A range-gated detection device and method of a pulsed iToF laser radar

Abstract

The application belongs to the technical field of ToF type laser radar ranging, and particularly relates to a distance gating detection device and method of a pulsed iToF laser radar.The device comprises a timing control unit, a laser light source, an iToF sensor, a signal processing unit, a transmitting lens module and a receiving lens.The timing control unit controls the driving timing of the laser light source and the iToF sensor, and realizes the distance gating detection function.The laser light source generates laser pulses with a pulse width of nanoseconds, the emitted laser light pulses are irradiated to the surface of a detection target after passing through the transmitting lens module, the laser echoes reflected by the target are received by the receiving lens, and are captured by the photosensitive pixel array of the iToF sensor.The signal processing unit calculates the phase shift of the emitted light pulses and the received light pulses, and obtains the distance information of the detection target in combination with the timing control information.The application can realize the distance gating function, and greatly improves the detection range.The device has a simple structure, is convenient to operate, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of ToF type lidar ranging technology, specifically relating to a range gating detection device and method for pulse iToF lidar. Background Technology

[0002] Indirect Time-of-Flight (iToF) calculates the distance to a target by measuring the relative phase difference between the emitted and reflected light. Due to its advantages such as simple basic principle, simple and stable structure, and high measurement accuracy, it has occupied an important position in lidar ranging technology and is gradually being applied to emerging fields such as 3D reconstruction, industrial inspection, attitude recognition, autonomous driving, and machine vision.

[0003] The simplified structure of the basic pixel unit of the pulsed iToF lidar sensor and typical detection timing are as follows: Figure 1 As shown. Specifically, the transmitted pulse width is t. p The laser signal controls the sensor pixels' G1 gating to synchronize with the emitted signal, and the G2 gating delay is t. p After activation, G3 is the reset gating. The transmitted signal passes through t d After the time of flight, the signal is received by the photodiode (PD) of the sensor pixel, and accumulated charge is formed successively in the floating diffusion regions FD1 and FD2. The time of flight t of the laser signal can be calculated by measuring the output voltage. d .

[0004] As can be seen from the above detection timing sequence, the ranging range of a pulsed iToF lidar is determined by the laser pulse width; a longer detection distance requires a wider laser pulse width. However, in practical applications, a longer pulse width requires higher power, which can lead to severe laser heating and affect system reliability. In addition, increasing the gated pulse width can also exacerbate the influence of ambient light, resulting in a decrease in the ranging accuracy of the lidar system.

[0005] Increasing the effective range of pulsed iToF lidar systems can expand application scenarios and help lidar better serve fields such as autonomous driving and intelligent security. Summary of the Invention

[0006] The purpose of this invention is to provide a range gating detection device and method for pulsed iToF lidar. The method performs range gating detection by adjusting the relative delay between the output pulse of the laser source and the sensor detection gate, and then splices together multiple gating distance intervals to improve the application range of iToF lidar.

[0007] The range-gated detection device for pulsed iToF lidar provided by this invention has the following structure: Figure 2As shown, it includes a timing control unit, a laser source, an iToF sensor, a signal processing unit, a transmitting lens module, and a receiving lens; wherein:

[0008] The timing control unit is used to control the driving timing of the laser source and the iToF sensor to achieve the distance gating detection function;

[0009] The laser source is used to generate laser pulses with a pulse width in the nanosecond range; the emitted laser pulses are emitted and then illuminate the surface of the target after passing through the emitting lens module. The laser echo reflected by the target is received by the receiving lens and captured by the photosensitive pixel array of the iToF sensor.

[0010] The signal processing unit is used to receive the output signal of the iToF sensor module, calculate the phase shift between the emitted and received light pulses, and obtain the distance information of the detected target by combining the timing control information.

[0011] The timing control unit 21 can be an FPGA, DSP, or designed as a dedicated ASIC chip; the laser source can be a vertical cavity surface-emitting laser (VCSEL), an edge-emitting laser (EEL), etc.

[0012] Accordingly, the present invention also provides a range-gated detection method for pulsed iToF lidar, comprising the following steps:

[0013] (1) Set the detection interval and the number of subframes; calculate the detection pulse width and delay time of each subframe;

[0014] (2) The timing control unit controls the laser source and iToF sensor to work according to the calculated pulse width and delay time respectively;

[0015] (3) The signal processing unit detects the output value of the iToF sensor. If the output value exceeds the saturation threshold of the iToF sensor, the current subframe data packet needs to be discarded, the exposure time of the iToF sensor is reduced, and the measurement of the current subframe interval is repeated. If the output value does not exceed the saturation threshold of the iToF sensor, the detection of the next subframe interval is performed.

[0016] (4) The signal processing unit calculates the target distance value based on the iToF sensor output value and the delay time;

[0017] (5) After each subframe is detected, the signal processing unit uses a weighted average to perform overlapping fusion and splicing of each subframe to correct the distance value and improve the accuracy of ranging.

[0018] This invention achieves range gating by controlling the relative delay between the laser source emission time and the iToF sensor signal reception time, significantly increasing the detection range without altering detection accuracy. The device has a simple structure, is easy to operate, and has broad application prospects in the field of lidar detection. Attached Figure Description

[0019] Figure 1 To simplify the structure and detection timing of the basic pixel unit of the iToF sensor.

[0020] Figure 2 A schematic diagram of the pulsed iToF lidar range gating detection device provided by the present invention.

[0021] Figure 3 This is a flowchart of a distance gating detection method provided in an embodiment of the present invention.

[0022] Figure 4 This is a schematic diagram of the gating and detection timing provided in an embodiment of the present invention.

[0023] Figure 5 This is a schematic diagram illustrating the influence of laser rise / fall times on measurement results, provided in an embodiment of the present invention.

[0024] Figure 6 This is a schematic diagram of a method for splicing overlapping subframe regions according to an embodiment of the present invention. Detailed Implementation

[0025] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0026] The range-gated detection device for pulsed iToF lidar provided by this invention has the following structure: Figure 2 As shown, it includes: a timing control unit 21, a laser source 22, a transmitting lens module 23, a target 24, a receiving lens 25, an iToF sensor 26, and a signal processing unit 27, wherein:

[0027] The timing control unit 21 is used to control the driving timing of the laser light source 22 and the iToF sensor 26 to realize the range gating detection function;

[0028] The light pulse emitted by the laser source 22 illuminates the surface of the detection target 24 after passing through the emitting lens module 23. The laser echo reflected by the detection target 24 is received by the receiving lens 25 and finally captured by the photosensitive pixel array of the iToF sensor 26.

[0029] The signal processing unit 27 calculates the phase shift between the transmitted and received light pulses and combines it with timing control information to obtain the distance information of the target 24.

[0030] The timing control unit 21 can be an FPGA, DSP, or designed as a dedicated ASIC chip; the laser source 22 can be a vertical cavity surface-emitting laser (VCSEL), an edge-emitting laser (EEL), or the like.

[0031] The laser source 22 can generate laser light with a pulse width in the nanosecond range and can be triggered by an external signal.

[0032] The range-gated detection method for pulsed iToF lidar provided by this invention is described in [reference needed]. Figure 3 As shown, the specific steps are as follows:

[0033] S51. For a detection interval of distance L, the entire detection interval is divided into N equally spaced sub-detection intervals. The specific calculation method for the division can be found in the following formula:

[0034] N = L / D, (1)

[0035] D is the single-frame detection distance, which should be less than or equal to the sensor's maximum detection distance; N is the result of dividing the distance L by the single-frame detection distance D and rounding it up.

[0036] S52. Calculate the detection pulse width tp and the delay step td. The detection pulse width tp should cover the range of a single sub-interval, i.e.: tp = 2D / (N*c). The delay step td should be equal to or slightly less than the detection pulse width tp. The timing control unit first controls the laser source to generate a pulse laser with a pulse width of tp, and then after a delay of td, opens the detection gate of the iToF sensor so that the iToF sensor can receive the laser echo signal reflected back from the target.

[0037] Specifically, such as Figure 4 As shown, both the laser pulse and the sensor detection gate pulse width are tp, but they differ by a delay time td. This delay time can be used to set and adjust the detection range. The laser pulse is received by the sensor after being reflected by the target, and accumulates within the sensor's G1 and G2 gate times, respectively, generating output signals FD1 and FD2. Under this detection method, the target distance calculation formula is:

[0038] Distance = c / 2 * [td + tp * FD2 / (FD1 + FD2)], (2)

[0039] Where c is the speed of light.

[0040] Since the delay td can be freely set, the detection range can be arbitrarily extended without adding any additional hardware. In some embodiments, the delay td and its corresponding adjustment step size can be set according to the estimated distance range of the target and the upper limit of the laser source power.

[0041] S53, the signal processing unit 27 allocates the starting address of each subframe register to receive and temporarily store the readout data of the iToF sensor;

[0042] S54, Subframe integration time: After the detection parameters are initialized, the subframe exposure integration stage begins. The iToF sensor starts to be exposed within the sub-interval, and then logical judgment is made based on the output value of the iToF sensor. This is to avoid overexposure of the iToF sensor, which would cause the readout value to be unable to correctly calculate the distance value.

[0043] The laser energy requirements for targets at close and long ranges differ significantly. Assuming the target is an extended target (capable of reflecting the entire laser beam and being received by the detector), according to the radar range equation, with a fixed output laser power, the received laser power is inversely proportional to the square of the target distance. Therefore, for wide-range ranging applications, the range-gated detection technology of this invention can be used to divide the detection intervals, and the output laser power and integration time length of each detection interval can be set separately.

[0044] Specifically

[0045] S55. Set a threshold Q_Thre, which can be set to 85%-95% of the iToF sensor output saturation voltage (usually set to 90%). Compare the iToF sensor readout value Q_n with the threshold Q_Thre. If the current readout value is greater than the threshold Q_Thre, it means that the iToF sensor is currently in an overexposed state. In this case, the current subframe data needs to be discarded, the unit integration time needs to be reduced, and then the measurement of the current subframe needs to be repeated.

[0046] Further steps include: calculating the average value of the output of a subset of pixels from the iToF sensor and comparing it with a threshold Q_Thre. This avoids the entire data packet being discarded due to an abnormality in a single pixel, thus preventing a decrease in the overall imaging frame rate. For subframes that meet the threshold requirement, step S56 is executed.

[0047] S56. Label the iToF sensor output data of this subframe with ID, and transmit the data packet to the signal processing unit 27. Then start the exposure stage of the next subframe and repeat steps S54 and S55.

[0048] S57. Determine whether all subframe data has been acquired. If all subframe data has been correctly received, proceed to step S58.

[0049] S58. Entering the data processing stage, the signal processing unit 27 calculates the distance information based on the intensity information of each subframe, and then stitches the subframes together to output a distance image.

[0050] Furthermore, during the exposure of the iToF sensor, the laser source cannot generate an ideal rectangular pulse, but rather a trapezoidal wave with certain rising and falling edges. Therefore, the output of the iToF sensor cannot maintain a linear relationship, and directly using the distance calculation formula will introduce errors. For example... Figure 5 As shown, the laser pulse returns to the iToF sensor at different times and is captured within the G1 and G2 gate times, respectively. If the laser pulse waveform is an ideal rectangular wave, the iToF sensor output value, as shown by the dashed line in the figure, exhibits a linear relationship. Under irrational conditions, the output, as shown by the solid line in the figure, shows a non-linear range at both near and far distances. Without correction, the calculated value at near distances will be greater than the true value, and the calculated value at far distances will be less than the true value.

[0051] To address the aforementioned problems, this invention corrects distance errors during the subframe stitching stage, as detailed below. Figure 6 Further explanation involves stitching together the detection intervals divided into 5 subframes. The pulse width of a single subframe detection interval is 60ns, and the gating delay between subframes is 40ns. Therefore, the detection ranges of the 5 subframes are 0–9m, 6–15m, 12–21m, 18–27m, and 24–33m, respectively. There is a 3m overlap between two subframe detection intervals, which can be used for distance correction. A single subframe can be further subdivided into: the front overlap region (near the subframe, where the output value is too high and needs correction by combining it with the back overlap region of the previous subframe); the back overlap region (far from the subframe, where the output value is too low and needs correction by combining it with the front overlap region of the following subframe); and the mask region (located in the linear region of this subframe, where data in this distance range should be discarded in other subframes). The formula for calculating distance correction using the weighted average method is as follows:

[0052] d corr = d n * t r / (t r + t f ) + d n+1 * t f / (t r + t f (3)

[0053] Among them, t r t is the rise time of the laser pulse. f The fall time of the laser pulse, d n d is the output value of the back overlap region of the nth subframe. n+1 This is the output value for the front overlap area of ​​the (n+1)th subframe.

[0054] As can be seen from the above, this invention only requires setting the relative delay between the laser output pulse and the sensor detection gate to perform range gating detection. Compared with traditional solutions, it avoids the defects of saturation at close range and insufficient energy at long range under long pulse width detection, thus expanding the application range of iToF lidar. In terms of data processing, to address nonlinear errors, the wide detection range is divided into multiple sub-ranges and weighted average correction is performed, improving the accuracy of ranging without requiring additional hardware configuration, thereby reducing system costs.

Claims

1. A range-gated detection method for pulsed iToF lidar, characterized in that, First, a range-gated detection device for a pulsed iToF lidar is constructed, specifically including a timing control unit, a laser source, an iToF sensor, a signal processing unit, a transmitting lens module, and a receiving lens; wherein: The timing control unit is used to control the driving timing of the laser source and the iToF sensor to achieve the distance gating detection function; The laser source is used to generate laser pulses with a pulse width in the nanosecond range; the emitted laser pulses are emitted and then illuminate the surface of the target after passing through the emitting lens module. The laser echo reflected by the target is received by the receiving lens and captured by the photosensitive pixel array of the iToF sensor. The signal processing unit is used to receive the output signal of the iToF sensor module, calculate the phase shift between the emitted and received light pulses, and obtain the distance information of the detected target by combining the timing control information. The specific steps of range-gated detection are as follows: (1) Set the detection interval and the number of subframes; calculate the detection pulse width and delay time of each subframe; (2) The timing control unit controls the laser source and iToF sensor to work according to the calculated pulse width and delay time; (3) The signal processing unit detects the output value of the iToF sensor. If the output value exceeds the saturation threshold of the iToF sensor, the current subframe data packet needs to be discarded, the exposure time of the iToF sensor is reduced, and the measurement of the current subframe interval is repeated. If the output value does not exceed the saturation threshold of the iToF sensor, the detection of the next subframe interval is performed. (4) The signal processing unit calculates the target distance value based on the iToF sensor output value and the delay time; (5) After each subframe is detected, the signal processing unit uses a weighted average to perform overlapping fusion and splicing of each subframe to correct the distance value and improve the accuracy of ranging. The specific process is as follows: S51. For a detection interval with a distance of L, the entire detection interval is divided into N equally spaced sub-detection intervals. The division method is as follows: N = L / D, (1) D is the single-frame detection distance, which is less than or equal to the sensor's maximum detection distance; N is the result of dividing the distance L by the single-frame detection distance D and rounding it up. S52. Calculate the detection pulse width tp and the delay time td. The detection pulse width tp covers a single sub-interval range, i.e.: tp = 2D / (N * c), where c is the speed of light; the delay time td is equal to or slightly less than the detection pulse width tp; the timing control unit first controls the laser source to generate a pulsed laser with a detection pulse width of tp, and then after a delay time td, opens the detection gate of the iToF sensor, so that the iToF sensor receives the laser echo signal reflected back from the target; Both the laser pulse and the iToF sensor's detection pulse width are tp, but they differ by a delay time td. This delay time is used to set and adjust the detection range. After the laser pulse is reflected by the target, it is received by the iToF sensor and accumulates within the sensor's G1 and G2 gate times, respectively, generating output signals FD1 and FD2. Under this detection method, the target distance is calculated using the following formula: Distance = c / 2 * [td + tp * FD2 / (FD1 + FD2)], (2) S53. The signal processing unit allocates the starting address of each subframe register to receive and temporarily store the readout data from the iToF sensor. S54, Subframe Integration Time: After the detection parameters are initialized, the subframe exposure integration phase begins. Within the sub-interval, the iToF sensor begins exposure, and then logical judgments are made based on the iToF sensor's output value to prevent overexposure and ensure the distance value is correctly calculated. Specifically: S55. Set a threshold Q_Thre, which is set to 85-95% of the saturation voltage of the iToF sensor output. Compare the iToF sensor readout value Q_n with the threshold Q_Thre. If the current readout value is greater than the threshold Q_Thre, it means that the iToF sensor is in an overexposed state. Then discard the current subframe data, reduce the unit integration time, and then re-measure the current subframe. In addition, the average value of the output of some pixels of the iToF sensor is calculated and compared with the threshold Q_Thre to avoid the entire data packet being dropped due to the abnormality of a single pixel, which would reduce the overall imaging frame rate; for subframes that meet the threshold requirements, step S56 is executed. S56. Label the iToF sensor output data of this subframe with ID, and transmit the data packet to the signal processing unit. Then start the exposure stage of the next subframe and repeat steps S54 and S55. S57. Determine whether all subframe data has been acquired. If all subframe data has been correctly received, proceed to step S58. S58. Entering the data processing stage, the signal processing unit calculates the distance information based on the intensity information of each subframe, and then stitches the subframes together to output a distance image.

2. The range-gated detection method according to claim 1, characterized in that, The timing control unit uses an FPGA, DSP, or is designed as a dedicated ASIC chip; the laser source uses a vertical cavity surface-emitting laser or a side-emitting laser.

3. The range-gated detection method according to claim 1, characterized in that, During the subframe stitching stage, distance errors are corrected using the following methods: For overlapping areas between the detection intervals of two subframes, distance correction is performed through this overlapping area; a single subframe is further subdivided into: the near end of the subframe (output values ​​tend to be higher and need to be corrected by combining the back overlapping area of ​​the previous subframe); the far end of the subframe (output values ​​tend to be lower and need to be corrected by combining the front overlapping area of ​​the next subframe); and the mask area (this part is within the linear region of the current subframe, and data in this distance range is discarded when other subframes contain this range). A weighted average method is used for distance correction, calculated using the following formula: d corr = d n * t r / (t r + t f ) + d n+1 * t f / (t r + t f ), (3) Among them, t r t is the rise time of the laser pulse. f The fall time of the laser pulse, d n d is the output value of the back overlap region of the nth subframe. n+1 This is the output value for the front overlap area of ​​the (n+1)th subframe.

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