CN114814881A

CN114814881A - Laser ranging method and laser ranging chip

Info

Publication number: CN114814881A
Application number: CN202210400118.8A
Authority: CN
Inventors: 唐佳捷; 张超
Original assignee: Shenzhen Adaps Photonics Technology Co ltd
Current assignee: Shenzhen Adaps Photonics Technology Co ltd
Priority date: 2022-04-15
Filing date: 2022-04-15
Publication date: 2022-07-29

Abstract

The invention discloses a laser ranging method and a laser ranging chip, wherein the method comprises the following steps: carrying out first exposure, detecting photon signals reflected by the target object, constructing a first histogram through TDC (time-to-digital converter) conversion, and obtaining the initial measurement distance of the target object, wherein the width of each time box of the first histogram is L1; judging whether the initial measurement distance is smaller than a preset value; if not, taking the initial measurement distance as the actual distance of the target object; and if the time bin width is smaller than the actual distance of the target object, performing second exposure, detecting photon signals reflected by the target object, constructing a second histogram through TDC conversion, obtaining the measured distance of the target object, and taking the measured distance as the actual distance of the target object, wherein the time bin width of each second histogram is L2, and L2 is smaller than L1. The embodiment of the invention can simultaneously give consideration to the long distance and high precision of laser detection on the premise of limited resources of a storage module for storing the histogram, and has lower cost and good ranging effect.

Description

Laser ranging method and laser ranging chip

Technical Field

The invention relates to the technical field of laser ranging, in particular to a laser ranging method and a laser ranging chip.

Background

Laser radar calculates the distance of an object by measuring the flight time of a light beam in space, and is widely applied to the fields of consumer electronics, automatic driving, remote sensing, AR/VR and the like due to the advantages of high precision, large measurement range and the like.

In the prior art, there are two different methods of time-of-flight measurement, direct tof (dTOF) and indirect tof (IndirectTOF). In the current dTOF-based laser radar measurement system, a transmitting module and a receiving module are generally included. Wherein the emitting module can be an EEL, a VCSEL, a picosecond laser and the like; the receiving module generally uses a single photon avalanche diode array (SPADARRAY) to receive the returned light signal, and converts the Time information into a quantized multi-bit Digital signal through a TDC (Time-to-Digital Converter), and then draws a distance-based dynamic histogram through long-Time exposure and a TDC trigger accumulated value, thereby obtaining distance information of the target object.

In the method based on Time-Correlated Single Photon Counting (TCSPC), the number of timebin (bin numbers) and the timebin width (bin width) of the histogram determine the farthest distance and accuracy that can be detected by the TOF system.

For TCSPC methods, a smaller histogram bin width (W) means higher range detection accuracy, and a larger bin number (N) means a longer detection range, but from a hardware perspective, a larger bin number means more memory space is needed, and memory space on the chip is always valuable and constant. Therefore, when the bin number is fixed, the accuracy and the distance become a pair of contradictions. Namely, when the binding (w) is smaller (the distance measurement accuracy is higher), the farthest detection distance d is correspondingly smaller; conversely, if the bin width is increased, the distance detection accuracy is lowered although the farthest detection distance is increased accordingly.

In the prior art, in order to balance the contradiction between long-distance ranging of a laser radar, high precision and limited storage resource space, a method of carrying out two times of exposure from a coarse histogram to a fine histogram, or a distance histogram or a multiple rolling method is generally adopted for solving the problem.

The method for carrying out two exposures from the coarse histogram to the fine histogram specifically comprises the following steps: one complete fullhistogram is divided into 2 histograms. First called coarsehistogram, i.e., a larger bin width (e.g., 4 ns). The coarsehistogram is analyzed first to find the bin of interest and then to perform the unfolding of the higher precision bin width (e.g., 250ps), i.e., the finehistogram stage. For example, 4nscoarsebin can be expanded into 16 (4ns/0.25 ns-16) finebins (bin width-250 ps). Since coarse and finecustom occur at different time stages, the SRAM in the TOFsensor chip is time-multiplexed. In this case, the requirement (determined by the searehistogram) for the maximum distance measurement is satisfied, and the requirement (determined by the finehistogram) for the distance measurement accuracy is satisfied.

However, coarse-fine histograms also have significant drawbacks:

(1) because only one or a plurality of coarsebin statistical data after expansion and re-exposure are obtained by the finehistogram, the information of the whole scene (fullhistogram) in the whole ranging range can not be seen, namely the data are lost;

(2) for complex application scenarios, especially scenarios with multiple peak peaks in the fullhistogram (e.g. transparent glass in the detection environment, multipath interference, rain and fog interference, dust interference, etc.), applying the coarse-fine approach is likely to find the wrong bin in the coarse phase, so that the data of the fine histogram is also inaccurate or not really interesting for the user; or even if the coarse can find pairs each time, if there are multiple peaks (coarse bins), the peaks formed by the coarse are required to be subjected to fine histogram expansion one by one, and the whole data amount may not be less than that of the full histogram from the system perspective, and the frame rate may be lower than that of the full histogram.

Ranging is performed by dividing a long-range into multiple segments, e.g., 0-2m for a first segment, 2-4m for a second segment, 4-6m for a third segment, and so on.

Because the distance corresponding to the fullhistogram to be processed each time is shortened, the bin width can be correspondingly reduced, and the ranging precision can be ensured on the premise of certain SRAM capacity; meanwhile, the long distance is divided into a plurality of short distances, but the farthest distance can still be reached after a plurality of times of processing (segmented exposure and processing). Therefore, the method also considers the farthest detection distance and the ranging precision, and the limit of the SRAM capacity.

However, this approach also has some disadvantages, as follows:

(1) in order not to lose the target object information at the distance cut-off (e.g. at 2m, 4m, 6m above), in practice there must be overlapping ranging regions between two consecutive segments, e.g. 0-2.2m, 2.0-4.2m, 4.0-6.2m, … …. In this way, the distance overlapping part is wasted, and the capacity requirement of the SRAM is additionally increased;

(2) if the distance is too large, the data frame rate is also greatly affected.

Multiple rolling (rolling measurement) is limited by the capacity of the SRAM, and the TOFsensor chip only processes a smaller number of pixel units at a time, so that the usage amount of the SRAM is saved by a time-to-space method. For example, for a TOFsensor with a resolution of 320 × 240 pixels, only 320 × 60 pixels may be processed at a time, with a total of 4 scrolling to complete a scan of 320 × 240 pixels.

The problem with this approach is also apparent:

(1) motion blur (motionblur) phenomenon by rolling shutter;

(2) the number of rolling times affects the data frame rate, the number of pixels increases, the pixel resolution increases, and the contradiction between the maximum detection distance and the distance accuracy cannot be solved.

Therefore, the existing laser ranging method cannot simultaneously acquire the farthest detection distance and the distance precision under effective storage resources.

Accordingly, the prior art is yet to be improved and developed.

Disclosure of Invention

In view of the above deficiencies of the prior art, the present invention provides a laser ranging method and a laser ranging chip, which aims to solve the problem that the laser ranging method in the prior art cannot simultaneously obtain the farthest detection distance and the distance accuracy under effective storage resources.

The technical scheme of the invention is as follows:

a first embodiment of the present invention provides a laser ranging method, including:

carrying out first exposure, wherein a receiving module detects photon signals reflected by a target object, and a first histogram is constructed through TDC conversion to obtain the initial measurement distance of the target object, and the width of each time box of the first histogram is L1;

judging whether the initial measurement distance is smaller than a preset value or not, and if the initial measurement distance is not smaller than the preset value, taking the initial measurement distance as the actual distance of the target object;

if the initial measured distance is smaller than a preset value, carrying out second exposure, detecting a photon signal reflected by the target object by the receiving module, constructing a second histogram through TDC conversion to obtain a fine measured distance of the target object, taking the fine measured distance as an actual distance of the target object, wherein the width of each time box of the second histogram is L2, and L2 is smaller than L1;

the preset value is set according to L2 and the depth of the storage module, and the storage module is used for storing the triggering data of the photons according to the value output by the TDC so as to construct a first histogram and/or a second histogram.

Furthermore, the storage module is divided into a first storage unit and a second storage unit, and the depths of the first storage unit and the second storage unit are the same;

the first storage unit is used for storing histogram data of all pixels of a first frame;

and the second storage unit is used for storing histogram data of all pixels of the second frame.

Further, the preset value is a product of L2, the depth of the storage module and a coefficient of 0.15.

Further, L1 is 2N times L2, N being an integer greater than 1.

Further, the accuracy of the TDC of the first exposure is a1, the accuracy of the TDC of the second exposure is a2, a1 is 2N times a2, L1 is a1, and L2 is a 2.

Further, the accuracy of the TDC of the first exposure is a2, and the accuracy of the TDC of the second exposure is a2, wherein the time bins are merged according to a merging technique during the first exposure, and after merging, L1 is 2N a2, and L2 is a 2.

Further, when there are M1+ M2 target objects, after determining whether the initial distance is smaller than a preset value, the method further includes:

if the initial measured distances of the M1 target objects are detected to be smaller than the preset value and the initial measured distances of the M2 target objects are detected to be larger than the preset value, the second exposure is carried out,

acquiring the detailed distances of the M1 target objects during the second exposure, wherein the detailed distances are the actual distances of the M1 target objects;

and acquiring initial measurement distances corresponding to the M2 target objects during the first exposure, wherein the initial measurement distances are actual distances of the M2 target objects.

Further, the exposure time of the second exposure is far shorter than that of the first exposure, the exposure times of the second exposure are far shorter than those of the first exposure, and the laser emission power during the second exposure is far shorter than that during the first exposure.

Another embodiment of the present invention provides a laser ranging chip, including:

the receiving module is used for detecting the photon signals reflected by the target object;

the TDC module is used for converting the photon signal into a digital signal;

the storage module is used for storing the triggering data of the photons according to the digital signals output by the TDC module so as to construct a first histogram and/or a second histogram;

the controller is used for setting exposure times, acquiring a primary measurement distance of a target object from the first histogram and judging whether the primary measurement distance is smaller than a preset value or not; if the initial measurement distance is smaller than a preset value, carrying out second exposure, detecting a photon signal reflected by the target object by the receiving module, constructing a second histogram through TDC conversion to obtain a fine measurement distance of the target object, taking the fine measurement distance as an actual distance of the target object, wherein the width of each time box of the second histogram is L2, and L2 is smaller than L1, and if the initial measurement distance is not smaller than the preset value, taking the initial measurement distance as the actual distance of the target object; wherein the preset value is set according to L2 and the depth of the storage module.

Further, L1 is 2N times L2, N being an integer greater than 1.

Has the advantages that: according to the embodiment of the invention, the initial measurement distance of the target object is obtained through one exposure, if the initial measurement distance is smaller than the preset value, the secondary exposure is carried out to generate the fine measurement distance of the target object, the fine measurement distance is used as the actual distance of the target object, and if the initial measurement distance is not smaller than the preset value, the initial measurement distance is used as the actual distance of the target object. By adopting two times of exposure, the limited storage capacity is fully utilized, the long distance detection and high precision are considered, and the distance measurement effect is better.

Drawings

The invention will be further described with reference to the accompanying drawings and examples, in which:

FIG. 1 is a flow chart of a preferred embodiment of a laser ranging method according to the present invention;

fig. 2 is a schematic diagram of a hardware structure of a laser ranging chip according to a preferred embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and effects of the present invention clearer and clearer, the present invention is described in further detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Embodiments of the present invention will be described below with reference to the accompanying drawings.

In view of the above problems, an embodiment of the present invention provides a laser ranging method, please refer to fig. 1, and fig. 1 is a flowchart of a preferred embodiment of the laser ranging method according to the present invention. As shown in fig. 1, it includes:

s100, carrying out first exposure, wherein a receiving module detects photon signals reflected by a target object, and a first histogram is constructed through TDC conversion to obtain an initial measurement distance of the target object, wherein the width of each time box of the first histogram is L1;

step S200, judging whether the initial measurement distance is smaller than a preset value, if so, executing step S300, and if not, executing step S400;

step S300, carrying out second exposure, wherein the receiving module detects photon signals reflected by the target object, a second histogram is constructed through TDC conversion to obtain the measured distance of the target object, the measured distance is taken as the actual distance of the target object, the width of each time box of the second histogram is L2, and L2 is smaller than L1;

and step S400, taking the initial measurement distance as the actual distance of the target object.

In specific implementation, the embodiment of the invention is based on a direct time-of-flight measurement method for ranging. Wherein the emitting module includes but is not limited to EEL, VCSEL, picosecond laser. The receive module includes, but is not limited to, a single photon avalanche diode array SPAD ARRAY. The EEL is an edge emitting laser, light emitted by the edge emitting laser is emitted along a direction parallel to the surface of the substrate, the VCSEL is a surface emitting laser, and the light emitting direction of the surface emitting laser is perpendicular to the surface of the substrate. SPAD ARRAY operate in Geiger mode. The SPAD can realize single photon detection theoretically and has the highest detection sensitivity.

When the transmitting module transmits pulse laser to the target object, the first exposure is carried out, the receiving module detects photon signals reflected by the target object, a first histogram is constructed through TDC conversion, and the initial measurement distance of the target object is calculated according to the time box width and the time box number of the first histogram.

The initial measured distance (in m) of the target object is represented by d1, the number of time bin bins of the first histogram is represented by N, the width of time bin of the first histogram is represented by L1, where the speed of light is 3 x 10 ⁸ m/s; then d1 is 0.15 × N × L1.

At this time, whether the initial measurement distance is smaller than a preset value is judged,

and if the time interval is smaller than the preset value, carrying out second exposure, so that the receiving module detects photon signals reflected by the target object, constructing a second histogram through TDC conversion, and calculating the fine measurement distance of the target object according to the time box width and the number of time boxes of the second histogram.

The subtlety distance (m) of the target object is represented by d2, and the second straight line is represented by NThe number of time bins bin number of the histogram, the time bin width (in ns) of the second histogram, is represented by L2, where the speed of light is 3 x 10 ⁸ m/s; then d2 is 0.15N L2, where L2 is smaller than L1, so that the accuracy of measuring the fine distance is higher than the accuracy of measuring the initial distance.

The preset value is set according to L2 and the depth of the storage module, and the storage module is used for storing the triggering data of the photons according to the value output by the TDC, so that a first histogram and/or a second histogram are/is constructed.

In the embodiment of the invention, a bin width (the bin width is larger) with low precision is firstly adopted to perform full histogram on the whole detectable distance once, and the bin width is uniformly distributed, for example, the depth of a memory is 256, N1 is 256, L1 is 1ns, and the initial measured distance corresponds to 38.4 m;

then, the full histogram is added once more specifically for the close range, and the bin widths are also uniformly distributed, but at this time, the bin widths are relatively small (high precision), for example, N1 is 256, L2 is 0.125, precision is 0.018 m-0.15 x 0.125ns, and the fine measurement distance corresponds to 4.8 m-0.15 x 0.125 ns. By adopting 2 exposures, the storage depth of 256 is fully utilized, the detection distance is long, 38.4m can be detected furthest, the precision is high in a short distance, and the precision can reach 0.018m within 4.8 m.

Preferably, the preset value is a product of L2, the depth of the storage module and a coefficient of 0.15.

Specifically, the preset value L2 stores the depth of the module 0.15. If the depth of the storage module is 256, the preset value L2 × 256 × 0.15.

In one embodiment, the memory module is divided into a first memory cell and a second memory cell, and the depths of the first memory cell and the second memory cell are the same;

In specific implementation, if the close-range accuracy is further improved, taking 256 bins as an example, 256 bins may be used, or 256 bins may be 0.0625ns — 16ns (corresponding to 2.4 m); or on the premise that the precision has met the requirement, the same bin number may provide 2 times of the pixel resolution, for example, the storage module includes a first storage unit and a second storage unit, each of the first storage unit and the second storage unit is 128 bins, that is, 256 bins is 128 bins +128 bins, and the 128bin corresponding to each storage unit adopts 125ps precision, so that the 256 bins of the storage module may store the histogram data of 2 frames.

Preferably, L1 is 2N times L2, N being an integer greater than 1.

Specifically, in order to improve the accuracy of measurement, the time bin width of the first histogram is 2N times the time bin width of the second histogram, the sampling width becomes wider, the sampling number becomes smaller, and thus the measurement distance becomes smaller and the measurement accuracy increases. For example, L1 may be 4 times, 8 times, 16 times, etc. greater than L2.

In one embodiment, the accuracy of the first exposure TDC is a1, the accuracy of the second exposure TDC is a2, a1 is 2N times a2, L1 is a1, and L2 is a 2.

In specific implementation, two TDC circuits with different accuracies can be adopted, the accuracy of the first exposure TDC is A1, the accuracy of the second exposure TDC is A2, and the first exposure TDC is adopted to construct the first histogram during the first exposure. A second histogram construction is performed at the second exposure using the second exposure TDC, where a1 is 2N times a2, L1 is a1, and L2 is a 2.

In some other embodiments, the accuracy of the TDC of the first exposure is a2 and the accuracy of the TDC of the second exposure is a2, wherein the time bins are merged according to a binning merging technique at the time of the first exposure, and L1 is 2N a2 and L2 is a 2.

In the specific implementation, in the distance measurement process, a TDC circuit is adopted by the distance measurement chip, the precision of the TDC is a2, the time boxes are fused by using a binning fusion technology in the first exposure, the fused precision is L1, L1 is 2N × a2, and the precision in the second exposure is L2, wherein L2 is a 2.

In one embodiment, when there are M1+ M2 target objects, after determining whether the initial distance is smaller than the preset value, the method further includes:

and acquiring initial distances corresponding to the M2 target objects during the first exposure, wherein the initial distances are the actual distances of the M2 target objects.

In specific implementation, when an object is measured once, a plurality of target objects exist, the number of the target objects is recorded as M1+ M2, wherein M1 is a positive number which is greater than or equal to 0, M2 is a positive number which is greater than or equal to 0,

and carrying out first exposure, detecting a photon signal reflected by the target object by the receiving module, constructing a first histogram through TDC conversion, and obtaining the initial measurement distance of the target object according to the first histogram.

If the initial measured distances of the M1 target objects are smaller than the preset value and the initial measured distances of the M2 target objects are not smaller than the preset value, carrying out second exposure,

acquiring the detailed distances of M1 target objects during the second exposure, wherein the detailed distances are the actual distances of M1 target objects;

In one embodiment, the exposure time of the second exposure is much shorter than that of the first exposure, the number of exposures of the second exposure is much smaller than that of the first exposure, and the laser emission power at the time of the second exposure is much smaller than that at the time of the first exposure.

In specific implementation, as the farther the target object is, the larger the emission power is, the close distance is specially aimed at in the second exposure, so that theoretically, the total exposure time and the exposure times, including the laser emission power, can be greatly reduced compared with the distance detection of the maximum range, and the system power consumption is saved. Wherein far less means that the exposure time of the first exposure is X times that of the second exposure, X is an integer greater than 2, and far less means that the latter is X times that of the former, and the numerical values are far apart.

It should be noted that, a certain order does not necessarily exist between the above steps, and those skilled in the art can understand, according to the description of the embodiments of the present invention, that in different embodiments, the above steps may have different execution orders, that is, may be executed in parallel, may also be executed interchangeably, and the like.

Another embodiment of the present invention provides a laser ranging chip, as shown in fig. 2, including:

the receiving module 11 is used for detecting a photon signal reflected by the target object;

a TDC module 12 for converting the photon signal into a digital signal;

the storage module 13 is used for storing the triggering data of the photons according to the digital signals output by the TDC module so as to construct a first histogram and/or a second histogram;

the controller 14 is configured to set exposure times, obtain an initial measurement distance of the target object from the first histogram, and determine whether the initial measurement distance is smaller than a preset value; if the initially measured distance is smaller than the preset value, performing second exposure, detecting a photon signal reflected by the target object by the receiving module, constructing a second histogram through TDC conversion to obtain a finely measured distance of the target object, taking the finely measured distance as an actual distance of the target object, wherein the width of each time box of the second histogram is L2, and L2 is smaller than L1, and if the initially measured distance is not smaller than the preset value, taking the initially measured distance as the actual distance of the target object; wherein the preset value is set according to L2 and the depth of the storage module.

In specific implementation, when the transmitting module transmits pulse laser to the target object, the receiving module detects a photon signal reflected by the target object, the TDC module converts the photon signal into a digital signal, and the storage module stores trigger data of the photon according to the digital signal output by the TDC module, so that a first histogram and/or a second histogram are/is constructed.

The controller sets the number of exposures and obtains the initial distance of the target object from the first histogram, and represents the initial distance of the target object by d1 (singly)Bit m), the number of time bins bin of the first histogram is denoted by N, the width of time bins bin of the first histogram is denoted by L1 (in ns), where the light speed is 3 x 10 ⁸ m/s; then d1 is 0.15 × N × L1.

The controller judges whether the initial measurement distance is smaller than a preset value,

The subtlety distance (in m) of the target object is represented by d2, the number of time bin bins of the second histogram is represented by N, the width of time bin of the second histogram is represented by L2, and the light speed is 3 x 10 ⁸ m/s; then d2 is 0.15N L2, where L2 is smaller than L1, so that the accuracy of measuring the fine distance is higher than the accuracy of measuring the initial distance.

In one embodiment, L1 is 2N times L2, N being an integer greater than 1.

In specific implementation, in order to improve the measurement accuracy, the time box width of the first histogram is 2N times of the time box width of the second histogram, the sampling width is increased, the sampling number is decreased, the measurement distance is decreased, and the measurement accuracy is increased. For example, L1 may be 4 times, 8 times, 16 times, etc. greater than L2.

Through the above description of the embodiments, it is clear to those skilled in the art that the embodiments may be implemented by software plus a general hardware platform, and may also be implemented by hardware. Based on such understanding, the above technical solutions essentially or contributing to the related art can be embodied in the form of a software product, which can be stored in a computer-readable storage medium, such as ROM/RAM, magnetic disk, optical disk, etc., and includes several instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods of the various embodiments or some parts of the embodiments.

Conditional language such as "can," "might," or "may" is generally intended to convey that a particular embodiment can include (yet other embodiments do not include) particular features, elements, and/or operations, among others, unless specifically stated otherwise or otherwise understood within the context as used. Thus, such conditional language is also generally intended to imply that features, elements, and/or operations are in any way required for one or more embodiments or that one or more embodiments must include logic for deciding, with or without input or prompting, whether such features, elements, and/or operations are included or are to be performed in any particular embodiment.

What has been described herein in the specification and drawings includes examples that can provide a laser ranging method and chip. It will, of course, not be possible to describe every conceivable combination of components and/or methodologies for purposes of describing the various features of the disclosure, but it can be appreciated that many further combinations and permutations of the disclosed features are possible. It is therefore evident that various modifications can be made to the disclosure without departing from the scope or spirit thereof. In addition, or in the alternative, other embodiments of the disclosure may be apparent from consideration of the specification and drawings and from practice of the disclosure as presented herein. It is intended that the examples set forth in this specification and the drawings be considered in all respects as illustrative and not restrictive. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. A laser ranging method, the method comprising:

2. The method of claim 1, wherein the memory module is divided into a first memory cell and a second memory cell, the first memory cell and the second memory cell having the same depth;

3. The method of claim 1, wherein the predetermined value is a product of L2, a depth of the memory module, and a coefficient of 0.15.

4. The method of claim 1, wherein L1 is 2N times L2, and N is an integer greater than 1.

5. The method of claim 4, wherein the accuracy of the first exposure TDC is A1, the accuracy of the second exposure TDC is A2, A1 is 2N times A2, L1 is A1, and L2 is A2.

6. The method of claim 4, wherein the accuracy of the first exposure TDC is A2 and the accuracy of the second exposure TDC is A2, and wherein the time bins are merged according to a merging technique during the first exposure, and the merged time bins are L1-2N A2 and L2-A2.

7. The method of claim 1, wherein when there are M1+ M2 target objects, determining whether the initial distance is less than a predetermined value further comprises:

8. The method of claim 1, wherein the exposure time of the second exposure is much shorter than the exposure time of the first exposure, the number of exposures of the second exposure is much shorter than the number of exposures of the first exposure, and the laser emission power at the second exposure is much shorter than the laser emission power at the first exposure.

9. A laser ranging chip, comprising:

the TDC module is used for converting the photon signal into a digital signal;

10. The ranging chip of claim 9, wherein L1 is 2N times L2, N being an integer greater than 1.