CN110308456B - Bias voltage adjusting device for improving detection distance and laser radar system - Google Patents
Bias voltage adjusting device for improving detection distance and laser radar system Download PDFInfo
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- CN110308456B CN110308456B CN201910667896.1A CN201910667896A CN110308456B CN 110308456 B CN110308456 B CN 110308456B CN 201910667896 A CN201910667896 A CN 201910667896A CN 110308456 B CN110308456 B CN 110308456B
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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
- G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
- G01S17/06—Systems determining position data of a target
- G01S17/08—Systems determining position data of a target for measuring distance only
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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
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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
- G01S7/481—Constructional features, e.g. arrangements of optical elements
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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
- G01S7/483—Details of pulse systems
- G01S7/486—Receivers
- G01S7/487—Extracting wanted echo signals, e.g. pulse detection
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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
- G01S7/491—Details of non-pulse systems
- G01S7/493—Extracting wanted echo signals
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Abstract
The invention discloses a bias voltage adjusting device and a laser radar system for improving detection distance, wherein the bias voltage adjusting device comprises: the signal acquisition unit is used for acquiring a target echo signal received by the APD detector, extracting an avalanche noise signal from the target echo signal and counting the number of the avalanche noise signals in a preset time period; the signal processor is used for calculating an actual bias voltage value according to the number of the avalanche noise signals and a preset first bias voltage correction model; the first bias voltage correction model is used for representing the mapping relation between the number of avalanche noise signals and an ideal bias voltage value in preset time; the bias voltage adjusting unit is used for generating and outputting an actual bias voltage value to the APD detector under the control of the signal processor; the invention keeps the bias voltage at a voltage value slightly lower than the avalanche voltage of the APD detector all the time by dynamically tracking the avalanche voltage working point of the APD detector, improves the detection sensitivity by improving the bias voltage of the APD detector and further increases the detection distance.
Description
Technical Field
The invention belongs to the technical field of laser ranging, and particularly relates to a high-sensitivity laser receiving device capable of improving the detection distance and anti-interference performance of a laser ranging radar and a laser radar system.
Background
The laser radar is an optical remote sensing technology which takes laser as a working beam and acquires target related information by detecting the characteristics of scattered light of a long-distance target, and has high measurement precision, fine time and space resolution and large detection span. The laser radar uses laser as a light source, and in order to ensure the safety of the laser radar to human eyes, the laser radar has extremely strict limits on the power or energy of the emitted laser light source. In most occasions, the laser radar is required to have the longest detection distance, for example, in the field of automatic driving of automobiles, the ideal requirement of the automobile enterprises on the detection distance of the laser radar is usually 300-500 meters or even more.
To further improve the range of the lidar, there are generally two embodiments. The first is to increase the emission power or energy of the laser, and the second is to improve the detection distance of the whole laser radar by improving the performance such as sensitivity of a laser receiving system, and the currently common receiving system is a detector based on avalanche photodiodes (hereinafter referred to as APDs). In the two improvements, the existing laser radar generally adopts the first scheme to improve the detection distance, but the following problems exist: because the peak power of the laser cannot be increased any more under the limit of the safety power of the laser eyes, the scheme is not easy to realize; and the high-power pulse laser has the problems of high heat dissipation, large volume, large power consumption, complex manufacturing process and the like, so that the scheme of the high-power pulse laser is difficult to realize.
The second scheme can improve the detection distance by improving the performance of the laser receiving system under the condition of not increasing the power of the laser, and has great advantages. The avalanche photodiode is a photoelectric device based on an internal photoelectric effect and has the functions of internal gain and amplification; the avalanche photodiode works under reverse bias, under a certain range of reverse bias, the higher the bias is, the larger the gain is, when the bias infinitely approaches to the avalanche voltage of the avalanche photodiode, the avalanche photodiode tends to the edge of avalanche, and the larger gain can be obtained; theoretically, the gain of an avalanche photodiode is affected by bias voltage and temperature; in order to improve the performance of an APD receiving system, the bias voltage of a photoelectric detector is generally adjusted according to the ambient temperature, so that the influence of temperature change on the sensitivity of the photoelectric detector is reduced, and the response speed and the sensitivity are improved; although this solution improves the sensitivity to some extent, it has the following disadvantages: firstly, an accurate APD temperature bias voltage curve and an accurate APD working temperature are required, and a temperature monitoring circuit is additionally arranged to test the temperature, so that the temperature monitoring circuit not only increases the complexity of a system structure, but also is easy to generate interference on the APD; secondly, the gain of the photoelectric detector is not only affected by the ambient temperature, but also affected by the ambient light such as sunlight, and if the influence of the ambient light on the sensitivity of the photoelectric detector is not considered, the photoelectric detector cannot perform the optimal performance, and the detection distance cannot be increased.
Disclosure of Invention
In view of at least one of the defects or the improvement requirements in the prior art, the present invention provides a bias voltage adjusting device and a laser radar system for increasing a detection distance, wherein an avalanche voltage operating point of an APD detector array is dynamically tracked, so that an APD bias voltage is always maintained at a voltage value slightly lower than the avalanche voltage of the APD detector array, and the detection sensitivity is increased by increasing the bias voltage of the APD detector array, thereby increasing the detection distance.
To achieve the above object, according to one aspect of the present invention, there is provided a bias voltage adjusting apparatus for increasing a detection distance, including a signal acquisition unit, a signal processor, and a bias voltage adjusting unit;
the signal acquisition unit is used for acquiring a target echo signal received by the APD detector, extracting an avalanche noise signal from the target echo signal and counting the number of the avalanche noise signals within a preset time;
the signal processor is used for calculating an actual bias voltage value according to the number of the avalanche noise signals and a preset first bias voltage correction model, so that the actual bias voltage value approaches to the avalanche voltage of the APD detector; the first bias voltage correction model is used for representing the mapping relation between the number of avalanche noise signals and an ideal bias voltage value within preset time;
the bias voltage adjusting unit is used for generating and outputting the actual bias voltage value to the APD detector under the control of the signal processor.
The signal processor dynamically tracks the avalanche voltage working point of the APD detector through the signal acquisition unit and generates APD bias voltage required by the APD detector, so that the APD bias voltage is always maintained at a voltage value slightly lower than the avalanche voltage of the APD detector, and the difference between the APD bias voltage and the avalanche voltage is between 0.5 and 3V; at this time, the APD is at the edge of the avalanche, and a larger gain can be obtained.
Preferably, the signal acquisition unit of the bias voltage adjustment device is further configured to extract a direct current signal corresponding to the background light from the target echo signal;
the signal processor calculates an actual bias voltage value according to the direct current signal corresponding to the background light and a preset second bias voltage correction model; the second bias voltage correction model is used for representing the mapping relation between the voltage value of the direct current signal corresponding to the background light and the ideal bias voltage value.
Preferably, the signal acquisition unit of the bias voltage adjusting device includes a multistage gain amplification module, a time discrimination module, a dc component extraction circuit, a pulse signal processing module, and a first analog-to-digital converter;
the multi-stage gain amplification module is used for amplifying a target echo signal and amplifying a laser pulse signal in the target echo signal to an amplitude which can be identified by the time identification module;
the time discrimination module is used for detecting the arrival time of the laser pulse signal and calculating the target distance according to the arrival time;
the direct current component extraction circuit is used for extracting a noise signal from the amplified target echo signal, wherein the noise signal comprises a direct current signal and an avalanche noise signal corresponding to background light; sending a direct current signal corresponding to the background light to a signal processor, and sending an avalanche noise signal to a pulse signal processing module;
the pulse signal processing module is used for shaping the avalanche noise signal into a standard pulse signal, and the standard pulse signal is processed by the first analog-to-digital converter and then is sent to the signal processor.
Preferably, the multi-stage gain amplification module of the bias voltage adjusting device includes a first-stage preamplifier, a second-stage gain variable amplifier, a third-stage fixed gain amplifier and a fourth-stage gain variable amplifier, which are connected in sequence;
the first-stage preamplifier is connected with the APD detector and is used for carrying out first-stage amplification on a target echo signal received by the APD detector;
the gain of the second-stage gain variable amplifier is controlled by the signal processor to be dynamically adjusted, and the signal stability is maintained in the process of amplifying the target echo signal output by the first-stage preamplifier;
the third-stage fixed gain amplifier further amplifies the target echo signal output by the second-stage variable gain amplifier and outputs a laser pulse signal in the amplified target echo signal to the time discrimination module;
the fourth-stage gain variable amplifier is used for amplifying the noise signal in the target echo signal output by the third-stage fixed gain amplifier.
Preferably, in the bias voltage adjusting apparatus, when the voltage value of the dc signal corresponding to the background light output by the dc component extracting circuit is greater than a preset voltage threshold, the signal processor calculates an actual bias voltage value output to the APD detector according to the second bias voltage correction model; otherwise, the signal processor calculates and outputs an actual bias voltage value to the APD detector according to the first bias voltage correction model; the voltage threshold is 80% of the maximum output voltage of the fourth stage variable gain amplifier.
Preferably, the bias voltage adjusting device, the time discrimination module thereof includes a timing circuit and an FPGA time discrimination circuit;
the timing circuit detects the arrival time of the laser pulse signal under the driving of the laser pulse signal;
and the FPGA time discrimination circuit calculates the target distance according to the arrival time.
Preferably, the bias voltage adjusting apparatus further includes a pulse signal processing module including a pulse signal forming circuit and a pulse signal shaping circuit;
the pulse signal forming circuit is used for arranging the avalanche noise signal into a pulse signal through an internal comparator;
the pulse signal shaping circuit is used for shaping the pulse signal output by the pulse signal forming circuit into a standard pulse signal.
Preferably, in the bias voltage adjusting device, the bias voltage adjusting unit includes a second analog-to-digital converter, a flyback BOOST booster and a bias voltage regulator;
the flyback BOOST booster is used for generating a voltage value higher than the avalanche voltage of the APD detector under the control of the signal processor;
the second analog-to-digital converter is used for sending the actual bias voltage value generated by the signal processor to the bias voltage regulator;
and the bias voltage regulator performs self-adaptive voltage division on the voltage value generated by the flyback BOOST booster according to the actual bias voltage value so as to output the actual bias voltage value to the APD detector.
Preferably, the signal processor of the bias voltage adjusting device is further configured to perform time domain calculation on a plurality of target echo signals received by the APD detector and generated when the target is measured for a plurality of times, generate a distribution curve of probability values of real or noise point distance values in the plurality of echo signals falling onto different distance segments, and take a distance value when the probability value is maximum as the real target distance.
By carrying out time domain calculation processing on a plurality of target echo signals, noise signals corresponding to false targets are filtered by utilizing the irrelevance of noise and the correlation of the target echo signals, the error probability of calculation of the distance value of the system is reduced, and the stability of the system is ensured.
According to another aspect of the present invention there is also provided a lidar system including a bias adjustment device as defined in any preceding claim.
Preferably, the laser radar system further includes a multi-channel laser transmitter and an APD detector;
the APD detector is an APD detection array packaged on a substrate, the APD detection array comprises a plurality of detection elements, and the size of the detection elements and the distance between adjacent detection elements have fixed values; therefore, the receiving system can realize regular spatial multi-channel receiving of a target space only by one optical focusing antenna, and the debugging difficulty of the receiving system is reduced;
the number of channels of the multichannel laser transmitter is the same as the number of detection elements in the APD detection array, so that a plurality of detection channels which are independent in transceiving are formed.
Preferably, the laser radar system further includes an optical collimating antenna and an optical focusing antenna;
the optical collimating antenna is used for collimating the light emitted by the multi-channel laser emitter and adjusting the direction of the emitted light beam so as to align the light beam with a target to be measured;
the optical focusing antenna is used for focusing the light rays reflected by the object to be detected on each detection element of the APD detector array.
In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:
(1) the invention provides a bias voltage adjusting device and a laser radar system for improving detection distance, wherein an avalanche photodiode, a signal acquisition unit, a signal processor and a bias voltage adjusting unit form a closed-loop control system, the signal processor controls the bias voltage applied to the avalanche photodiode in a closed-loop manner by measuring and acquiring the avalanche noise characteristic of the current avalanche photodiode, and the system can track the state of the avalanche photodiode in real time to carry out dynamic adjustment of the bias voltage, so that the avalanche photodiode works in a working state approaching to avalanche voltage, and the state can obtain larger signal gain, thereby improving the detection distance.
(2) The bias voltage adjusting device and the laser radar system for improving the detection distance, provided by the invention, take the influence of background light on the gain of the avalanche photodiode into consideration, correct the bias voltage value input to the avalanche photodiode according to the direct current signal corresponding to the background light in the target echo signal, reduce the influence of the background light on the avalanche photodiode, and improve the response speed and the sensitivity.
(3) According to the bias voltage adjusting device and the laser radar system for improving the detection distance, provided by the invention, the time domain correlation distance calculation method is adopted to process a plurality of target echo signals, and the noise signals corresponding to the false targets are filtered by utilizing the irrelevance of the noise and the correlation of the target echo signals, so that the error probability of calculation of the distance value of the system is reduced, and the stability of the system is ensured.
(4) According to the bias voltage adjusting device and the laser radar system for improving the detection distance, all laser emitting units in the multi-channel laser emitter emit laser beams simultaneously, the spatial position relationship between the optical focusing antenna and the APD detector array and the spatial position relationship between the optical collimating antenna and the multi-channel laser emitter ensure that each path of laser emitting and receiving system is spatially independent, space laser interference is reduced, and the detection efficiency of laser scanning ranging is greatly improved.
(5) According to the bias voltage adjusting device and the laser radar system for improving the detection distance, the space position between each detection element in the APD detection array is fixed, so that the optical axis direction of the receiver does not need to be adjusted, only the plurality of discrete transmitters are adjusted to enable the emission direction of each discrete transmitter to be overlapped with the corresponding detection element of the APD detection array, and half of debugging workload and optical debugging difficulty are reduced.
(6) According to the bias voltage adjusting device and the laser radar system for improving the detection distance, the detection distance and the anti-interference performance of the laser ranging radar are improved by introducing novel hardware designs and algorithms such as an APD detector array, a multi-channel laser transmitter, the bias voltage adjusting device and a time domain related distance calculating algorithm, so that a very beneficial effect is obtained, and the bias voltage adjusting device and the laser radar system have great significance for improving the overall performance of a laser scanning distance detection system; the invention can be directly applied to laser scanning distance detection systems with various wavelengths, and has wide adaptability.
Drawings
Fig. 1 is a schematic structural diagram of a laser radar system according to an embodiment of the present invention; in the figure, 1-APD detector; 2-detecting element; 3-an optical focusing antenna; 4-a multi-channel laser transmitter; 5-optical collimating antenna
FIG. 2 is a schematic structural diagram of a bias voltage adjustment apparatus according to an embodiment of the present invention;
fig. 3 is a distribution graph of a plurality of echo signals generated based on time domain correlation distance solution according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
The invention provides a laser radar system based on a pulse Time of Flight (TOF) method; the multichannel laser beam is subjected to collimation emission through a multichannel laser emitter and an optical collimation antenna, and angle adjustment is carried out, so that the multichannel laser beam is emitted towards a specified direction; the emitted laser beam is scattered with a distant target, 180-degree backward scattering laser is reflected to an optical focusing antenna of a laser radar system, and the return light beams of all channels are respectively focused to each receiving unit of the APD detector array through the focusing action of the optical focusing antenna.
Fig. 1 is a schematic structural diagram of a laser radar system provided in an embodiment of the present invention, as shown in fig. 1, the laser radar system includes an APD detector 1, a multi-channel laser transmitter 4, an optical focusing antenna 3, an optical collimating antenna 5, and a bias voltage adjusting device;
in this embodiment, the multichannel laser transmitter 4 is used to transmit a 905nm short pulse laser with a pulse width of about 3ns, and the laser radar performs a ranging operation using the pulse.
The APD detector 1 is an APD detection array which is packaged on a substrate and provided with a plurality of detection elements, and the size of the detection elements and the distance between the adjacent detection elements are completely fixed; this embodiment employs 32-channel APD array product from HAMAMATSU corporation of japan. The multichannel APD detection array is located behind the optical focusing antenna 3, since the spatial position between each detection element 2 in the APD detection array is known; therefore, the receiving angle and the receiving field of view corresponding to each detecting element 2 in the APD detecting array can be determined by selecting the optical focusing antennas 3 with different focal lengths and different calibers; the receiving field and angle of each detecting element 2 of the APD detecting array correspond to the transmitting divergence angle and angle of the multi-channel laser transmitter 4 one by one, so that the laser radar system can work only when the receiving and transmitting light beams are aligned.
The multi-channel laser transmitter 4 is positioned behind the optical collimating antenna 5, and when debugging, the light beam emitted by the transmitting system is enabled to be in the view field of the corresponding detecting element 2 on the APD detecting array by adjusting the three-axis direction and the angle of the multi-channel laser transmitter 4; the difficulty of system debugging is greatly simplified because only the multichannel laser transmitter 4 needs to be adjusted and the APD detection array does not need to be adjusted.
The multi-channel laser emitter 4 of the laser radar emits short pulse laser, the short pulse laser is collimated by the optical collimating antenna 5, an emitted light beam is collimated into a laser beam with a smaller divergence angle, and the collimated laser beam is emitted from the optical collimating antenna 5 and then travels along the direction at the light speed until colliding with a distant target and scattering; the 180 ° backward scattering light returns along the original path, the returned echo optical signal is focused by the optical focusing antenna 3 in fig. 1, so as to obtain a certain optical gain, and the optical focusing antenna 3 focuses the echo optical signal on each detection element of the APD detection array. The measurement of the target distance can be completed by measuring the time difference between the emission of the laser pulse and the reception of the laser pulse.
It should be particularly noted that, in the embodiment, the laser receiving system employs the APD detection array instead of the discrete APD detector unit, the spatial position between each detection element 2 in the APD detection array is fixed, and the fixity is ensured by the production process of the manufacturer thereof, and the APD detection array has the advantage of reducing the difficulty of optical debugging; if the traditional discrete transmitter and the discrete receiver are adopted, the optical axes of the plurality of discrete transmitters are required to be adjusted firstly and then the optical axes of the plurality of discrete receivers are required to be adjusted in the production process, so that the transmitting directions of the plurality of transmitters are ensured to be respectively coaxial with the field directions of the plurality of receivers corresponding to the transmitters, and the debugging is very troublesome; and adopt the APD to survey the array, because the spatial position between every detection element 2 is fixed, therefore the optical axis direction of receiver just can need not adjust, only need adjust a plurality of discrete transmitters, make every discrete transmitter the emission direction with APD survey the array correspond survey the element coincidence can, reduced half debugging work load.
Fig. 2 is a schematic structural diagram of a bias voltage adjusting device provided in the present embodiment; as shown in fig. 2, the bias voltage adjusting apparatus includes a signal acquisition unit, a signal processor, and a bias voltage adjusting unit;
the signal acquisition unit is used for acquiring a target echo signal received by the avalanche photodiode, extracting an avalanche noise signal from the target echo signal and counting the number of the avalanche noise signals within preset time;
the signal processor is used for calculating an actual bias voltage value according to the number of the avalanche noise signals and a preset first bias voltage correction model, so that the actual bias voltage value approaches to the avalanche voltage of the avalanche photodiode; the first bias correction model is used for representing the mapping relation between the number of the avalanche noise signals in the preset time and an ideal bias value;
the bias voltage adjusting unit is used for generating and outputting the actual bias voltage value to the avalanche photodiode under the control of the signal processor.
The signal processor dynamically tracks the avalanche voltage operating point of the avalanche photodiode through the signal acquisition unit and generates the APD bias voltage required by the avalanche photodiode, so that the APD bias voltage is always maintained at a voltage value slightly lower than the avalanche voltage of the avalanche photodiode, and the difference between the APD bias voltage and the avalanche voltage is between 0.5 and 3V; at this time, the APD is at the edge of the avalanche, and a larger gain can be obtained, thereby improving the detection distance of the laser radar system.
Furthermore, the signal acquisition unit is also used for extracting a direct current signal corresponding to the background light from the target echo signal; the signal processor calculates an actual bias voltage value according to the direct current signal corresponding to the background light and a preset second bias voltage correction model; the second bias correction model is used for representing the mapping relation between the background light direct current voltage and the ideal bias voltage value. The invention takes the influence of the background light on the gain of the avalanche photodiode into consideration, corrects the bias value input to the avalanche photodiode according to the direct current signal corresponding to the background light in the target echo signal, reduces the influence of the background light on the avalanche photodiode, and improves the response speed and the sensitivity.
As shown in fig. 2, the signal acquisition unit includes a multistage gain amplification module, a time discrimination module, a dc component extraction circuit, a pulse signal processing module, and a high-speed analog-to-digital converter; the multistage gain amplification module comprises a first-stage preamplifier, a second-stage gain variable amplifier, a third-stage fixed gain amplifier and a fourth-stage variable gain amplifier; the time identification module comprises a TDC timing circuit and a high-speed FPGA time identification circuit; the pulse signal processing module comprises a pulse signal forming circuit and a pulse signal shaping circuit;
the signal processor is an FPGA master control processing circuit board; the bias voltage regulation unit comprises a digital-to-analog converter, a flyback BOOST voltage booster and a bias voltage regulator.
The first-stage preamplifier is connected with the avalanche photodiode and is used for carrying out first-stage amplification on an echo signal received by the avalanche photodiode, the first-stage preamplifier can be a trans-resistance amplifier or a pre-stage resistance load and is used for converting a current signal of the avalanche photodiode into a voltage signal, and the trans-resistance amplifier or the pre-stage resistance load is connected with the voltage amplifier. The second-stage gain variable amplifier is connected with the first-stage preamplifier, the gain of the second-stage gain variable amplifier is controlled by the FPGA master control processing circuit board, the control mode may be digital or analog voltage control, and the embodiment is not limited specifically. The purpose of the gain adjustable mode adopted by the second-stage gain variable amplifier is to avoid ranging errors caused by near fog of the laser radar or scattering of the near environment. The working process adopts a low-gain and high-gain near-linear transition mode, when the laser is just emitted, the gain of the second-stage gain variable amplifier is lower, and the gain of the second-stage gain variable amplifier is gradually improved along with the increasing and decreasing distance of the laser beam, so that the problems are avoided. In addition, the gain may be changed in the same laser pulse propagation time, or may be changed by a plurality of pulses, that is, a low gain is used in the detection of the first laser pulse, and a high gain is used in the detection of the second laser pulse. The third stage fixed gain amplifier is connected with the second stage variable gain amplifier and is used for further amplifying the weak signals of the echo and isolating the second stage variable gain amplifier.
And the first output end of the third-stage fixed gain amplifier is connected with the TDC timing circuit, and the TDC timing circuit is connected with the high-speed FPGA time discrimination circuit. The TDC timing circuit and the high-speed FPGA time discrimination circuit function to resolve the target distance, and the specific resolving mode may be a comparator time discrimination mode or a high-speed analog-to-digital converter mode, which is not specifically limited in this embodiment. The first-stage preamplifier, the second-stage gain variable amplifier and the third-stage fixed gain amplifier are mainly used for amplifying the laser pulse signal in the target echo signal to the amplitude which can be identified by the time identification module; if the signal is not amplified, the signal output by the avalanche photodiode is weak, and the TDC timing circuit cannot be directly driven. The TDC timing circuit detects the arrival time of the laser pulse signal under the drive of the amplified laser pulse signal; and the FPGA time discrimination circuit calculates the target distance according to the arrival time.
The second output end of the third stage fixed gain amplifier is connected with the fourth stage variable gain amplifier, the fourth stage variable gain amplifier is mainly used for amplifying an avalanche noise signal of the avalanche photodiode, when the bias voltage applied to the avalanche photodiode is too high and almost reaches the avalanche voltage value of the avalanche photodiode, the noise output by the avalanche photodiode is rapidly and greatly increased, the noise is amplified by the first stage preamplifier and the second stage variable gain amplifier, and the third stage fixed gain amplifier, particularly the fourth stage variable gain amplifier, the avalanche noise output by the avalanche photodiode accounts for the vast majority of the whole signal noise, and the noise signal is used for obtaining the ideal avalanche voltage of the current avalanche photodiode. In this embodiment, the amplification factor of the fourth-stage variable gain amplifier is about 4-6 times of the sum of the amplification factors of the first-stage preamplifier, the second-stage variable gain amplifier, and the third-stage fixed gain amplifier; without the fourth stage variable gain amplifier, the avalanche noise signal is still weak and cannot form a standard pulse.
The direct current component extraction circuit is connected with the fourth-stage variable gain amplifier, and the signal received by the avalanche photodiode comprises background light, such as sunlight, besides the laser pulse emitted by the laser radar, and the background light has a great influence on the bias control of the avalanche photodiode. The direct current component extraction circuit is used for distinguishing the echo signal into a short pulse signal, namely laser emitted by the multichannel laser transmitter 4 with the pulse width of about 4ns, and a direct current signal and an avalanche noise signal corresponding to background light.
The first output end of the direct current component extraction circuit is connected with the FPGA master control processing circuit board, and the direct current signal corresponding to the background light is transmitted to a low-speed analog-to-digital converter in the FPGA master control processing circuit board and serves as a first input variable of the FPGA master control processing circuit board. The second output end of the direct current component extraction circuit is connected with the pulse signal forming circuit, the direct current component extraction circuit transmits the avalanche noise signal of the avalanche photodiode to the pulse signal forming circuit, and the pulse signal forming circuit arranges the avalanche noise signal into a pulse signal through an internal comparator; the pulse signal shaping circuit is connected with the pulse signal forming circuit and is used for shaping the irregular pulse signals output by the comparator in the pulse signal forming circuit into standard pulse signals so as to be used for subsequent processing. The high-speed analog-to-digital converter is connected with the pulse signal shaping circuit and used for collecting the noise standard pulse signal generated by the avalanche noise shaping filtering into the FPGA master control processing circuit board as a second input variable of the FPGA master control processing circuit board.
The FPGA master control processing circuit board calculates the actual bias voltage value which needs to be added to the avalanche photodiode currently under the control of two input variables of a direct current signal and a noise standard pulse signal corresponding to background light; specifically, the FPGA master control processing circuit board calculates an actual bias voltage value according to the number of avalanche noise signals in a preset time and a preset first bias voltage correction table; or calculating an actual bias voltage value according to the direct current signal corresponding to the background light and a preset second bias voltage correction table; the first bias voltage correction table stores the corresponding relation between the number of noise pulses in a certain time period (such as 1ms) and the current ideal bias voltage value of the APD, and the second bias voltage correction table stores the corresponding relation between the voltage value of the direct current signal corresponding to the background light and the current ideal bias voltage value of the APD; when the voltage value of the direct current signal corresponding to the background light output by the direct current component extraction circuit is larger than a preset voltage threshold value, the situation that the background light signal is too strong at the moment is shown, the receiving of the laser pulse signal by the avalanche photodiode is seriously influenced, and the actual bias voltage value output to the APD detector is calculated by the FPGA master control processing circuit board according to a second bias voltage correction table; otherwise, the signal processor calculates and outputs the actual bias voltage value to the APD detector according to the first bias voltage correction table; in this embodiment, the voltage threshold is set to 80% of the maximum output voltage of the fourth-stage variable gain amplifier.
The flyback BOOST booster is connected with the FPGA master control processing circuit board and is used for generating a high voltage which is higher than the avalanche voltage of the avalanche photodiode and is generated under the control of the FPGA master control processing circuit board; the FPGA master control processing circuit board generates PWM pulse width modulation signals to control a switching tube in the flyback BOOST booster, so that output voltage is controlled. The control is closed loop, the high voltage generated by the flyback BOOST booster is fed back to the FPGA master control processing circuit board through a low-speed analog-to-digital converter in the FPGA master control processing circuit board, and the FPGA master control processing circuit board enables the generated high voltage to be always maintained at the avalanche voltage higher than the avalanche photodiode through the closed loop control; the digital-to-analog converter is connected with the FPGA master control processing circuit board, the FPGA master control processing circuit board outputs the calculated actual bias voltage value to the bias regulator through the digital-to-analog converter, and the bias regulator immediately carries out self-adaptive voltage division on the fixed high voltage generated by the flyback BOOST booster so as to generate an actual bias voltage value and apply the actual bias voltage value to the avalanche photodiode.
The system can track the state of the avalanche photodiode in real time to dynamically adjust the bias voltage, so that the avalanche photodiode works in a working state approaching the avalanche voltage, and the state can obtain larger signal gain, thereby improving the detection distance.
It should be noted that the higher the bias voltage applied to the avalanche photodiode, the larger the signal gain is, which is advantageous, but the closer to the avalanche voltage of the avalanche photodiode, the larger the avalanche noise of the avalanche photodiode itself is, which is disadvantageous. In order to solve the problem of avalanche noise increase caused by signal gain increase, the embodiment processes the target echo signal by using a time domain correlation distance calculation method. Specifically, the FPGA master control processing circuit board performs time domain calculation processing on a plurality of target echo signals received by the avalanche photodiode and generated when the targets are measured for a plurality of times, generates a distribution curve of probability values of the plurality of echo signals falling into different distance segments, and takes a distance value of the distribution curve when the probability value is maximum as a real target distance. By carrying out time domain calculation processing on a plurality of target echo signals, noise signals corresponding to false targets are filtered by utilizing the irrelevance of noise and the correlation of the target echo signals, the error probability of calculation of the distance value of the system is reduced, and the stability of the system is ensured.
The laser radar in this embodiment is a 360-degree rotating laser radar, that is, the part described in fig. 1 and fig. 2 is installed as a whole ranging system on a 360-degree rotating platform, the rotating platform rotates at a speed of 10-25 circles per second, and the laser transmitting and receiving system continuously performs multi-channel distance detection. The laser radar performs distance measurement each time it rotates to 0 °, but since the bias voltage of the avalanche photodiode after the bias voltage adjustment device is used is relatively high, avalanche noise is large, and a false ranging target may be caused. Therefore, the multi-channel laser transmitter transmits laser for N times at the same angle, and measures N echo signals and corresponding distance values. Specifically, when the value N is set to 3, the ranging is performed once when the laser radar rotates through an angle of 0 ° for the first time, but the result is not output, and the ranging is performed for the second time when the laser rotates through an angle of 0 ° for the second time, and the result is not output. When the laser rotates for the third time by an angle of 0 degrees, the third time of distance measurement is carried out, and the time domain correlation processing is carried out on the distance measurement result and the distance measurement results of the previous two times; the target distance is detected by utilizing the irrelevance of noise and the correlation of target echo signals, and the distance of a real target is extracted from a false target by utilizing the principle, so that the target distance detection is completed.
Fig. 3 is a distribution graph of a plurality of echo signals generated based on time domain correlation distance solution according to this embodiment, in which a horizontal axis represents a distance value, and a vertical axis represents a probability that a distance value of a real or noise signal in the echo signals falls into different distance segments, and since noise has irrelevance, a probability that a noise signal in echo signals measured 3 times appears in the same distance segment is very small, and a distance where a real target is located can be determined by using a magnitude of the probability value.
In addition, because the noise signals are filtered by adopting time domain correlation distance calculation, the same target needs to be measured for many times in the process, the measurement time can be prolonged, and the measurement efficiency is reduced; in order to solve the problem, in the embodiment, all laser emitting units in the multi-channel laser emitter 4 emit laser beams at the same time, rather than sequentially emitting light, and the spatial position relationship between the optical focusing antenna 3 and the APD detector array 1 and the spatial position relationship between the optical collimating antenna 5 and the multi-channel laser emitter 4 ensure that each laser emitting and receiving system is spatially independent, and there is less spatial laser interference. The mode of sequentially emitting and detecting light from a plurality of detecting elements is shortened to one-time simultaneous light emission for detection, and the detection efficiency of laser scanning distance measurement is greatly improved. The improvement of the detection efficiency can greatly compensate the slight reduction of the efficiency caused by the time domain correlation distance calculation algorithm adopted for pursuing distance improvement.
According to the bias voltage adjusting device and the laser radar system for improving the detection distance, laser scanning ranging is carried out by using the APD detector array and the multi-channel laser transmitter which are packaged on the substrate, the size of the detection elements of the APD detector array and the distance between the adjacent detection elements are completely fixed, and therefore the receiving system can achieve spatial multi-channel receiving of a target space only by one optical focusing antenna. The multichannel laser emitter emits light simultaneously, and the APD detector array measures the distance of the target space simultaneously, so that the scanning speed is improved. On the other hand, the sensitivity of the APD detector array is improved by improving the bias voltage of the APD detector; this may generate unnecessary noise, but the temporal correlation distance solution method is used to detect the target distance by using the irrelevancy of the noise and the correlation of the target echo signal. By the scheme, on one hand, the detection sensitivity is improved by improving the bias voltage of the APD detector array, and further the detection distance is improved. On the other hand, the error probability of the distance value calculation of the system is reduced through the time domain correlation distance calculation method, and the stability of the system is ensured.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (8)
1. A bias voltage adjusting device for improving detection distance is characterized by comprising a signal acquisition unit, a signal processor and a bias voltage adjusting unit;
the signal acquisition unit is used for acquiring a target echo signal received by the APD detector, extracting an avalanche noise signal from the target echo signal and counting the number of the avalanche noise signals in a preset time period; extracting a direct current signal corresponding to background light from the target echo signal;
the signal processor is used for calculating an actual bias voltage value according to the number of the avalanche noise signals and a preset first bias voltage correction model; the first bias voltage correction model is used for representing the mapping relation between the number of avalanche noise signals and an ideal bias voltage value within preset time; calculating an actual bias voltage value according to the direct current signal corresponding to the background light and a preset second bias voltage correction model; the second bias correction model is used for representing the mapping relation between the voltage value of the direct current signal corresponding to the background light and the ideal bias value;
when the voltage value of the direct current signal corresponding to the background light is larger than a preset voltage threshold value, the signal processor calculates and outputs an actual bias voltage value to the APD detector according to a second bias voltage correction model; otherwise, the signal processor calculates and outputs an actual bias voltage value to the APD detector according to the first bias voltage correction model;
the bias voltage adjusting unit is used for generating and outputting the actual bias voltage value to the APD detector under the control of the signal processor.
2. The bias voltage adjusting device according to claim 1, wherein the signal acquisition unit includes a multi-stage gain amplification module, a time discrimination module, a dc component extraction circuit, a pulse signal processing module, and an analog-to-digital converter;
the multi-stage gain amplification module is used for amplifying a target echo signal so as to amplify a laser pulse signal in the target echo signal to an amplitude which can be identified by the time identification module;
the time discrimination module is used for detecting the arrival time of the laser pulse signal and calculating the target distance according to the arrival time;
the direct current component extraction circuit is used for extracting a noise signal from the amplified target echo signal, wherein the noise signal comprises a direct current signal and an avalanche noise signal corresponding to background light; sending the direct current signal corresponding to the background light to a signal processor, and sending the avalanche noise signal to a pulse signal processing module;
the pulse signal processing module is used for shaping the avalanche noise signal into a standard pulse signal, and the standard pulse signal is processed by the analog-to-digital converter and then is sent to the signal processor.
3. The bias voltage adjusting device according to claim 2, wherein the multi-stage gain amplifying module comprises a first-stage preamplifier, a second-stage variable gain amplifier, a third-stage fixed gain amplifier and a fourth-stage variable gain amplifier connected in sequence;
the first-stage preamplifier is connected with the APD detector and is used for carrying out first-stage amplification on a target echo signal received by the APD detector;
the gain of the second-stage gain variable amplifier is controlled by the signal processor to be dynamically adjusted, and the signal stability is maintained in the process of amplifying the target echo signal output by the first-stage preamplifier;
the third-stage fixed gain amplifier further amplifies the target echo signal output by the second-stage variable gain amplifier and outputs a laser pulse signal in the amplified target echo signal to the time discrimination module;
and the fourth-stage gain variable amplifier is used for amplifying the noise signal in the target echo signal output by the third-stage fixed gain amplifier and outputting the amplified noise signal to the direct-current component extraction circuit.
4. A bias voltage regulating device according to claim 1 or 3, wherein the bias voltage regulating unit comprises a digital-to-analog converter, a flyback BOOST booster and a bias voltage regulator;
the flyback BOOST booster is used for generating a voltage value higher than the avalanche voltage of the APD detector under the control of the signal processor;
the digital-to-analog converter is used for performing digital-to-analog conversion on the actual bias voltage value generated by the signal processor and then sending the actual bias voltage value to the bias voltage regulator;
and the bias voltage regulator performs self-adaptive voltage division on the voltage value generated by the flyback BOOST booster according to the actual bias voltage value so as to output the actual bias voltage value to the APD detector.
5. The bias voltage adjusting device according to claim 1 or 3, wherein the signal processor is further configured to perform time domain solution processing on a plurality of target echo signals received by the APD detector and generated when a target is measured for a plurality of times, generate a distribution curve of probability values that distance values corresponding to real target signals or noise signals in the plurality of target echo signals fall onto different distance segments, and take the distance value at the maximum probability value as the real target distance.
6. A lidar system comprising the bias voltage adjustment apparatus of any one of claims 1-5.
7. The lidar system of claim 6, further comprising a multi-channel laser transmitter and an APD detector;
the APD detector is an APD detection array packaged on the substrate, the APD detection array comprises a plurality of detection elements, and the size of the detection elements and the distance between the adjacent detection elements are fixed;
the number of channels of the multichannel laser transmitter is the same as the number of detection elements in the APD detection array, so that a plurality of detection channels which are independent in transceiving are formed.
8. The lidar system of claim 7, further comprising an optically collimating antenna and an optically focusing antenna;
the optical collimating antenna is used for collimating the light emitted by the multi-channel laser emitter and adjusting the direction of the emitted light beam so as to align the light beam with a target to be measured;
the optical focusing antenna is used for focusing the light reflected by the object to be detected on each detection element of the APD detection array.
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