A method and system for frequency scanning of a laser radar

Abstract

This invention relates to the field of lidar technology and discloses a lidar frequency conversion scanning method and system. The system includes a laser emitting device, a scanning device, and an echo signal receiving and processing device. The scanning device is a galvanometer-type galvanometer or a MEMS galvanometer. The galvanometer-type galvanometer consists of an X-axis lens that controls the laser pulse for lateral scanning and a Y-axis lens that controls the laser pulse for longitudinal scanning. Both the X-axis and Y-axis lenses are equipped with a high-precision motor and a drive circuit board. Compared with scanning methods that simply use a single-frequency drive signal to control the galvanometer movement, this invention can significantly improve the angular resolution within the lidar scanning field of view, increase the proportion of the effective scanning area to the overall field of view, improve the scanning effect, reduce the area of ​​the scanning blind zone, and facilitate the subsequent processing to obtain more uniformly distributed three-dimensional point cloud data and highly restored outer contour features of the scanned target object.

Description

Technical Field

[0001] This invention relates to the field of lidar technology, and specifically to a lidar frequency conversion scanning method and system. Background Technology

[0002] As an active detection device, lidar is widely used in fields such as autonomous driving and terrain mapping. Lidar can be broadly classified into three types based on its scanning method: mechanical, semi-solid-state, and solid-state. Semi-solid-state lidar maintains an overall stationary exterior but contains some movable scanning components internally. Based on its ranging principle, lidar can be broadly classified into TOF (Time of Flight) lidar and FMCW (Frequency-Modulated Continuous Wave) lidar. The TOF ranging principle directly calculates the distance based on the time difference between pulse transmission and reception, as shown in equation (1).

[0003]

[0004] Where L is the distance between the pulse transmitter and the target, in meters. c is the speed of light, which is 299,792,458 meters per second in a vacuum. ΔT is the time difference between pulse transmission and pulse reception, in seconds.

[0005] A semi-solid-state lidar system based on the Time-of-Flight (TOF) principle typically consists of three parts: a laser emitting device, a scanning device, and an echo signal receiving and processing device. Its overall structural block diagram is shown below. Figure 1 The laser emitting device emits laser pulse signals of a certain frequency. After being reflected by continuously moving mirrors in the scanning device, these pulses form a specific scanning trajectory, enabling a two-dimensional scan of the target. When the laser pulse strikes the target, it reflects to form a laser echo signal. After receiving the laser echo, the echo signal receiving and processing device calculates the distance to the target object by measuring the time difference between the laser emission and the laser echo. Combined with the current motion state of the scanning device, i.e., the elevation and azimuth angles of the emitted laser pulse, the three-dimensional point cloud data of the target can be calculated using the polar coordinate transformation formula.

[0006] A laser emitting device refers to a device containing only one laser emitting diode, while an echo signal receiving and processing device refers to a device containing only one photodetector. Optionally, the photodetector can be a PD (photodiode), APD (avalanche photodiode), etc. In this type of single-transmitter, single-receiver lidar system, to achieve two-dimensional scanning of the target, the movement of the scanning device must be used to form various scanning trajectories to complete the scanning task. The step angle, stability, and motion characteristics of the scanning device directly affect the scanning effect of the entire lidar system.

[0007] Optionally, semi-solid-state lidar commonly uses rotating and oscillating scanning mirrors. These mirrors typically have high reflectivity for laser pulse signals, reducing energy loss after reflection. Rotating scanning mirrors mainly include various prisms, such as hexagonal prisms; oscillating scanning mirrors include MEMS (Micro Electromechanical System) galvanometer mirrors and galvanometer-type galvanometer mirrors.

[0008] In the LiDAR industry, angular resolution is a crucial parameter for evaluating the performance of LiDAR hardware. When the scanning mirror operates under a single-frequency drive signal, its scanning path is relatively fixed. Therefore, the angular resolution within the LiDAR's scanning field of view is also fixed. As scanning time accumulates, its effective scanning area becomes limited, resulting in scanning blind spots. With a low number of scanning lines, especially during long-distance scanning and large-field-of-view scanning, large scanning blind spots are easily generated, making it impossible to dynamically achieve higher angular resolution scanning.

[0009] In summary, existing lidar scanning schemes suffer from limitations such as limited angular resolution, limited effective scanning area, and large scanning blind spots due to the relatively fixed scanning path. To address these issues, we propose a lidar frequency conversion scanning method and system. Summary of the Invention

[0010] (a) Technical problems to be solved

[0011] To address the shortcomings of existing technologies, this invention provides a laser radar frequency conversion scanning method and system to solve the aforementioned problems.

[0012] (II) Technical Solution

[0013] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0014] A laser radar frequency conversion scanning system includes a laser emitting device, a scanning device, and an echo signal receiving and processing device. The scanning device is either a galvanometer-type galvanometer or a MEMS galvanometer.

[0015] The galvanometer-type galvanometer consists of an X-axis lens that controls the laser pulse for horizontal scanning and a Y-axis lens that controls the laser pulse for vertical scanning. Both the X-axis and Y-axis lenses are equipped with a high-precision motor and a drive circuit board.

[0016] A laser radar frequency conversion scanning method includes the following steps:

[0017] S1: In the first second, the rotation frequency of the X-axis lens is set to 65Hz and the rotation frequency of the Y-axis lens is set to 4Hz. After one second of scanning time, 4 frames of 16-line non-repeating scanning trajectories will be formed. After superposition, an equivalent 64-line two-dimensional scanning trajectory will be formed.

[0018] S2: In the second second, set the rotation frequency of the X-axis lens to 66Hz and the rotation frequency of the Y-axis lens to 4Hz. After one second of scanning time, 4 frames of 16-line scanning trajectory will be formed. Because half of the scanning trajectory is repeated, the superposition will form an equivalent 32-line two-dimensional scanning trajectory.

[0019] S3: In the third second, the rotation frequency of the X-axis lens is set to 67Hz, and the rotation frequency of the Y-axis lens is still set to 4Hz. After one second of scanning time, 4 frames of 16-line non-repeating scanning trajectories will be formed. After superposition, an equivalent 64-line two-dimensional scanning trajectory will be formed.

[0020] S4: In the fourth second, the rotation frequency of the X-axis lens is set to 66Hz and the rotation frequency of the Y-axis lens is set to 4Hz. The scanning trajectory formed in this second will completely overlap with the scanning path in the second second.

[0021] S5: In the fifth second, the rotation frequency of the X-axis lens is also set to 65Hz and the rotation frequency of the Y-axis lens is set to 4Hz. Then the scanning trajectory formed in this second completely overlaps with the scanning path in the first second.

[0022] S6: After a frequency conversion scan with a cycle of 5 seconds, 10 non-repeating 16-line scan trajectories will be generated. After being superimposed, they form an equivalent 160-line scan effect, which is a two-dimensional non-repeating scan path.

[0023] Preferably, the rotation frequency of the X-axis lens and the Y-axis lens is changed by the frequency of the triangular wave drive signal, and the frequency of the triangular wave drive signal used to control the rotation of the X-axis lens and the Y-axis lens should maintain a continuous change.

[0024] Preferably, the method for changing the triangular wave driving signal is as follows:

[0025] S1: The Matlab program generates the triangular waveform information to be output and saves it as a MIF file. During programming, it is stored in the memory of the FPGA control board as the waveform data table of the DDS signal generator.

[0026] S2: Under the clock signal of the FPGA control board, the phase accumulator continuously accumulates the pre-set frequency control word. After a certain number of clock signals, the phase accumulator will be full and overflow. The overflow frequency of the phase accumulator is the signal frequency output by the DDS signal generator.

[0027] S3: Using the data output by the phase accumulator as the sampling address of the waveform data table, the waveform sample values ​​stored in the waveform data table can be found, and the phase to amplitude conversion can be completed.

[0028] S4: The amplitude data output from the waveform data table is input to the D / A converter, which converts the digital signal into an analog signal, which is then used as the output of the DDS signal generator.

[0029] Preferably, the frequency of the output signal can be calculated using the following formulas (2) and (3): If the clock signal frequency of the FPGA control board is Fclk and the frequency control word Fword is 1, then the frequency of the output signal Fout is:

[0030]

[0031] Where N is the number of bits in the phase accumulator defined in the FPGA program. If the frequency control word Fword is modified to any positive integer (let B represent any positive integer), the output frequency Fout will be:

[0032]

[0033] (III) Beneficial Effects

[0034] Compared with the prior art, the laser radar frequency conversion scanning method and system provided by the present invention have the following beneficial effects:

[0035] 1. The laser radar frequency conversion scanning method and system can achieve a scanning effect with a high equivalent line count after the scanning path is superimposed, which can improve the angular resolution within the laser radar scanning field of view and increase the proportion of the effective scanning area to the overall scanning field of view.

[0036] 2. This laser radar frequency conversion scanning method and system, based on an FPGA control board and utilizing DDS technology to generate the frequency conversion drive signal, can achieve precise timing and precise frequency output, outputting drive signals of different frequencies in different time slots. This allows for smooth changes in the motion state of the scanning device, helping to maintain its service life unaffected by the frequency conversion drive.

[0037] 3. In practical use, the frequency conversion scanning method and system of this lidar can be increased / decreased and the number of scanning lines increased / decreased according to the situation, and the frequency conversion scanning method provided by this invention is still applicable.

[0038] 4. This laser radar frequency conversion scanning method and system is not limited to use on scanning devices of galvanometer-type galvanometers, but also includes other types of laser radar scanning devices, such as two-dimensional MEMS galvanometers, one-dimensional galvanometers combined with rotating mirrors, etc., and has wide applicability.

[0039] 5. The laser radar frequency conversion scanning method and system are not limited to use on scanning devices driven by triangular wave signals. For common scanning methods such as Lissajous pattern scanning, the method of this invention can still be used to reduce the blind zone of scanning and increase the proportion of the effective scanning area to the overall scanning field of view. In some other application scenarios of laser radar, such as a scanning scenario that requires extremely high angular resolution, the method of this invention or similar modifications can also significantly improve the scanning effect. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the composition and structure of a lidar system according to an embodiment of the present invention;

[0041] Figure 2 This is a schematic diagram of the structure of a galvanometer-type galvanometer according to an embodiment of the present invention;

[0042] Figure 3 This is a scan trajectory diagram of a single frame scanning 16 lines according to an embodiment of the present invention;

[0043] Figure 4 This is a scanning effect diagram of an embodiment of the present invention with X:65Hz / Y:4Hz equivalent to 64 lines;

[0044] Figure 5 This is a scanning effect diagram of an embodiment of the present invention with X:66Hz / Y:4Hz equivalent to 32 lines;

[0045] Figure 6 This is a scanning effect diagram of an embodiment of the present invention with X:67Hz / Y:4Hz equivalent to 64 lines;

[0046] Figure 7 This is a diagram illustrating the scanning effect of a frequency conversion scanning system generating an equivalent 160 lines, as described in an embodiment of the present invention.

[0047] Figure 8 This is a schematic flowchart of the frequency conversion scanning process according to an embodiment of the present invention. Detailed Implementation

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] Example

[0050] Please see Figures 1-2 The laser radar frequency conversion scanning system provided by this invention includes a laser emitting device, a scanning device, and an echo signal receiving and processing device. The scanning device is either a galvanometer-type galvanometer or a MEMS galvanometer. The galvanometer-type galvanometer mainly consists of two rotatable mirrors, each equipped with a high-precision motor and a drive circuit board. The mirror that controls the laser pulse for horizontal scanning is called the X-axis mirror, and the mirror that controls the laser pulse for vertical scanning is called the Y-axis mirror. An external voltage signal within a certain range can control the mirror to rotate within a certain angle range. If a triangular wave signal of a specific frequency is input to the X-axis mirror drive circuit board and the Y-axis mirror drive circuit board respectively, the two scanning mirrors will move back and forth at a specific frequency, so that the laser pulse signal emitted by the laser emitting device forms a "Z"-shaped scanning trajectory.

[0051] The triangular wave drive signal is generated using DDS (Direct Digital Frequency Synthesis) technology, jointly produced by an FPGA (Field Programmable Gate Array) control board and an AD9767 high-speed dual-channel DAC module. DDS technology is a frequency synthesis technique, primarily composed of a phase accumulator, a phase modulator, a waveform data table, and a D / A converter. A device that uses DDS technology to generate various signal waveforms can be called a DDS signal generator. The specific implementation process is as follows: First, the triangular wave waveform information to be output is generated by a Matlab program and saved as a MIF file. During programming, this MIF file is stored in the memory of the FPGA control board as the waveform data table for the DDS signal generator. Second, under the clock signal of the FPGA control board, the phase accumulator continuously accumulates a pre-set frequency control word. After a certain number of clock signals, the phase accumulator will overflow, and the overflow frequency of the phase accumulator is the signal frequency output by the DDS signal generator. Using the data output by the phase accumulator as the sampling address of the waveform data table, the waveform sample values ​​stored in the waveform data table can be retrieved, completing the phase-to-amplitude conversion. Finally, the amplitude data output by the waveform data table is input to the D / A converter, which converts the digital signal into an analog signal, serving as the output of the DDS signal generator. The frequency of the output signal can be calculated using the following formulas (2) and (3):

[0052] If the clock signal frequency of the FPGA control board is Fclk and the frequency control word Fword is 1, then the frequency of the output signal Fout is:

[0053]

[0054] Where N is the number of bits in the phase accumulator defined in the FPGA program. If the frequency control word Fword is changed to B (where B represents any positive integer), the output frequency Fout is:

[0055]

[0056] By using time-slotted input of triangular wave drive signals of different frequencies to the scanning device, different scanning frequencies and paths can be achieved at different time periods. This is accomplished through FPGA programming and the use of DDS technology. As mentioned above, the frequency of the output waveform can be controlled by adjusting the frequency control word size of the DDS signal generator. Therefore, when programming the FPGA control board, different values ​​can be assigned to the frequency control word of the DDS signal generator in time slots. The overall output signal will then exhibit the following behavior: outputting triangular wave drive signals of different frequencies at different time periods. After a complete frequency conversion scanning cycle, the triangular wave drive signal will be output at the initial set frequency at the beginning of the next scanning cycle.

[0057] Regarding the specific scanning method of lidar, this invention provides a specific embodiment. Taking a 4-frame 16-line scan as an example, this means completing 4 scans of the scanning field of view per second, with each scan having 16 lines. By setting the rotation frequency of the X-axis mirror to 65Hz and the rotation frequency of the Y-axis mirror to 4Hz, under the synchronous operation of the X-axis and Y-axis mirrors, a non-repeating "Z"-shaped scan of the target can be achieved, effectively producing a scanning effect of 64 lines per second. A simulation diagram of the scanning effect is shown below. Figure 3 , Figure 4 As shown in the simulation diagram, after four non-repeating scans, the angular resolution and effective scanning area within the scanning field of view are improved compared to 16-line scanning. However, when the scanning device is stationary, if the rotation frequency of the X-axis lens is maintained at 65Hz and the rotation frequency of the Y-axis lens at 4Hz, the scanning trajectory of the next second will overlap with the scanning trajectory of the previous second, resulting in a certain scanning blind zone. This blind zone will persist and not disappear as the scanning time accumulates. In the case of long-distance scanning, the area of ​​this blind zone will be amplified; the farther the LiDAR scans, the larger the area of ​​the scanning blind zone becomes, eventually becoming non-negligible.

[0058] See Figures 3-8 The laser radar frequency conversion scanning method provided by the present invention includes the following steps:

[0059] S1: In the first second, the rotation frequency of the X-axis lens is set to 65Hz, and the rotation frequency of the Y-axis lens is set to 4Hz. After one second of scanning time, four frames of 16-line non-repeating scan trajectories will be formed. After superposition, an equivalent 64-line two-dimensional scan trajectory will be formed. See the simulation diagram of this scanning effect. Figure 4 ;

[0060] S2: In the second second, the rotation frequency of the X-axis lens is set to 66Hz, while the rotation frequency of the Y-axis lens remains at 4Hz. After one second of scanning time, four frames of 16-line scanning trajectories will be formed. Because half of the scanning trajectories are repeated, they are superimposed to form an equivalent 32-line two-dimensional scanning trajectory. However, since the scanning field of view of the lidar is fixed, changing the rotation frequency of the X-axis will cause the scanning trajectory of the laser pulse signal to differ from the scanning trajectory of the previous second. See the simulation diagram for the scanning effect. Figure 5 ;

[0061] S3: In the third second, the rotation frequency of the X-axis mirror is set to 67Hz, while the rotation frequency of the Y-axis mirror remains at 4Hz. After one second of scanning time, four frames of 16-line non-repeating scan trajectories will be formed, which, when superimposed, form an equivalent 64-line two-dimensional scan trajectory. Furthermore, because the scanning field of view of the lidar is fixed, the scan trajectory of the laser pulse signal within this second after changing the X-axis rotation frequency differs from the scan trajectory of the previous two seconds. See the simulation diagram for the scanning effect. Figure 6 ;

[0062] S4: Considering the lifespan of the scanning device, the frequency of the triangular wave drive signal used to control the lens rotation should maintain a relatively consistent change. If the rotation frequency of the X-axis lens is set to 66Hz and the rotation frequency of the Y-axis lens is set to 4Hz in the fourth second, the scanning trajectory formed in this second will completely overlap with that in the second second. See the simulation diagram for this scanning effect. Figure 5 ;

[0063] S5: In the fifth second, the rotation frequency of the X-axis lens is set to 65Hz, and the rotation frequency of the Y-axis lens is set to 4Hz. The scanning trajectory formed in this second completely overlaps with that of the first second. See the simulation diagram for the scanning effect. Figure 4 ;

[0064] S6: After a frequency conversion scan with a 5-second cycle, 10 non-repeating 16-line scan paths will be generated. These are then superimposed to form an equivalent 160-line scan effect. It is evident that the scanning blind zone generated after the previous scans is reduced after non-repeating superposition. Under the relatively fixed scanning field of view mentioned earlier, the angular resolution is significantly improved. A simulation diagram of the scanning effect after superimposing non-repeating scan paths can be found... Figure 7 .

[0065] See the schematic flowchart of the frequency conversion scanning process. Figure 8 .

[0066] The lidar frequency conversion scanning method and system provided in the above embodiments of the present invention, within the fixed scanning field of view of the lidar, significantly improves the angular resolution within the lidar scanning field of view compared to single-frame 16-line scanning or 4-frame 16-line non-repeating superposition resulting in an equivalent 64-line scanning method, and increases the proportion of the effective scanning area to the overall field of view. In some complex lidar application scenarios, such as scenarios requiring high-resolution scanning and long-distance scanning, the method of the present invention can effectively improve the scanning effect, reduce the area of ​​the scanning blind zone, and facilitate the subsequent processing to obtain more uniformly distributed 3D point cloud data and highly restored outer contour features of the scanned target object. It should be noted that, in order to ensure the service life of the scanning device, the frequency of the driving signal should change relatively smoothly, and the continuity of frequency change should be maintained.

[0067] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A laser radar frequency conversion scanning method, characterized in that, Includes the following steps: S1: In the first second, the rotation frequency of the X-axis lens is set to 65Hz and the rotation frequency of the Y-axis lens is set to 4Hz. After one second of scanning time, 4 frames of 16-line non-repeating scanning trajectories will be formed. After superposition, an equivalent 64-line two-dimensional scanning trajectory will be formed. S2: In the second second, set the rotation frequency of the X-axis lens to 66Hz and the rotation frequency of the Y-axis lens to 4Hz. After one second of scanning time, 4 frames of 16-line scanning trajectory will be formed. Because half of the scanning trajectory is repeated, the superposition will form an equivalent 32-line two-dimensional scanning trajectory. S3: In the third second, the rotation frequency of the X-axis lens is set to 67Hz, and the rotation frequency of the Y-axis lens is still set to 4Hz. After one second of scanning time, 4 frames of 16-line non-repeating scanning trajectories will be formed. After superposition, an equivalent 64-line two-dimensional scanning trajectory will be formed. S4: In the fourth second, the rotation frequency of the X-axis lens is set to 66Hz and the rotation frequency of the Y-axis lens is set to 4Hz. The scanning trajectory formed in this second will completely overlap with the scanning path in the second second. S5: In the fifth second, the rotation frequency of the X-axis lens is also set to 65Hz and the rotation frequency of the Y-axis lens is set to 4Hz. Then the scanning trajectory formed in this second completely overlaps with the scanning path in the first second. S6: After a frequency conversion scan with a cycle of 5 seconds, 10 non-repeating 16-line scan trajectories will be generated. After being superimposed, they form an equivalent 160-line scan effect, which is a two-dimensional non-repeating scan path.

2. The laser radar frequency conversion scanning method according to claim 1, characterized in that: The rotation frequencies of the X-axis and Y-axis lenses are changed by the frequency of the triangular wave drive signal. The frequency of the triangular wave drive signal used to control the rotation of the X-axis and Y-axis lenses should change continuously.

3. The laser radar frequency conversion scanning method according to claim 2, characterized in that: The method for changing the triangular wave driving signal is as follows: S1: The Matlab program generates the triangular waveform information to be output and saves it as a MIF file. During programming, it is stored in the memory of the FPGA control board as the waveform data table of the DDS signal generator. S2: Under the clock signal of the FPGA control board, the phase accumulator continuously accumulates the pre-set frequency control word. After a certain number of clock signals, the phase accumulator will be full and overflow. The overflow frequency of the phase accumulator is the signal frequency output by the DDS signal generator. S3: Using the data output by the phase accumulator as the sampling address of the waveform data table, the waveform sample values ​​stored in the waveform data table can be found, and the phase to amplitude conversion can be completed. S4: The amplitude data output from the waveform data table is input to the D / A converter, which converts the digital signal into an analog signal, which is then used as the output of the DDS signal generator.

4. The laser radar frequency conversion scanning method according to claim 3, characterized in that: The frequency of the output signal can be calculated using the following formulas (2) and (3): If the clock signal frequency of the FPGA control board is Fclk and the frequency control word Fword is 1, then the frequency of the output signal Fout is: ； Where N is the number of bits in the phase accumulator defined in the FPGA program, if the frequency control word Fword is modified to any positive integer B, then the output frequency Fout is: 。 5. A laser radar frequency conversion scanning system, applied to the laser radar frequency conversion scanning method as described in any one of claims 1-4, characterized in that, It includes a laser emitting device, a scanning device, and an echo signal receiving and processing device. The scanning device is either a galvanometer-type galvanometer or a MEMS galvanometer. The galvanometer-type galvanometer consists of an X-axis lens that controls the laser pulse for horizontal scanning and a Y-axis lens that controls the laser pulse for vertical scanning. Both the X-axis and Y-axis lenses are equipped with a high-precision motor and a drive circuit board.

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