CN114035197A - Detection method for obtaining distance information - Google Patents

Abstract

The invention discloses a detection method for obtaining distance information, which comprises a light source module, a detection module and a control module, wherein the light source module can output emitted light signals with different phases; the receiving module is used for acquiring a return light signal which is emitted and returned by the detected object in the field of view through the emitted light and converting the return light signal into an electric signal; the processing module obtains the distance signal of the detected object according to the electric signal converted from the return light signal acquired by the receiving module, and the processing module comprises a correction function which is used for calculating the distance signal according to the electric signal, and the processing module obtains the distance information of the detected object according to the correction function.

Description

Detection method for obtaining distance information

Technical Field

The present application relates to the field of detection technologies, and in particular, to a detection method for obtaining distance information.

Background

As a method of measuring a distance from an object in a scene, a time of flight (TOF) technique is developed. Such TOF technology can be applied in various fields such as the automotive industry, human-machine interfaces, games, robotics, security, and the like. In general, TOF technology operates on the principle of illuminating a scene with modulated light from a light source and observing the reflected light reflected from objects in the scene. In order to ensure that a detection system has a wider field of view while obtaining higher detection efficiency in a detection process in the conventional detection system, an array-type receiving module is mostly adopted at present, the array-type receiving module may have thousands of pixel units, each pixel unit may be a diode of a charge-coupled semiconductor CCD or a complementary metal oxide semiconductor CMOS type, and the like, and the array-type receiving module is not limited to be formed by only the two types of diodes.

In order to obtain distance information, in the detection of TOF, delay information of emitted light and returned light is obtained indirectly, so that delay phase or phase shift is obtained, and then phase shift is converted into final result information. In actual use, the method of receiving the return light signal with complementary phases and obtaining the distance information is called a two-phase scheme, and a scheme of obtaining the target distance with four phases of 0 °, 90 °, 180 ° and 270 ° is also used, and of course, a scheme of obtaining the distance of the detected object by trying a 3-phase or even 5-phase scheme is also used in the literature, and a phase-shifted electrical signal is obtained, and the electrical signal needs to be processed by a processing unit to obtain the final distance information, but the electrical signal corresponding to the actually obtained return light signal is due to environmental factors including but not limited to temperature and ambient lighting conditions. For example, temperature changes in the sensor array may increase the so-called dark current of the pixels, which in turn may change the phase shift of the measurement, which may show large distance fluctuations in the measurement results, and a specific conversion relationship is required to eliminate such influences due to the aforementioned dark current and the like, so as to obtain an accurate detection distance. The TOF distance calibration and correction method commonly used at present is a lookup table method, and the lookup table method is to obtain calibration and correction data information through a large amount of test data and store the calibration and correction data information in a memory. When the correction data is used, the independent variable is used as an address to read the memory, and corresponding correction data information in the memory is obtained and used for correction. The size of the look-up table is exponential to the data bit width and therefore the required memory space is rather large.

In the above analysis, it is a technical problem to be solved urgently to design a detection method and a detection system for obtaining distance information so as to accurately and stably output stable and accurate distance results of detected objects in various distance ranges in a field of view, and to solve huge burden on memory capacity and memory bandwidth requirements.

Disclosure of Invention

The present application aims to overcome the above drawbacks of the prior art, and provide a method for acquiring distance information to reduce the storage pressure of a TOF chip, so that the TOF chip is miniaturized and feasible, and stable and accurate distance results of an object to be detected in each distance range in a field of view can be accurately output.

In order to achieve the above purpose, the technical solutions adopted in the embodiments of the present application are as follows:

the embodiment of the application provides a detection method for obtaining distance information, which comprises a light source module, a first detection module and a second detection module, wherein the light source module can output emitted light signals with different phases; the receiving module is used for acquiring a return light signal which is emitted and returned by the detected object in the field of view through the emitted light and converting the return light signal into an electric signal; and the processing module is used for obtaining a distance signal of the detected object according to an electric signal converted from the return light signal acquired by the receiving module, and comprises a correction function for calculating the distance signal according to the electric signal, and the processing module is used for obtaining the distance information of the detected object according to the correction function.

Optionally, the correction function is obtained by looking up a relationship between theoretical distances and measured distances in a table.

Optionally, the look-up table information of measured distances of different theoretical distances in the look-up table is obtained by adjusting the phase of the emitted light.

Optionally, adjusting the phase of the emitting light source keeps the distance between the light source and the detected object fixed.

Optionally, the correction function is saved in a memory space.

Optionally, the processing unit generates calibration information in real time according to the calibration function to calibrate the distance and obtain final target distance information.

Optionally, the processing unit generates a temporary lookup table according to the calibration function to calibrate the distance to obtain the final target distance information.

Optionally, the temporary lookup table is not saved after the correction is completed.

Optionally, the calibration function is a fourier series.

Optionally, the fourier series is a fourier series with an order of eight or less.

The beneficial effect of this application is:

the detection method for obtaining the distance information comprises a light source module, a detection module and a control module, wherein the light source module can output emitted light signals with different phases; the receiving module is used for acquiring a return light signal which is emitted and returned by the detected object in the field of view through the emitted light and converting the return light signal into an electric signal; the processing module obtains the distance signal of the detected object according to the electric signal converted from the return light signal acquired by the receiving module, and the processing module comprises a correction function which is used for calculating the distance signal according to the electric signal, and the processing module obtains the distance information of the detected object according to the correction function.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings that are required to be used in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present application and therefore should not be considered as limiting the scope, and for those skilled in the art, other related drawings can be obtained from the drawings without inventive effort.

FIG. 1 is a schematic diagram illustrating the operation of a detection system according to the prior art;

FIG. 2 is a schematic diagram of an ITOF acquisition time-of-flight signal provided by the prior art;

FIG. 3 is a diagram illustrating actual range and detected phase shift results provided by an embodiment of the present application;

FIG. 4A is a diagram illustrating a test system for obtaining lookup table information according to an embodiment of the present disclosure;

FIG. 4B is a schematic diagram of adjusting the phase of the emitting light source according to an embodiment of the present disclosure;

FIG. 5 is a schematic diagram of a lookup table workflow provided by an embodiment of the present application;

FIG. 6A is a schematic diagram illustrating lookup table information of a pixel according to an embodiment of the present disclosure;

fig. 6B is a schematic diagram of a relationship between a theoretical distance and a measured distance provided in the embodiment of the present application;

FIG. 7 is a flowchart of a calibration function according to an embodiment of the present application;

fig. 8 is a flowchart of a calibration function according to an embodiment of the present application.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are some embodiments of the present application, but not all embodiments. The components of the embodiments of the present application, generally described and illustrated in the figures herein, can be arranged and designed in a wide variety of different configurations.

Thus, the following detailed description of the embodiments of the present application, presented in the accompanying drawings, is not intended to limit the scope of the claimed application, but is merely representative of selected embodiments of the application. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, it need not be further defined and explained in subsequent figures.

The detection systems currently used basically comprise: an ITOF ranging module, a processing module, and a light receiving module, where the ITOF ranging module is taken as an example for illustration, the light emitting module includes but is not limited to a semiconductor laser, a solid-state laser, and may also include other types of lasers, when the semiconductor laser is used as a light source, a Vertical-cavity surface-emitting laser (VCSEL) or an edge-emitting semiconductor laser (EEL) may be used, which is only exemplary and not specific, the light emitting module emits a sine wave, a square wave, a triangle wave, etc., in the ranging application, most of the lasers with a certain wavelength, such as 950nm infrared laser (preferably near infrared laser), are emitted light projected into a field of view, and an object to be detected in the field of view may reflect the projected laser to form a return light, and the return light enters the detecting system and is captured by the light receiving module, the light receiving module may include a photoelectric conversion portion, such as an array type sensor composed of CMOS, CCD, etc., and may further include a plurality of lenses that may form more than one image plane, that is, the receiving module includes more than one image plane, the photoelectric conversion portion of the receiving module is located at one of the image planes, and may receive the signals in the most common four-phase scheme to obtain delayed received signals of 0 °, 90 °, 180 °, and 270 °, and the four-phase distance calculation scheme is used to illustrate the sine wave method, and the amplitude of the received signals is measured at four equidistant points (e.g., at intervals of 90 ° or 1/4 λ):

the ratio of the difference between a1 and A3 to the difference between a2 and a4 is equal to the tangent of the phase angle. ArcTan is in fact a bivariate arctangent function, which can be mapped to the appropriate quadrant, defined as 0 ° or 180 ° when a2 ═ a4 and a1> A3 or A3> a1, respectively.

The distance to the target is determined by the following formula:

so far, the frequency of the emitted laser light needs to be determinedMeasuring the line distance, wherein c is the speed of light,

is the phase angle (measured in radians) and f is the modulation frequency. The scheme can realize the effect of detecting the distance of the detected object in the field of view, the scheme is called as a four-phase delay scheme to obtain a detection result, the receiving module generates different information through photoelectric conversion, the information acquisition of the detected object is realized by using a 0-degree and 180-degree two-phase scheme under certain conditions, three-phase schemes of 0 degree, 120 degrees and 240 degrees are disclosed in documents to obtain target information, even a five-phase delay scheme is disclosed in documents.

Fig. 2 is a schematic diagram showing that the emission wave is a square wave, the detection system emits emission light, where different regions indicate that the emission light is different in color, and the emission light may be uniform light or non-uniform light in practical use, and is not limited herein, but the actually used emission light is small in wavelength variation and does not show color difference, further, the actually used emission light is infrared laser light with higher eye safety, and the wavelength range may be 900nm to 1000nm, and is not limited herein, and the emission light is reflected by the detected object to form return light, and fig. 2 shows the phase shift of the emission light (1) and the return light (2), which actually represents the time-of-flight phase shift related to the distance, and distance information can be obtained as long as the phase shift can be obtained.

Fig. 3 illustrates a relationship between phase information and real distance information of a light source for converting return light of emitted light, in which the emission power of the light source is generally low, the peak power is in the order of hundreds of milliwatts to several watts, the laser emission frequency can be set according to different situations and different precision requirements, for example, 20MHz can be increased to hundreds of MHz, different detection precisions are generally corresponding to different laser emission frequencies, 25MHz belongs to low frequency detection, the precision is low, but the farthest distance that can be detected in one period is far, for example, the farthest detection distance is calculated according to the flight time in the range of 6m, for hundreds of MHz laser such as 120MHz, the farthest detection distance range is only 1.25m according to the flight time scheme, and the precision for the distances in different distance ranges is close to 5 times, a general detection system has certain requirements for both detection range and detection precision, for example, the situation in a general field of view is complex, the detection system is required to realize detection at a longer distance, and the application scenario of the system requires the detection system to obtain a more accurate scenario at a higher precision for different distance information, so a detection method of two or more frequencies is proposed, wherein the highest frequency detection laser usually corresponds to a higher precision, for example, a scheme of finding a common multiple of a high frequency result and a low frequency result to obtain final distance information may be adopted, or another scheme of firstly detecting the approximate position of an object by using different low frequency emitted lights and then obtaining final accurate distance information by using a scheme similar to a caliper to obtain a distance result within a micro range by using a high frequency laser related to precision, and the scheme of realizing a final range and accuracy in a certain manner is not limited herein to realize obtaining a distance result by using a scheme of taking account of a final range and accuracy And (6) taking. The phase shift calculated from the electrical signal converted from the return light of the emitted light after returning through the detected object does show positive correlation with the real distance result, but it can be seen from fig. 3 that the obtained phase shift does not completely reflect the real distance information of the detected object, and the difference mainly comes from the following influences: 1. influence of electrical characteristics due to internal structure and defects of the sensor; 2. the light source emitter emits light with a waveform which has defects and introduces high-order terms, so in order to establish a real result of obtaining phase and distance, a corresponding relation between a measured distance and a theoretical distance needs to be obtained before actual ranging, so that the measured distance is calibrated in the actual ranging to obtain an accurate measured distance value. In the invention, the corresponding relation between the measured distance and the theoretical distance, namely a lookup table, is obtained through experimental tests. In order to meet different actual measurement requirements, a large number of relationships between different measured distances and theoretical distances are required. This requires that the distance between the object to be measured and the light source is constantly moved to obtain the information of the lookup table at different distances. However, the manual movement of the object to be measured may introduce new non-ideal factors to affect the ranging accuracy and is not beneficial to the operation. The invention achieves the effect of moving the distance between the light source and the object to be measured by adjusting the phase of the transmitting light source. Fig. 4A shows a test system for obtaining the look-up table information, which includes a probe system 401 and an object 402 to be tested, wherein the distance between the probe system 401 and the object 402 to be tested is fixed and known. The probing system 401 and the object to be tested 402 remain stationary throughout the experimental testing. The detection system 401 includes a transmitting light source and a receiving device. Wherein the receiving modulation wave in the receiving device remains unchanged, or respectively receives the echo signal of the object 402 to be measured according to the four-phase principle described in fig. 1. It can be known from fig. 3 that the distance and the phase have a corresponding relationship, so that the effect of changing the distance between the detection system 401 and the object 402 can be achieved by changing the phase of the emitting light source, as shown in fig. 4B, different phases of the emitting light source represent different distances between the detection system 401 and the object 402. This distance is obtainable from the phase information, i.e. the theoretical distance. Of course, calibration of different distances may also be achieved by directly moving the distance of the object to be measured, which has to be a predetermined distance, i.e. a theoretical distance. And then, obtaining the actual measurement distance through the echo of the object to be measured, thereby obtaining the correction information between the theoretical distance and the actual measurement distance at different distances. Calibration information at each distance can be calibrated through a large number of test processes, and a relatively complete lookup table is obtained. However, in the actual measurement process, it is not possible to calibrate the look-up table information at all distances, and if the required correction information for the distance is not calibrated in the look-up table, it is obtained by the difference between the two values that are nearest to each other.

Fig. 5 is a workflow of the lookup table method. As shown in fig. 5, includes:

s501: adjusting the phase difference between the light source and the modulation signal, and driving the light source to emit a detection light signal;

s502: the emitted detection light returns to pixels on the sensor through the target to be detected, and the detector calculates the distance according to the echo signal;

s503: establishing an error lookup table according to the theoretical distance between the targets to be detected and the distance obtained according to the echo signal;

s504: and performing distance compensation on each pixel according to the error lookup table in subsequent actual measurement.

Fig. 6A shows the look-up table calibration data for one pixel. Taking 320 × 240 QVGA pixels as an example, the step of the required phase delay is calibrated to be 156ps, and 320 distance data are required for each pixel in the range of 7.5 m. For single precision floating point type stores (4B) by one distance, the data store size is 94MB, 320 × 240 × 320 × 4 ═ 94 MB. In a typical TOF ranging chip, the storage space is typically only 5MB due to area constraints. Therefore, the huge storage space required by the lookup table becomes a huge resistance for restricting the miniaturization of the TOF chip, and a solution is needed urgently.

In order to solve the technical problem, the invention provides a scheme for replacing the lookup table by the correction function. As shown in fig. 6B, 601 is a theoretical distance value 602 which is an actual measurement value, and it can be seen from the figure that 601 and 602 are deviated in initial positions and the starting point positions are different, and a detailed description of the reason is explained in chinese patent CN 202110103547.4. It can be seen from fig. 6B that if the relationship between 601 and 602 can be replaced by a function, and the actually measured distance is used as an input in the function, the output of the function can be obtained, and the calibration information of the distance can be obtained, so that the ranging accuracy is improved. The correction function is fitted with the look-up table information as shown in fig. 6A. The function fitted in the invention is an eighth order Fourier series:

a0+a1*cos(x*w)+b1*sin(x*w)+a2*cos(2*x*w)+b2*sin(2*x*w)+a3*cos(3*x*w)+b3*sin(3*x*w)+a4*cos(4*x*w)+b4*sin(4*x*w)+a5*cos(5*x*w)+b5*sin(5*x*w)+a6*cos(6*x*w)+b6*sin(6*x*w)+a7*cos(7*x*w)+b7*sin(7*x*w)+a8*cos(8*x*w)+b8*sin(8*x*w) (1)

where different pixels correspond to different coefficients. It can be seen from formula 1 that 320 distance data are represented by functional relationships, where 18 of a0, a1, b1, a2, b2, a3, b3, a4, b4, a5, b5, a6, b6, a7, b7, a8, b8, and w are coefficients, and 18 different coefficients correspond to different pixels. The functional relation can be expressed by 18 coefficients, the single-precision floating-point type storage (4B) is still realized according to one coefficient, and the size of a data storage space is 5.3MB

320 × 240 × 18 × 4 ═ 5.3 MB. Of course to further reduce the pressure on the storage space, a fourier series of lower order may be used when fitting the correction function, for example a fourier series of sixth order expressing the function in 14 coefficients, although such a correction function would be less accurate than a correction function of 18 coefficients. The size of the data storage space required for a correction function of 14 coefficients is: 320 × 240 × 14 × 4 ═ 4.1 MB.

The pressure of the storage space is greatly reduced by the correction function as described above, so that the miniaturization of the TOF chip is not limited by the storage space.

Fig. 7 is a working flow of a correction function provided by the present invention. As shown in fig. 7, the method comprises:

s701: adjusting the phase difference between the light source and the modulation signal, and driving the light source to emit a detection light signal;

s702: the emitted detection light returns to pixels on the sensor through the target to be detected, and the detector calculates the distance according to the echo signal;

s703: establishing an error lookup table according to the theoretical distance between the targets to be detected and the distance obtained according to the echo signal;

s704: generating a fitting correction function according to the lookup table information;

s705: storing the correction function in a memory;

s706: and calculating correction information in real time according to the correction function to correct the measured distance.

In the calibration method shown in fig. 7, the CPU is required to calculate the calibration information in real time according to the calibration function, which is high and slow. The correction by means of a simple look-up table can be implemented quite quickly, which is usually limited only by the access time of the ROM or RAM, typically less than 20ns, depending on the device used. But the time required to correct the function (e.g., an eighth order fourier series) in real time for a measured distance as an input is much greater than 20 ns. In order to solve this technical problem, the present invention proposes another distance correction method as shown in fig. 8. As shown in fig. 8, includes:

s801: adjusting the phase difference between the light source and the modulation signal, and driving the light source to emit a detection light signal;

s802: the emitted detection light returns to pixels on the sensor through the target to be detected, and the detector calculates the distance according to the echo signal;

s803: establishing an error lookup table according to the theoretical distance between the targets to be detected and the distance obtained according to the echo signal;

s804: generating a fitting correction function according to the lookup table information;

s805: storing the correction function in a memory;

s806: generating a temporary lookup table according to the correction function;

s807: saving the temporarily generated lookup table;

s808: correcting according to the temporarily generated lookup table;

s809: and releasing the stored temporary lookup table after the correction is completed.

In the method shown in fig. 8, only the correction function is stored, but the lookup table is temporarily generated when the ranging correction is needed, or the correction is performed according to the lookup table method, so that the storage pressure is reduced, and the correction speed is increased.

It is to be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.

The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application. It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, it need not be further defined and explained in subsequent figures. The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims (10)

1. A detection method for obtaining distance information is characterized by comprising a light source module, a light source module and a detection module, wherein the light source module can output emitted light signals with different phases; the receiving module is used for acquiring a return light signal which is emitted and returned by the detected object in the field of view through the emitted light and converting the return light signal into an electric signal; and the processing module is used for obtaining a distance signal of the detected object according to an electric signal converted from the return light signal acquired by the receiving module, and comprises a correction function for calculating the distance signal according to the electric signal, and the processing module is used for obtaining the distance information of the detected object according to the correction function.

2. The detection method for obtaining distance information according to claim 1, wherein the correction function is obtained by a relationship between a theoretical distance and a measured distance in a look-up table.

3. The detection method for obtaining distance information according to claim 2, wherein the look-up table information of the measured distances of different theoretical distances in the look-up table is obtained by adjusting the phase of the emitted light.

4. A detection method for obtaining distance information according to claim 3, wherein adjusting the phase of said light source for emission keeps the distance between said light source and the object to be detected constant.

5. The detection method for obtaining distance information according to claim 1, wherein the correction function is stored in a memory space.

6. The detection method for obtaining distance information according to claim 1, wherein the processing unit generates calibration information in real time according to the calibration function for calibrating the distance to obtain the final target distance information.

7. The detection method for obtaining distance information according to claim 1, wherein the processing unit generates a temporary lookup table for calibrating the distance according to the calibration function to obtain the final target distance information.

8. The detection method of obtaining distance information according to claim 7, wherein the temporary lookup table is not saved after completion of the correction.

9. A detection method for obtaining distance information according to claim 1, wherein said calibration function is a fourier series.

10. The detection method for obtaining distance information according to claim 9, wherein the fourier series is a fourier series having an order of eight or less.

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