitting unit 21 comprises an array of lasers configured to emit detection laser beams for detecting the object OB. The laser beam encounters the object OB and is diffusely reflected, and a part of the reflected echo returns to the laser radar and is received by the receiving unit 22. The receiving unit 22, as previously indicated, comprises an array of single photon avalanche diodes SPAD, which can receive echoes of the laser beam reflected by the detection object OB and convert them into digital signals for output. Although not shown, it is easily understood by those skilled in the art that the transmitting unit 21 may further include a transmitting lens group located downstream of the optical path of the laser array for collimating the laser beam emitted from the laser array into parallel light and emitting into the environmental space around the laser radar 20. Similarly, the receiving unit 22 may also include a receiving lens group, and the single photon avalanche diode is located on a focal plane thereof for converging an echo reflected by the outgoing laser beam through the detection object OB onto the single photon avalanche diode. As shown in the figure, the laser beam L1 emitted by the emitting unit 21 is projected on the object OB, and is diffusely reflected, and a part of the laser beam is reflected back to form an echo L1'. The receiving unit 22 receives the reflected echo L1' and converts it into an electrical signal. The processing unit 23 is coupled to the emitting unit 21 and the receiving unit 22, and is configured to perform the photon counting correction method as described above to calculate the photon count received by the single photon avalanche diode array, and calculate the distance to the target object and/or the reflectivity of the target object according to the corrected photon count.
According to an embodiment of the invention, the processing unit is configured to calculate the arrival time of the echo pulse from the corrected photon count. By means of a photon counting correction method, after the waveform shape is recovered, a more accurate arrival time of the echo pulse is obtained by means of a leading edge method, a gravity center method or an interpolation filtering method, wherein the leading edge method is to use the time when the rising edge of the pulse waveform exceeds a preset threshold value as the pulse arrival time; the gravity center method is that the ratio of the sum of the time and amplitude products of each signal in discrete signals to the sum of the amplitudes of each signal is used as the pulse arrival time; the interpolation filtering method is to compare the current pulse signal with various preset signal models to obtain the best matched signal model to determine the pulse arrival time.
Fig. 6 shows a logical structure of the processing unit 23 according to a preferred embodiment of the present invention. As shown in fig. 6, the processing unit 23 is coupled to the receiving unit 22, and receives the electric signals output by the individual single photon avalanche diodes SPAD from the receiving unit 22. The processing unit 23 may include a statistics module 231 and a control module 232, wherein the statistics module 231 may include a detection module and a SUM module. The detection module is used for detecting the number A of newly triggered single-photon avalanche diodes in the current time periodiThe SUM module is used for obtaining S by using the accumulated SUM of the number of newly added photons in k time periods before the current time periodi. The control module receives AiAnd Si. The control module 232 may include an arithmetic module and a query module. Wherein the operation module can be based on AiAnd SiCalculating triggered single photon avalanche diodes on single photon avalanche diode arrayThe number y of the tubes can be used for the query module to obtain a photon count x actually incident on the single photon avalanche diode array in the current time period through a query table, the query table is obtained by actual measurement of the device, and the query table can also be used for further calculating the photon count x through the operation module (the calculation formula is an ideal calculation model, such as the calculation formula mentioned above). In addition, the operation module can also calculate the distance and the reflectivity of the target object based on the photon count x.
Fig. 7 illustrates a detection method 30 of lidar 20 in accordance with one embodiment of the present invention.
In step S31, a set of detection light pulses is emitted, and the single photon avalanche diode array receives the echo signal.
In step S32, it is determined whether a corresponding echo signal is received. If the corresponding echo signal is received, proceed to step S33, otherwise return to step S31. According to one embodiment of the invention, the probe light pulses are time-coded and/or amplitude-coded. After the echo signal is received, whether the echo signal corresponds to the transmitting pulse or not can be judged through the time and/or the amplitude of the echo signal, so that the crosstalk of the laser radar is reduced.
Step S33, obtaining the number A of the single photon avalanche diodes triggered in the current time periodiSimultaneously calculating the number A of single photon avalanche diodes triggered in the previous k time periodsi-1、Ai-2、…、Ai-kIt is accumulated.
Step S34, obtaining a recovery signal including the photon count of the incident light pulse in the current time period by using the photon count correction method.
Step S35, performing distance calculation using the restored signal, for example, calculation using a leading edge method, a center of gravity method, an interpolation filter method, or the like; the leading edge method is to use the time when the rising edge of the pulse waveform exceeds a preset threshold value as the pulse arrival time; the gravity center method is that the ratio of the sum of the time and amplitude products of each signal in discrete signals to the sum of the amplitudes of each signal is used as the pulse arrival time; the interpolation filtering method is to compare the current pulse signal with various preset signal models to obtain the best matched signal model to determine the pulse arrival time.
In step S36, a reflectance calculation is performed using the restored signal, for example, a signal peak or an integral amount calculation.
The invention also relates to a computer readable storage medium having stored thereon a computer program code which, when executed by a processor, may cause the processor to perform a photon count correction method as described above.
More accurate echo signal shape information can be recovered through a photon counting correction method, so that more accurate distance information can be calculated by using the echo signal shape information. More accurate echo signal strength values can also be obtained, and more accurate reflectivity information can be calculated by using the echo signal strength values.
In the existing single photon avalanche diode array, when an optical pulse is received, a part of single photon avalanche diodes in the array are triggered due to the fact that photons are received, but because the signal is possibly too strong, more single photon avalanche diodes are triggered and then are in a recovery state, and therefore newly incident photons cannot trigger the single photon avalanche diodes on the single photon avalanche diode array to emit light with a high probability. The present invention solves this problem by: the probability that the single photon avalanche diode cannot be triggered to emit light is in direct proportion to the number of the triggered single photon avalanche diodes, a certain relation exists between the number x of actually arriving photons in a short time and the number y of the triggered single photon avalanche diodes, and correction is carried out through a preset relation. When the signal is received each time, the number y of the single photon avalanche diodes triggered each time can be obtained, and the number x of actually arriving photons can be obtained through a preset relation. The waveform diagram output by the number of the triggered single photon avalanche diodes is distorted, and the distorted waveform can be recovered by correcting according to the calculated number of actually arriving photons.
By the photon counting correction method, the shape of the signal can be well restored under the conditions of stronger noise and stronger signal, so that the restored information can be fully utilized by a subsequent ranging and reflectivity algorithm, more accurate distance and reflectivity results can be obtained, and the detection capability of the laser radar based on the distance and reflectivity results can be improved. The photon counting correction method is simple to realize on hardware, small in calculation amount, low in cost and low in requirements on power consumption and area of a chip. The incident optical pulse signals can be restored more accurately, and therefore more accurate ranging information can be obtained. If the device characteristics are changed, for example, single photon avalanche diodes on the SPAD array are damaged or some special effects occur, the parameters of the lookup table can be modified through actual tests to realize corresponding matching adjustment; the compensation can be started after the optical pulse signal is found, so that the power consumption of the circuit can be reduced.
Finally, it should be noted that: although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments and/or equivalents thereof without departing from the spirit and scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (12)
1. A photon count correction method for a single photon avalanche diode array, comprising:
a: receiving incident light pulses by the single photon avalanche diode array;
b: obtaining photon counts of the incident light pulses according to the output of the single photon avalanche diode array; and
c: and correcting the photon count of the incident light pulse according to a preset condition.
2. The photon count correction method according to claim 1, said step c comprising: and acquiring the corrected photon count of the incident light pulse based on the photon count acquired by the output of the single photon avalanche diode array.
3. The photon count correction method according to claim 2, further comprising a look-up table, the corrected photon counts of the incident light pulses being obtained based on the photon counts obtained from the single photon avalanche diode array and the look-up table.
4. The photon count correction method according to claim 1, wherein the step c comprises:
photon counting total sum X in the first k corrected time periods corresponding to the current time periodiPhoton count sum X in corrected first k time periods corresponding to the previous time periodi-1And acquiring the corrected photon count of the incident light pulse, thereby correcting the photon number of the incident light pulse.
5. The photon counting correction method according to claim 4, wherein the k is equal to a recovery time of the single photon avalanche diode divided by the period length rounded.
6. A lidar comprising:
a transmitting unit including a laser array configured to emit a probe beam to detect a target object;
the receiving unit comprises a single photon avalanche diode array and is configured to receive the echo of the detection light beam reflected on the target object and convert the echo into an electric signal;
a processing unit coupled to the transmitting unit and the receiving unit and configured to perform the photon count correction method of any one of claims 1 to 5 to correct the photon count of the light pulses incident to the single photon avalanche diode array and to calculate the distance to the target and/or the reflectivity of the target from the corrected photon count of the incident light pulses.
7. The lidar of claim 6, wherein the processing unit comprises a statistic module and a control module, wherein the statistic module counts the number A of newly-triggered single photon avalanche diodes in the current time periodiAnd Sum Sum of newly-increased triggered single photon avalanche diode number in the first k time periods of the current time periodiThe k is equal to the recovery time of the single photon avalanche diode divided by the time period length and then is rounded, and the control module obtains the AiAnd SumiAnd according to said AiAnd SumiA corrected photon count of the incident light pulse is obtained.
8. The lidar of claim 7, wherein the statistics module comprises a detection module and a SUM module, the detection module is configured to detect a number a of newly-triggered single photon avalanche diodes in a current time periodiThe SUM module is used for calculating the SUM Sum of the number of newly-increased triggered single-photon avalanche diodes in the first k time periods of the current time periodi。
9. The lidar of claim 8, the control module comprising an arithmetic module and a query module, wherein based on the aiAnd SumiThe calculation module is configured to calculate a photon count obtained from the single photon avalanche diode array output, the lookup module obtains a corrected photon count of the incident light pulse based on the photon count obtained from the single photon avalanche diode array output and a lookup table, and/or the calculation module calculates the corrected photon count of the incident light pulse based on the photon count.
10. Lidar according to any of claims 6-9, wherein the processing unit is configured to calculate the echo arrival time by a leading edge method, a barycentric method or an interpolation filtering method based on the corrected photon counts.
11. A method of detection by a lidar according to any of claims 6 to 10.
12. A computer readable storage medium having stored thereon computer program code which, when executed by a processor, can cause the processor to perform a photon count correction method according to any of claims 1-5.
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