Method for measuring the reflectivity of a target using a laser radar and laser radar

Abstract

The method (10) for measuring the reflectivity of a target using a laser radar (100) includes the steps of: a step S101 in which a detector of the laser radar (100) receives an echo reflected by a target with a detection beam, converts the reflected echo into an electrical signal, and obtains an echo photocurrent integral; and a step S102 in which the reflectivity of the target is determined based on a preset reflectivity calibration curve and the echo photocurrent integral. The present invention provides a method for measuring the reflectivity of a target by photocurrent integration, which is based on the long-term accumulation of a response current of a silicon photomultiplier tube to photons. The provided method has a higher accuracy rate, a higher measurement precision, and a larger dynamic range than a method for measuring reflectivity by measuring a short-time echo signal such as a pulse width or a leading edge tilt.

Description

[Technical field]

[0001] The present invention relates to the technical field of laser detection, and in particular to a method for measuring the reflectivity of a target using a laser radar. [Background technology]

[0002] Laser radar transmits a detection light pulse by a laser device, converts the echo light pulse reflected by the target into an echo electric pulse by a light detector, calculates the time of flight (ToF) of light according to the sending time of the detection light pulse and the receiving time of the echo light pulse, and obtains the target distance in combination with the speed of light, and characterizes the echo light intensity according to the signal characteristics of the echo electric pulse, and calculates the reflectivity of the target according to the echo light intensity and the equivalent emitted light intensity attenuated with distance.

[0003] Conventional optical detectors are avalanche photon diodes (APDs), which have low photoelectric gain and are disadvantageous for long-distance detection by radar. In order to increase the detection distance, the aperture of the light receiving lens must be enlarged, which increases the volume of the laser radar. In addition, a high-bandwidth amplifier circuit must be placed for each detector in the system to amplify the electrical pulse signal, which increases the volume, power consumption, and cost of the laser radar.

[0004] Photodiodes operating in Geiger mode use an avalanche discharge mechanism to obtain high gain and are called Single Photon Avalanche Diodes (SPADs). When the reverse bias exceeds its rated breakdown voltage, the necessary high electric field gradient is created at the PN junction, and when a photon enters the SPAD, the carriers excited by the photon will cause an avalanche effect under the action of the electric field. However, once such a current is activated or flowing, it should be stopped or "quenched". A quenching resistor reduces the voltage across the diode below the breakdown voltage, thereby stopping the avalanche. The SPAD is then recharged to a reverse bias above the breakdown voltage and used to detect subsequent photons. During the period between when the SPAD avalanches and when it re-recovers to a reverse bias above the breakdown voltage, the SPAD does not respond to subsequent photons.

[0005] To overcome the drawback of a single SPAD not being able to measure multiple photons simultaneously, a silicon photomultiplier (SiPM) integrates a high-density, small, independent SPAD sensor array, each with its own quenching resistor, and the SPAD and quenching resistor operating independently are called microunits. When one microunit in a SiPM responds to an absorbed photon, the other microunits still have the photon detection capability.

[0006] SiPM has extremely high photoelectric gain (10 6 SiPMs have a power consumption of 1000 W (nW level) and can detect echo energy at the nW level, whereas APDs require echo energy at the mW level for effective detection. Compared with APDs, using SiPMs as optical detectors can effectively reduce the power consumption of laser radars. In addition, SiPMs can be manufactured using CMOS processes, which is advantageous for chip formation and integration, reducing the volume of the radar and improving the efficiency of assembly and debugging, making them suitable for large-scale mass production.

[0007] Laser radars using SiPMs as detectors usually use the slope and pulse width of the leading edge of the echo electrical pulse as signal features to measure the reflectivity. However, the response signal of SiPMs to photon flow has high statistical variability and nonlinear characteristics, and the signal features may randomly jitter for the same intensity of echo energy, resulting in low reflectivity measurement accuracy. In addition, the number of microunits in SiPMs is limited, and under strong light, almost all microunits in SiPMs are triggered within the recovery time (a few tens of nanoseconds), which makes it very easy to saturate the detector, and at this time, the echo electrical signal features hardly change with the target reflectivity, resulting in a very small dynamic range of reflectivity measurement. Therefore, devices based on SiPMs face the challenges of low accuracy and small dynamic range in reflectivity measurement, which is one of the main obstacles to the application of SiPMs in laser radar products.

[0008] The contents of the Background Art section are merely technologies known to the discloser and do not necessarily represent prior art in the field. Summary of the Invention [Means for solving the problem]

[0009] In view of at least one shortcoming of the prior art, the present invention provides a method for manufacturing a semiconductor device comprising: The laser radar detector receives an echo reflected by a target with a detection beam, converts the echo into an electrical signal, and obtains an integral of the echo photocurrent; and determining a reflectivity of the target based on a preset reflectivity calibration curve and the echo photocurrent integral.

[0010] According to one aspect of the present invention, step S101 includes: The method further includes integrating the electrical signal within a first predetermined time period to obtain a first photocurrent integral corresponding to the electrical signal.

[0011] According to one aspect of the present invention, step S101 includes: The method further includes obtaining a second photocurrent integral corresponding to ambient light, and determining the difference between the first photocurrent integral and the second photocurrent integral as the echo photocurrent integral.

[0012] According to one aspect of the present invention, step S101 includes: The detector receives ambient light and converts it into an electrical signal; and integrating the electrical signal generated by the ambient light over a second predetermined time period to obtain the second photocurrent integral.

[0013] According to one aspect of the invention, the first predetermined time is equal to or greater than the total time it takes the detector to make one measurement.

[0014] According to one aspect of the present invention, the detector is a silicon photomultiplier tube, and the electrical signal is an electrical signal output from an anode of the silicon photomultiplier tube.

[0015] According to one aspect of the present invention, the laser radar determines a target distance based on a transmission time of the detection beam and a reception time of the echo, and step S102 includes: The method further includes determining the reflectivity of the target based on a preset calibration curve of reflectivity, photocurrent integral and target distance, and the echo photocurrent integral.

[0016] According to one aspect of the present invention, step S102 includes: The method further includes a step of calculating the target reflectivity corresponding to the echo photocurrent integral by interpolation based on preset calibration curves of photocurrent integral and target distance respectively corresponding to a plurality of predetermined reflectivities.

[0017] According to one aspect of the invention, the echo includes a plurality of light pulses, the detector converts the plurality of light pulses into a plurality of electrical pulses, and the echo photocurrent integral is a cumulative integral of the plurality of electrical pulses.

[0018] According to one aspect of the invention, the method comprises the steps of: The method further includes the step of acquiring a temperature of the laser radar detector and converting the echo photocurrent integral to an echo photocurrent integral at an equivalent calibration temperature.

[0019] According to one aspect of the invention, the method comprises the steps of: The method further includes converting the echo photocurrent integral into an echo photocurrent integral in an equivalent calibration PDE when the second photocurrent integral is greater than a preset threshold.

[0020] According to one aspect of the present invention, the laser radar includes a plurality of detectors, and the method further comprises: The method further includes the step of setting calibration curves of reflectance, photocurrent integral and target distance corresponding to each detector, respectively.

[0021] The present invention relates to a transmitting unit configured to transmit a detection beam to detect a target; a receiving unit including at least one detector and configured to receive an optical signal and convert the optical signal into an electrical signal; The present invention further provides a laser radar including: a processing unit configured to acquire an echo photocurrent integral corresponding to an echo reflected by a target from the detection beam, and determine a reflectivity of the target based on a preset reflectivity calibration curve and the echo photocurrent integral.

[0022] According to one aspect of the present invention, the laser radar comprises: The detector further includes an integration unit coupled to the detector and configured to integrate the electrical signal within a predetermined time period.

[0023] According to one aspect of the invention, the detector is a silicon photomultiplier tube and the integration unit is coupled to the anode of the detector.

[0024] According to one aspect of the invention, the integration unit includes an RC integration circuit.

[0025] According to one aspect of the present invention, the integration unit further includes an operational amplifier, the RC integration circuit is coupled to a first input terminal of the operational amplifier, and the output terminal of the operational amplifier is coupled to a second input terminal of the operational amplifier via a resistor.

[0026] According to one aspect of the present invention, the laser radar comprises: The system further includes a sampling unit coupled to the integration unit and configured to sample an output of the integration unit.

[0027] According to one aspect of the present invention, the laser radar comprises: The detector further includes a temperature detection unit for detecting a temperature of the detector.

[0028] A preferred embodiment of the present invention provides a method for measuring the reflectance of a target by integrating photocurrent, which is based on the long-term accumulation of the response current of a photodetector to photons, and the method provided by the present invention has lower power consumption, higher accuracy, higher measurement precision, and larger dynamic range than the prior art method for measuring reflectance by measuring signal features such as the peak intensity, pulse width, and leading edge slope of an echo electric pulse. In particular, it can overcome the adverse effects on reflectance measurement caused by defects such as random jitter, nonlinearity, saturation tendency, and small dynamic range of the echo electric pulse signal features, and thus plays a good role in promoting the wide application of silicon photomultiplier tube devices in the field of laser radar.

[0029] The drawings are intended to provide a further understanding of the invention, to form part of the specification, and to illustrate the invention together with the examples thereof, but are not intended to limit the invention. [Brief description of the drawings]

[0030] [Figure 1] The internal structure of a silicon photomultiplier tube is shown diagrammatically. [Diagram 2]13A and 13B are schematic diagrams showing output signal waveforms of a high-speed output port of a silicon photomultiplier tube. [Diagram 3] 1 illustrates a method for measuring the reflectivity of a target using laser radar, according to a preferred embodiment of the present invention. [Figure 4] 3 illustrates a schematic diagram of a preset reflectance calibration curve according to a preferred embodiment of the present invention; [Diagram 5] 1 shows a schematic diagram of the degradation of detection efficiency of a silicon photomultiplier tube in strong ambient light. [Figure 6A] 1 illustrates a method for calibrating a preset reflectance according to a preferred embodiment of the present invention. [Figure 6B] 6B shows a schematic diagram of relevant equipment used in the calibration method of FIG. 6A; [Figure 7] 1 illustrates a schematic diagram of a laser radar according to a preferred embodiment of the present invention. [Figure 8A] 1 shows a schematic diagram of an integrator circuit according to a preferred embodiment of the present invention; [Figure 8B] 8B illustrates a schematic diagram of an output waveform of the integrating circuit of FIG. 8A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] In the following, only some exemplary embodiments are briefly described. As can be understood by those skilled in the art, the described embodiments can be modified in various ways without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be understood as illustrative in nature and not restrictive.

[0032] In describing the present invention, it is to be understood that the orientations or positional relationships indicated by the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc. are based on the drawings and are merely for the purpose of easily explaining and simplifying the description of the present invention, and do not indicate or imply that the devices or elements described necessarily have a particular orientation or are constructed or operated in a particular orientation, and should not be understood as limiting the present invention. In addition, the terms "first" and "second" are merely for descriptive purposes and should not be understood as indicating or implying a relative importance or the number of technical features indicated. Thus, a feature limited as "first" or "second" may explicitly or implicitly include one or more of the said features. In describing the present invention, unless expressly and specifically limited, "plurality" means two or more than two.

[0033] In the description of the present invention, unless otherwise clearly specified or limited, the terms "attach", "couple" and "connect" should be understood in a broad sense, for example, fixedly connected, detachably connected, integrally connected, mechanically connected, electrically connected, or in communication with each other, directly connected, or indirectly connected through an intermediate medium, or the insides of two elements may communicate with each other, or the relationship of interaction between two elements. Those skilled in the art may understand the specific meaning of the above terms in the present invention according to the specific situation.

[0034] In the present invention, unless otherwise clearly specified or limited, a first feature being "above" or "below" a second feature may include a case where the first and second features are in direct contact with each other, or may include a case where the first and second features are not in direct contact with each other but are in contact with each other through another feature between them. Furthermore, a first feature being "above," "above," and "on the upper surface" of a second feature may include a case where the first feature is directly above and diagonally above the second feature, or may simply mean that the horizontal height of the first feature is higher than that of the second feature. A first feature being "below," "below," and "on the lower surface" of a second feature may include a case where the first feature is directly below and diagonally below the second feature, or may simply mean that the horizontal height of the first feature is smaller than that of the second feature.

[0035] In the following disclosure, many different embodiments or examples are provided to realize different structures of the present invention. In order to simplify the disclosure of the present invention, the following describes specific example components and configurations. Of course, they are merely examples and are not intended to limit the present invention. In addition, the present invention may duplicate reference numerals and / or alphabets in different examples, and such duplication is for the sake of brevity and clarity, and does not in itself indicate a relationship between the various embodiments and / or configurations described. In addition, the present invention provides examples of various specific processes and materials, but those skilled in the art may envision the application of other processes and / or the use of other materials.

[0036] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. It should be understood that the embodiment described herein is merely for the purpose of explaining and interpreting the present invention and is not intended to limit the present invention.

[0037] As shown in FIG. 1, the SiPM includes multiple avalanche photodiodes and quenching circuits (illustrated as quenching resistors) that are coupled in one-to-one correspondence with the avalanche photodiodes, and one avalanche photodiode and quenching circuit constitute one micro unit, with multiple micro units connected in parallel to form the SiPM.

[0038] Some SiPM devices have a third output terminal (Fast Output, not shown) in addition to the cathode and anode, and each microunit has a capacitively coupled output, and the capacitively coupled output terminals of all microunits of one SiPM are connected in parallel to the Fast Output. By outputting pulses at high speed, ultra-fast time measurements can be performed.

[0039] Figure 2 shows a typical waveform of the electrical pulse signal measured at the fast output terminal after the SiPM receives the optical pulse and converts the optical pulse into an electrical pulse signal. The same target is measured multiple times at the same distance, and each measurement corresponds to a different echo energy intensity. The corresponding signal waveform distribution fast output from the SiPM is shown in Figure 2. As the echo energy intensity increases, the pulse intensity fast output improves and the slope of the leading edge increases. If the ordinate 20 is the noise threshold, the time difference between the leading edge reaching the noise threshold for the electrical pulses generated by echoes of different energy intensities is 350ps.

[0040] However, by measuring the same intensity signal multiple times and performing data calculations and statistics, we found that the slope of the leading edge of the electric pulse has random jitter of several picoseconds to more than 1 nanosecond, which is due to the photon flow following a Poisson distribution and the existence of optical crosstalk between the microunits of the SiPM. Since the bottom width of the echo signal (the time width when the signal amplitude is greater than zero) is only a few nanoseconds, such random jitter cannot be ignored and reduces the accuracy of the reflectivity measurement.

[0041] On the other hand, with the change of signal intensity, the echo electric pulse feature does not change monotonically. When the echo energy intensity is low, the slope of the leading edge of the electric pulse increases with the increase of the echo energy intensity, but when the echo energy intensity is very high, the overwhelming majority of the micro units of the silicon photomultiplier are all triggered in a short time and cannot return within the pulse time, so the phenomenon occurs that the slope of the leading edge of the echo electric pulse does not change with the echo energy intensity, and even the slope of the leading edge decreases. At this time, the echo electric pulse feature has ambiguity (one echo electric pulse feature corresponds to two or more echo energy intensities), and the change of the echo energy cannot be characterized, and the silicon photomultiplier reaches the limit of the dynamic range. Because the number of micro units of the silicon photomultiplier is limited (several hundred), it is very easy to cause reception saturation, so the dynamic range of the energy characterization by the echo pulse feature is very small, and the dynamic range of the reflectance measurement is very small.

[0042] In order to solve the technical problems of large random jitter of electrical pulse signal characteristics and small dynamic range faced by reflectance measurement by laser radar based on silicon photomultiplier tubes, the present invention provides a method for reflectance calibration and measurement based on photocurrent integration of silicon photomultiplier tubes.

[0043] As shown in FIG. 3, according to a preferred embodiment of the present invention, the present invention provides a method 10 for measuring the reflectivity of a target using a laser radar, which includes step S101 and step S102.

[0044] In step S101, the detector of the laser radar receives an echo optical signal reflected by a target from a detection beam, converts the received echo optical signal into an electrical signal, and obtains an echo optical current integral.

[0045] In the application of laser radar based on ToF measurement, the detection beam may include one or more light pulses, and accordingly, the echo light signal is one or more light pulses corresponding to the detection beam, and the photodetector responds to the light pulses, converts it into an electrical signal, and generates one or more electrical pulses corresponding thereto. The electrical pulses may be represented as a curve of electrical signal amplitude over time, and the echo photocurrent integral may be represented as the area covered by the electrical pulse signal curve, which can be used to characterize the echo energy intensity.

[0046] In step S102, the reflectance of the target is determined based on a preset reflectance calibration curve and the echo photocurrent integral.

[0047] According to a preferred embodiment of the present invention, for a target with a certain reflectivity, a corresponding relationship between the target distance and the echo photocurrent integral is preset, and for targets with different reflectivities, a corresponding relationship between the target distance and the echo photocurrent integral is respectively obtained, and the above-mentioned corresponding relationship is stored in the processing unit of the laser radar, and the processing unit can determine the reflectivity of the target according to the echo photocurrent integral obtained by actual detection and the preset reflectivity calibration curve.

[0048] In a preferred embodiment, the corresponding relationship between the target distance and the echo photocurrent integral can be fitted as a calibration curve. As shown in Fig. 4, the calibration curves of the target distance and the echo photocurrent integral corresponding to three targets with different reflectances are preset.

[0049] The method 10 for measuring the reflectivity of a target using a laser radar measures the reflectivity of the target by obtaining an echo photocurrent integral, in which the echo photocurrent integral does not have random jitter and can accurately reflect the echo energy intensity, thereby improving the accuracy of the reflectivity measurement, and the echo photocurrent integral is single-valued and changes monotonically with the echo energy intensity, thereby increasing the dynamic range of the reflectivity measurement.

[0050] According to a preferred embodiment of the present invention, in a method 10 for measuring the reflectivity of a target using a laser radar, step S101 includes: The method further includes integrating the electrical signal output from the laser radar detector within a first predetermined time period to obtain a first photocurrent integral corresponding to the electrical signal.

[0051] By integrating the electrical signal output from the detector within a first predetermined time period, the area covered by the time-varying curve of the amplitude of the electrical signal output from the detector can be obtained, which can be used to characterize the energy intensity of the electrical signal output from the detector.

[0052] According to a preferred embodiment of the present invention, in the method 10 for measuring the reflectivity of a target using a laser radar, step S101 further includes the steps of obtaining a second photocurrent integral corresponding to the ambient light, and determining the difference between the first photocurrent integral and the second photocurrent integral as the echo photocurrent integral.

[0053] The laser radar detector converts the received optical signal into an electrical signal and integrates it to obtain the photocurrent integral (referred to as the first photocurrent integral) which actually corresponds to the sum of the echo optical signal and the ambient optical signal. The photocurrent integral corresponding to the echo optical signal is difficult to measure directly, because the intensity of the ambient light is stable within a short time, the photocurrent integral corresponding to the ambient optical signal is basically kept constant within a unit time, therefore, the photocurrent integral corresponding to the ambient optical signal (referred to as the second photocurrent integral) is obtained by further measurement, and the echo photocurrent integral corresponding to the echo optical signal is obtained by calculating the difference.

[0054] According to a preferred embodiment of the present invention, in a method 10 for measuring the reflectivity of a target using a laser radar, step S101 includes: The detector receives the ambient light and converts it into an electrical signal of the ambient light; and integrating the ambient light electrical signal within a second predetermined time period to obtain a second photocurrent integral.

[0055] Furthermore, the first photocurrent integral includes information on the electrical signal generated on the photodetector by the echo of the detection beam reflected by the target and information on the electrical signal generated on the photodetector by the ambient light. The second photocurrent integral is obtained only from the electrical signal generated on the photodetector by the ambient light. Therefore, by calculating the difference between the first photocurrent integral and the second photocurrent integral, the electrical signal strength due to the echo optical signal can be accurately characterized and the influence of the ambient light can be removed.

[0056] In one embodiment, the time interval of the first predetermined time is equal to or longer than the total time for the detector to perform one measurement. Alternatively, at least one laser device of the laser radar is configured to emit detection light for detection, and at least one detector is configured to receive an echo of the detection light reflected by a target, and the at least one laser device and the at least one detector constitute one detection channel of the laser radar. The time interval of the first predetermined time is equal to or longer than the total measurement time of one detection channel.

[0057] Specifically, within the total measurement time of one detection channel of the laser radar, the detector receives an optical signal and converts it into an electrical signal, the optical signal includes an echo optical signal and an ambient light signal, and the integration circuit outputs the integration of the electrical signal within the total time, i.e., the first photocurrent integration. The total measurement time may include the reserved ToF time of the detection channel, the total pulse width of a plurality of optical pulses, and the sum of the time intervals between the plurality of optical pulses. The reserved ToF time is the flight time corresponding to the farthest target detection distance of the radar after the detection light is emitted from the laser device of the detection channel. For example, the detection light emitted from one detection channel includes three optical pulses, each pulse width is 4ns, the time interval between two adjacent optical pulses is 100ns, and the reserved ToF time is 400ns, so that the first predetermined time interval is ≧(400+3*4+2*100)ns.

[0058] During the total measurement time of one detection channel, the SiPM still generates a weak photocurrent due to the action of the ambient light, and the integration circuit then integrates the electrical signal of the ambient light within a second predetermined time to obtain a second photocurrent integral.

[0059] Preferably, the time interval between the first and second predetermined times is approximately the same, and more preferably, the time interval between the first and second predetermined times is on the order of 100 ns.

[0060] In an embodiment where the SiPM is used as a detector, the photocurrent integral is the time integral of the photocurrent output from the anode of the SiPM. Although the fast output allows for ultrafast time measurement, it is not suitable for measuring the echo energy intensity because a lot of information has been filtered out. The time integral of the photocurrent output from the anode of the SiPM increases as the echo energy intensity increases, which can accurately reflect the magnitude of the echo energy intensity and is useful for obtaining highly accurate reflectance measurement results.

[0061] In another embodiment, when an echo signal is determined as an echo reflected by a target by signal processing of a laser radar, and target distance information is obtained by this (for example, when the strength of the echo signal exceeds a noise threshold), an integral within a first predetermined time corresponding to the reception time of the echo signal is taken as a first photocurrent integral, and an integral of an integrating circuit within a second predetermined time is obtained in a time range other than the reception time of the echo signal, to obtain a second photocurrent integral.

[0062] According to a preferred embodiment of the present invention, the laser radar determines the target distance based on the transmission time of the search beam and the reception time of the echo. In the method 10 for measuring the reflectivity of a target using a laser radar, step S102 includes: The method further includes determining the reflectivity of the target based on a preset calibration curve of reflectivity, photocurrent integral and target distance, and echo photocurrent integral.

[0063] As shown in FIG. 4, the reflectance calibration curves prestored in the processing unit of the laser radar include curves in which the value of the photocurrent integral changes with distance at different reflectances.

[0064] According to a preferred embodiment of the present invention, the reflectance calibration curve includes a plurality of different predetermined reflectance photocurrent integral-distance curves. According to a preferred embodiment of the present invention, the reflectance calibration curve includes a calibrated predetermined reflector 1 reflectance value R1, a predetermined reflector 2 reflectance value R2, and a predetermined reflector 3 reflectance value R3, and the echo photocurrent integral V d0 From the target distance d, the target reflectance R can be determined by linear interpolation.

[0065] From the following linear interpolation formula (Equation 1),

[0066]

number

[0067]

number

[0068] Although the preset echo photocurrent integral curve is performed at a certain calibration temperature, the gain of the photodetector changes at different temperatures, and the photocurrent integral generated when receiving the same intensity of optical signal is different, so that a drift phenomenon occurs in the actually measured reflectance value. Therefore, it is necessary to perform temperature compensation on the echo photocurrent integral obtained by the actual measurement to obtain the echo photocurrent integral at the equivalent calibration temperature, so that the reflectance of the target can be accurately calculated based on the calibration curve at the calibration temperature.

[0069] For silicon photomultipliers, the photocurrent integral and temperature have the following relationship (Equation 3):

[0070]

number

[0071] Using the above formula, temperature compensation can be performed on the photocurrent integral of the silicon photomultiplier tube, and the echo photocurrent integral V measured in real time within the operating temperature range of the laser radar can be calculated. d is the echo photocurrent integral V at the equivalent calibration temperature T0 d,T0 can be converted to

[0072] The temperature compensation method provided in the preferred embodiment of the present invention reduces the temperature drift of the actual measured reflectance by 80%.

[0073] In accordance with one preferred embodiment of the present invention, a method 10 for measuring target reflectivity using laser radar includes: The method further includes converting the echo photocurrent integral into an echo photocurrent integral in an equivalent PDE if the second photocurrent integral is greater than a preset threshold.

[0074] When the ambient light is weak, the photocurrent integral corresponding to the echo optical signal and the photocurrent integral corresponding to the ambient light signal can be linearly superimposed. However, when the ambient light is strong, as shown in FIG. 5, the ambient light signal causes some SPADs in the silicon photomultiplier tube to be saturated, and these SPADs are not triggered within the recovery time, so that the number of SPADs triggered by the echo optical signal decreases, and the response ability of the detector to the echo optical signal decreases, which is equivalent to a decrease in the photon detection efficiency (PDE). Therefore, when the ambient light is strong, it is necessary to compensate the equivalent PDE affected by the ambient light for the echo photocurrent integral so as to obtain the echo photocurrent integral in the equivalent calibrated PDE.

[0075] For a given silicon photomultiplier, the equivalent PDE is only related to the first photocurrent integral corresponding to the output electrical signal of the silicon photomultiplier and the second photocurrent integral corresponding to the ambient light, and is unrelated to the target reflectance and distance, but is affected by temperature.

[0076] According to a preferred embodiment of the present invention, the equivalent PDE=echo photocurrent integral / second photocurrent integral. In a constant temperature environment, the equivalent PDE is calibrated for each of the multiple silicon photomultipliers of the laser radar, and the calibration relationship between the equivalent PDE and the first photocurrent integral, the second photocurrent integral, or the calibration relationship between the equivalent PDE and the echo photocurrent integral (the difference between the first photocurrent integral and the second photocurrent integral), the first photocurrent integral is determined, and ambient light compensation is performed on the echo photocurrent integral to obtain the echo photocurrent integral in the equivalent calibrated PDE.

[0077] Specifically, the first photocurrent integral and the second photocurrent integral are measured under different ambient light intensities, respectively, and the equivalent PDE is calculated. Alternatively, the equivalent PDE is calculated based on the echo photocurrent integral and the second photocurrent integral. Thus, a calibration relationship between the equivalent PDE and the first photocurrent integral and the second photocurrent integral, or a calibration relationship between the equivalent PDE and the echo photocurrent integral and the second photocurrent integral is obtained.

[0078] Here, the critical value of the ambient light current integral V B0 In one embodiment, a threshold value of the ambient light intensity V can be set at a calibration condition. B0 corresponds to the second photocurrent integral of the calibration environment. The second photocurrent integral V corresponds to the ambient light signal. B is the critical value V B0 If it is higher, the ambient light signal exceeds the ambient light intensity under the calibration condition, and the equivalent PDE of the silicon photomultiplier tube is considered to be lower than the calibrated value. In this case, ambient light compensation must be performed.

[0079] According to a preferred embodiment of the present invention, an equivalent PDE corresponding to the actually measured photocurrent integral is determined based on the above calibration relationship, the ratio of the calibration PDE and the equivalent PDE is taken as a compensation coefficient, and the echo photocurrent integral is multiplied by the compensation coefficient to obtain the echo photocurrent integral in the equivalent calibration PDE. For example, when the equivalent PDE under the calibration condition is 1, the equivalent PDE corresponding to the actually measured first photocurrent integral and second photocurrent integral is 0.9, and the compensation coefficient is 1 / 0.9, the echo photocurrent integral in the equivalent calibration PDE after compensation is (the first photocurrent integral - the second photocurrent integral) / 0.9.

[0080] The second photocurrent integral V corresponds to the ambient light signal. B is the threshold V B0 At lower levels, the ambient light signal is believed to have essentially no effect on the detection efficiency of the silicon photomultiplier tube.

[0081] In accordance with a preferred embodiment of the present invention, a laser radar includes a plurality of detectors, and a method 10 for measuring target reflectivity using a laser radar includes: The method further includes the step of setting calibration curves of reflectance, photocurrent integral and target distance corresponding to each detector, respectively.

[0082] As shown in FIGS. 6A and 6B, the present invention provides a method 20 for a laser radar to perform reflectivity calibration for each detection channel, which includes the following steps S201 to S207.

[0083] In step S201, a laser radar and a plate with a specified reflectivity are installed.

[0084] In step S202, the laser radar is incident at a normal angle on a plate with a predetermined reflectivity.

[0085] In step S203, under weak ambient light (cloudy or indoor light), the reflectance plate is moved, the first photocurrent integral curve and the second photocurrent integral curve are measured, and their difference is calculated to obtain a curve that changes according to the distance of the echo photocurrent integral corresponding to the reflectance of a given reflectance plate.

[0086] In step S204, under strong ambient light, the first photocurrent integral and the second photocurrent integral are measured, and their difference is taken to obtain the echo photocurrent integral.

[0087] In step S205, the relationship between the equivalent PDE and the echo photocurrent integral and the second photocurrent integral is calibrated.

[0088] In step S206, the plate with the predetermined reflectance is replaced, and steps S201 to S205 are repeated.

[0089] In step S207, a calibration table is generated and stored in the laser radar.

[0090] Here, in step S205, the equivalent PDE is equal to the ratio of the echo photocurrent integral to the second photocurrent integral.

[0091] According to a preferred embodiment of the present invention, as shown in FIG. 7, the present invention provides a laser radar 100, which includes a transmitting unit 110, a receiving unit 120 and a processing unit .

[0092] The transmitting unit 110 is configured to transmit a detection beam to detect a target.

[0093] The receiving unit 120 includes at least one detector and is configured to receive an optical signal and convert the optical signal into an electrical signal.

[0094] The processing unit 130 is configured to obtain an echo photocurrent integral corresponding to an echo of the detection beam reflected by the target, and determine the reflectivity of the target based on a preset reflectivity calibration curve and the echo photocurrent integral.

[0095] According to a preferred embodiment of the present invention, as shown in FIG. 7, a laser radar 100 includes: The detector further includes an integration unit 140 coupled to the detector of the receiving unit 120 and configured to integrate the electrical signal within a predetermined time period.

[0096] Preferably, the input terminal of the integration unit 140 is coupled to the anode or cathode of the laser radar detector to obtain an integral of the electrical signal output from the detector.

[0097] According to a preferred embodiment of the present invention, the detector is a silicon photomultiplier tube and the integration unit 140 is coupled to the anode of the silicon photomultiplier tube.

[0098] According to a preferred embodiment of the present invention, the integration unit 140 includes an RC integration circuit.

[0099] According to another preferred embodiment of the present invention, the integration unit 140 further includes an operational amplifier, and the RC integrator circuit is coupled to a first input terminal of the operational amplifier.

[0100] Preferably, the output terminal of the operational amplifier is coupled to the second input terminal of the operational amplifier via a resistor.

[0101] According to a preferred embodiment of the present invention, as shown in FIG. 7, a laser radar 100 includes: It further includes a sampling unit 150 coupled to the output terminal of the integration unit 140 and configured to sample the output of the integration unit 140 to obtain a photocurrent integral.

[0102] FIG. 8A illustrates an integration unit 140 according to one embodiment of the present invention.

[0103] Fig. 8A is a schematic diagram of an integrating circuit used in the integrating unit 140 of the present invention, and Fig. 8B is an output signal diagram of the integrating circuit. Referring to Fig. 8A and Fig. 8B, a laser radar detector (preferably a silicon photomultiplier tube SiPM in the figure) is triggered by a photon avalanche with a reverse bias Vs, and the anode of the silicon photomultiplier tube SiPM outputs an electric pulse, which is integrated through an RC circuit consisting of a resistor r1 and a capacitor C, and converted into a voltage signal V by an operational amplifier.

[0104] 8A, the sampling unit 150 is an analog digital converter (ADC), more preferably a low-speed ADC, which can sample the voltage signal output from the operational amplifier and obtain the voltage signal peak as the photocurrent integral.

[0105] On the other hand, for a SiPM that uses a fast output electrical signal to calculate the target distance, the integrating circuit of the present invention is connected to the anode of the SiPM and has no effect on the timing measurement of the Fast Output. According to a preferred embodiment of the present invention, the target distance can be quickly measured and accurate reflectance information can be obtained based on the photocurrent integral output from the integrating circuit.

[0106] Preferably, the integration period of the integration circuit is the same as the detection period of the silicon photomultiplier tube SiPM, and can reach the hundreds of nanoseconds level, which significantly improves the measurement accuracy compared with the prior art in which the reflectivity of the target is determined by measuring the gradient of the leading edge of the echo pulse or the pulse width at the nanosecond level. When the echo electrical pulse signal has random jitter, the reflectivity is measured by measuring the echo photocurrent integral, which can eliminate the influence of the electrical pulse signal of the device, and further improve the accuracy of the reflectivity measurement. In addition, the echo photocurrent integral changes monotonically according to the echo energy intensity, has no ambiguity, is not affected by the saturation of the device, and improves the dynamic range of the reflectivity measurement.

[0107] According to the characteristics of SPAD, when an avalanche occurs, any defect in the avalanche multiplication region can become a carrier capture region. When a photon is incident on the SPAD, the charge generated based on the photoelectric effect passes through the avalanche multiplication region of the detector, some carriers are captured by these defects, and when the photoelectric conversion process is finished, the carriers released from these defects are accelerated by the electric field, which causes another avalanche, generating a rear pulse related to the previous avalanche pulse. The rear pulse causes jitter in the electrical pulse waveform output from the detector, which causes a large error in the calculation of the reflectivity using the leading edge slope and pulse width.

[0108] However, the intensity of the rear pulse increases with the increase in the intensity of the incident light. The present invention not only can eliminate the influence of the waveform jitter generated by the rear pulse by skillfully integrating the electrical signal in time, but also can utilize the law that the intensity of the rear pulse increases with the increase in echo energy, so as to more accurately reflect the intensity of the echo light, and improve the calculation accuracy of the reflectance of the target.

[0109] In one embodiment, the laser devices of the laser radar correspond one-to-one to the detectors (SiPMs), and when one laser device is strobed to emit light, the corresponding SiPM is activated to receive the echo signal. Before switching to the next silicon photomultiplier, the switch K of the integration circuit is turned on once to clear the integrated charge on the capacitor C.

[0110] As can be understood by those skilled in the art, FIG. 8A only shows one type of integration circuit adopted in the integration unit of the present invention, and other circuit forms for realizing integration of the output photocurrent of the laser radar detector are also within the scope of protection of the present invention.

[0111] In accordance with a preferred embodiment of the present invention, the return echo of the search beam includes a plurality of light pulses, and the echo photocurrent integral is the cumulative integral of a plurality of electrical pulses to which the plurality of light pulses correspond.

[0112] As shown in Fig. 8B, in one detection period, the transmitting unit 110 transmits multiple laser pulse signals (may be 2 to 4), and at least one detector of the corresponding receiving unit 120 receives multiple echo light pulse signals even during detection, and integrates all echo electrical pulses in one detection period. That is, in each ToF measurement, the transmitting unit 110 transmits 2 to 4 laser pulse signals, and the receiving unit 120 receives the reflected echoes of the 2 to 4 pulse signals and accumulates and integrates them. In a preferred embodiment of the present invention, the cumulative integration method is used to significantly reduce the measurement error caused by single pulse jitter, and further improve the measurement accuracy of the reflectance.

[0113] Preferably, the laser radar 100 includes a plurality of integration units 140 respectively coupled to the plurality of detectors of the receiving unit 120, integrating the photocurrents output by the corresponding detectors during detection and outputting the integration results. Alternatively, the integration units 140 are respectively coupled to the plurality of detectors of the receiving unit 120, integrating the photocurrents output by the strobed detectors during detection and outputting the integration results.

[0114] According to a preferred embodiment of the present invention, the integration unit 140 of the laser radar 100 further comprises: integrating the electrical signal generated by the ambient light over a second predetermined time period to obtain a second photocurrent integral corresponding to the ambient light; The detector of the laser radar is configured to integrate the electrical signal when receiving the echo within a first predetermined time to obtain a first photocurrent integral.

[0115] According to a preferred embodiment of the present invention, the processing unit 130 further comprises: The device is configured to obtain a first photocurrent integral and a second photocurrent integral, and to determine a difference between the first photocurrent integral and the second photocurrent integral as an echo photocurrent integral.

[0116] Preferably, the first and second predetermined times are approximately the same in time interval, and more preferably, the first and second predetermined times are on the order of hundreds of nanoseconds.

[0117] According to a preferred embodiment of the present invention, the processing unit 130 further comprises: determining a target distance based on the transmission time of the search beam and the reception time of the echo; The reflectance of the target is determined based on a preset reflectance, a calibration curve of photocurrent integral and target distance, and the echo photocurrent integral.

[0118] According to a preferred embodiment of the present invention, the processing unit 130 further comprises: The reflectance of the target corresponding to the echo photocurrent integral is calculated by an interpolation method based on preset calibration curves of photocurrent integral and target distance respectively corresponding to a plurality of predetermined reflectances.

[0119] According to a preferred embodiment of the present invention, the laser radar further includes a detection unit for acquiring a temperature of the detector.

[0120] The processing unit 130 further comprises: The laser radar detector is configured to convert the echo photocurrent integral into an echo photocurrent integral at an equivalent calibration temperature based on the temperature of the laser radar detector.

[0121] According to a preferred embodiment of the present invention, the processing unit 130 further comprises: When the second photocurrent integral is greater than a preset threshold, the echo photocurrent integral is configured to be converted into an echo photocurrent integral in an equivalent calibration PDE.

[0122] According to a preferred embodiment of the present invention, the processing unit 130 further comprises: Each detector is configured to calculate a target reflectivity based on the echo photocurrent integral corresponding to each detector.

[0123] A preferred embodiment of the present invention provides a method for measuring the reflectance of a target by integrating photocurrent, which is based on the long-term accumulation of the response current of a photodetector to photons, and the method provided by the present invention has lower power consumption, higher accuracy, higher measurement precision, and larger dynamic range than the prior art method for measuring reflectance by measuring signal features such as the peak intensity, pulse width, and leading edge slope of an echo electric pulse. In particular, it can overcome the adverse effects on reflectance measurement caused by defects such as random jitter, nonlinearity, saturation tendency, and small dynamic range of the echo electric pulse signal features, and thus plays a good role in promoting the wide application of silicon photomultiplier tube devices in the field of laser radar.

[0124] Finally, it should be explained that the above are only preferred embodiments of the present invention, and are not intended to limit the present invention, and the present invention has been described in detail with reference to the above embodiments, but those skilled in the art can still modify the technical solutions described in the above embodiments or make equivalent substitutions for some of the technical features therein. All modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for measuring the reflectivity of a target using a laser radar, comprising: Step S101: receiving an echo reflected by a target with a detection beam of the laser radar detector, converting the echo into an electrical signal, and acquiring an integral of the echo photocurrent; determining a reflectance of the target based on a preset reflectance calibration curve and the echo photocurrent integral; The step S101 further includes integrating the electrical signal within a first predetermined time to obtain a first photocurrent integral corresponding to the electrical signal; The step S101 further includes the steps of obtaining a second photocurrent integral corresponding to ambient light, and setting the difference between the first photocurrent integral and the second photocurrent integral as the echo photocurrent integral; The method further includes the step of acquiring a temperature of a detector of the laser radar and converting the echo photocurrent integral to an echo photocurrent integral at an equivalent calibration temperature; The method further includes converting the echo photocurrent integral to an echo photocurrent integral in an equivalent calibrated PDE (Photon Detection Efficiency) if the second photocurrent integral is greater than a preset threshold.

2. The step S101 is receiving ambient light and converting it into an electrical signal by said detector; 2. The method of claim 1, further comprising: integrating the electrical signal generated by the ambient light over a second predetermined time period to obtain the second photocurrent integral.

3. 3. The method of claim 1 or 2, wherein the first predetermined time is equal to or greater than the total time it takes for the detector to make one measurement.

4. 3. The method of claim 1 or 2, wherein the detector is a silicon photomultiplier tube, and the electrical signal is an electrical signal output from an anode of the silicon photomultiplier tube.

5. the laser radar determines a distance from a target based on a time of transmission of the detection beam and a time of reception of the echo; 3. The method according to claim 1 or 2, wherein step S102 further comprises determining the reflectivity of the target based on a preset calibration curve of reflectivity, photocurrent integral and target distance, and the echo photocurrent integral.

6. The step S102 is 6. The method of claim 5, further comprising: calculating a target reflectance corresponding to the echo photocurrent integral by interpolation based on a preset calibration curve of a photocurrent integral and a distance from the target, each corresponding to a plurality of predetermined reflectances.

7. 3. The method of claim 1, wherein the echo comprises a plurality of light pulses, the detector converts the plurality of light pulses into a plurality of electrical pulses, and the echo photocurrent integral is a cumulative integral of the plurality of electrical pulses.

8. The laser radar includes a plurality of detectors; 3. The method according to claim 1, further comprising the step of setting calibration curves of reflectance, photocurrent integral and target distance corresponding to each detector, respectively.

9. a transmitting unit configured to transmit a detection beam to detect a target; a receiving unit including at least one detector and configured to receive an optical signal and convert the optical signal into an electrical signal; a processing unit configured to obtain an echo photocurrent integral corresponding to an echo reflected by the detection beam from a target, and determine a reflectivity of the target based on a preset reflectivity calibration curve and the echo photocurrent integral; an integration unit coupled to the detector; a temperature detection unit for detecting a temperature of the detector; The integration unit further comprises: integrating the electrical signal over a first predetermined time period to obtain a first photocurrent integral corresponding to the electrical signal; configured to obtain a second photocurrent integral corresponding to the ambient light; The processing unit further comprises: a difference between the first photocurrent integral and the second photocurrent integral is defined as the echo photocurrent integral; converting the echo photocurrent integral to an echo photocurrent integral at an equivalent calibration temperature based on a temperature of the detector; A laser radar configured to convert the echo photocurrent integral into an echo photocurrent integral in an equivalent calibration PDE (Photon Detection Efficiency) when the second photocurrent integral is greater than a preset threshold.

10. 10. The laser radar of claim 9, wherein the detector is a silicon photomultiplier tube and the integration unit is coupled to an anode of the detector.

11. The laser radar according to claim 9 or 10, wherein the integration unit includes an RC integration circuit.

12. 12. The laser radar of claim 11, wherein the integration unit further includes an operational amplifier, the RC integration circuit being coupled to a first input terminal of the operational amplifier, and an output terminal of the operational amplifier being coupled to a second input terminal of the operational amplifier via a resistor.

13. 11. The laser radar according to claim 9 or 10, further comprising a sampling unit coupled to the integration unit and configured to sample an output of the integration unit.

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