A method for measuring flight time and a time-to-digital converter

Abstract

The embodiment of the present application provides a time-to-digital converter, characterized in that it includes: a statistical module based on a first clock, used to count the first time interval in which the input signal of the time-to-digital converter is located; and a measurement module based on a second clock, used to measure the first moment of the input signal in the first time interval. By dividing the measured distance into multiple time intervals, the flight time is obtained by only one statistical operation and one measurement, which saves the data resources of the TDC compared to the traditional one-time measurement method. Therefore, the amount of data transmission can be reduced to a certain extent.

Description

Technical Field

[0001] The present application relates to the field of detection technology, and in particular to a method for measuring flight time and a time-to-digital converter. Background Art

[0002] The principle of time of flight (TOF) is to continuously send light pulses to the target, then use a sensor to receive the light returned from the object, and detect the flight (round-trip) time of the light pulses to obtain the distance of the target. Direct time of flight (DTOF) is a type of TOF. DTOF technology directly obtains the target distance by calculating the emission and reception time of light pulses. It has the advantages of simple principle, good signal-to-noise ratio, high sensitivity, and high accuracy, and has received more and more attention. In other words, direct time of flight detection determines the distance to the target by directly measuring the length of time between the emission of radiation and the detection of radiation after reflection from an object or other target.

[0003] In some applications, sensing of reflected radiation may be performed using a photodetector array including single photon detectors (e.g., single photon avalanche diode (SPAD) arrays. One or more photodetectors may define a detector pixel of the array. SPAD arrays may be used as solid-state photodetectors in imaging applications where high sensitivity and timing resolution may be required. SPADs are based on semiconductor junctions (e.g., pn junctions) that can detect incident photons when biased out of their breakdown region, for example, by or in response to a gating signal having a desired pulse width. A high reverse bias voltage generates an electric field of sufficient magnitude so that a single charge carrier introduced into the depletion layer of the device can cause a self-sustaining avalanche by impact ionization. The avalanche can be quenched actively (e.g., by reducing the bias voltage) or passively (e.g., by using a voltage drop across a series resistor) by a quenching circuit to "reset" the device for further photon detection. The starting charge carrier can be photogenerated by a single incident photon striking a high electric field region. It is this feature that has given rise to the name "single photon avalanche diode." This single photon detection mode of operation is often referred to as "Geiger mode."

[0004] To count the photons incident on the SPAD array, some ToF pixel approaches may use digital counters or analog counters to indicate the detection and arrival times of the photons, also known as timestamps. Digital counters may be easier to implement and scale, but may be more expensive in terms of area (e.g., relative to the physical size of the array). Analog counters may be more compact, but may be limited by photon counting depth (bit depth), noise, and / or uniformity issues.

[0005] To time-stamp the incoming photons, some ToF pixel approaches based on SPAD arrays use a time-to-digital converter (TDC). TDCs can be used in time-of-flight imaging applications to improve the timing resolution of a single clock cycle. Some advantages of this digital approach may include that the size of the TDC tends to scale with technology nodes, and the stored values ​​can be more robust to leakage.

[0006] However, the TDC circuitry may only be able to process one event measurement cycle in a single event, so a row of SPADs may require multiple TDCs. TDCs may also be power hungry, making larger arrays more difficult to implement. TDCs may also generate relatively large amounts of data, for example, a 16-bit timestamp per photon. A single SPAD connected to a TDC may generate millions of these timestamps per second. Thus, relative to the available input /

[0007] Output bandwidth or functionality, imaging arrays larger than 100,000 pixels would render them infeasible

[0008] Big data rate. But it is totally impractical and the TDC measurement accuracy is not achievable. Summary of the invention

[0009] The purpose of the present application is to provide a method for measuring flight time and a time-to-digital converter to address the deficiencies in the above-mentioned prior art, so as to solve the technical problem of the pressure of TDC transmitting data in the existing pixel array.

[0010] To achieve the above purpose, the technical solution adopted in the embodiment of the present application is as follows:

[0011] In a first aspect, an embodiment of the present application provides a time-to-digital converter, characterized in that it includes: a statistical module based on a first clock, used to count the first time interval in which an input signal of the time-to-digital converter is located; and a measurement module based on a second clock, used to measure the first moment of the input signal in the first time interval.

[0012] Optionally, the statistical module includes: a first statistical module, which counts the first time interval in which the input signal of the time digital converter is located based on the first clock; and a second statistical module, which counts the first sub-time interval in the first time interval in which the input signal of the time digital converter is located based on the third clock; wherein the clock period of the third clock is smaller than the clock period of the first clock.

[0013] Optionally, a clock period of the second clock is smaller than a clock period of the third clock.

[0014] Optionally, the measurement module uses a vernier caliper type time-to-digital converter.

[0015] Optionally, the number of time intervals of the first statistical module is smaller than the number of time intervals of the second statistical module.

[0016] In a second aspect, an embodiment of the present application provides a method for measuring flight time, characterized in that it includes: dividing a distance to be measured into multiple first distances, each of the first distances corresponding one-to-one to a first time interval; counting the first time interval in which an echo signal is located based on a first clock; measuring the first moment in the first time interval in which the echo signal is located based on a second clock; and determining the flight time based on the first time interval and the first moment.

[0017] Optionally, the first time interval in which the statistical echo signal is located also includes: dividing each first distance into multiple first sub-distances, the first sub-distances corresponding one-to-one to the first sub-time intervals; based on the first sub-time interval in the first time interval in which the statistical echo signal is located, based on the third clock; wherein the clock period of the third clock is smaller than the clock period of the first clock.

[0018] Optionally, a clock period of the second clock is smaller than a clock period of the third clock.

[0019] Optionally, the number of first time intervals is smaller than the number of first sub-time intervals.

[0020] Optionally, the method for measuring the echo signal is a vernier caliper method.

[0021] The beneficial effect of the present application is that by dividing the measured distance into multiple time intervals, the flight time is obtained by only one statistics and one measurement, which saves TDC data resources compared to the traditional one-time measurement method. Therefore, the amount of data transmission can be reduced to a certain extent. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0023] Figure 1 A distance measurement principle diagram provided for an embodiment of the present application;

[0024] Figure 2 A structural block diagram of a TDC circuit provided in an embodiment of the present application;

[0025] Figure 3 A structure and timing diagram of a TDC circuit provided in an embodiment of the present application;

[0026] Figure 4 A structural block diagram of another TDC circuit provided in an embodiment of the present application;

[0027] Figure 5 A distance measurement method provided in an embodiment of the present application;

[0028] Figure 6 Another distance measurement method provided in an embodiment of the present application. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations.

[0030] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application for which protection is sought, but merely represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0031] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0032] Figure 1 This is a functional module diagram of a detection device provided in an embodiment of the present application. Figure 1 As shown, the detection device includes: a pulse light source 101 , an object to be detected 102 , a detector array 103 and a processing module 104 .

[0033] Among them, the pulse light source 101 is used to emit a detection pulse to the object to be tested 102, and the object to be tested 102 reflects part of the pulse light source to the detector array 103. The detection array 103 can be a SPAD array, and the detection array 103 receives the reflected photons. When the reflected photons hit the high electric field area, photoelectric generation causes an avalanche of the SAPD. Each pixel of the SPAD array detects the avalanche time caused by the arrival of photons during a certain window detection period. When the avalanche caused by the photons is detected within a certain detection window time period, it is considered that an event is detected, and it is marked that the event is detected during the detection window period. The marking method can be cumulative plus 1 or other identification, which is not limited in the embodiments of the present invention.

[0034] The processing module 103 can determine in which detection window the reflected photon is according to the detection events identified during the statistically analyzed detection windows. The arrival time range of the reflected photon is determined, and the arrival time of the emitted photon can be further detected within the time range. When further detecting the arrival time of the reflected photon, a TDC module can be used. The TDC module generates a time code according to the arrival time of the reflected photon, and the processing module can generate a histogram according to the time code, and finally obtain the precise time of arrival of the reflected photon according to the histogram.

[0035] After obtaining the arrival time of the reflected photon, the distance of the object to be measured can be detected based on the arrival time of the photon. The distance D can be calculated by the following formula:

[0036] D=c·t / 2 (1)

[0037] Where c is the speed of light.

[0038] Figure 2 The module block diagram of the time-to-digital converter provided in the present application includes a statistical module 201 and a measurement module 202, which receives start and stop signals and outputs digital time. The statistical module 201 receives a first clock signal, and the measurement module 202 receives a second clock signal. The statistical module performs statistics on the echo signal reflected back by the measured signal based on the first clock signal to obtain the time interval where the echo signal is located. For example, the measurement distance of the detector can be divided into several parts, each distance corresponding to a time interval where the return pulse is located, and the several parts can be equidistantly distributed or unequally distributed, which is not specifically limited in the present application. For example, in a DTOF detector with a ranging range of 5 meters, the detection distance is divided into 5 parts, representing the echo signal reflected back to the detector from 1 meter, 2 meters, 3 meters, 4 meters, and 5 meters away, respectively. The 5 detection distances correspond to 5 time intervals based on the clock cycle of the first clock, for example, 1ps, 2ps, 3ps, 4ps and 5ps respectively. The statistical module 201 counts the echo signal received by the pixels in the SPAD array to a specific time in the 5 time intervals. For example, when the echo signal returns from 3 meters, the statistical module 201 counts the echo signal as a time interval of 3ps. Then the measurement module 202 starts from 3ps and calculates the specific time of the echo signal based on the second clock. For example, the measurement module 202 measures the echo signal as 0.8ps, and the digital time output by the TDC is 3.8ps. It can be seen that compared with the traditional TDC, the TDC provided in the present application does not need to measure from time 0, but directly starts measuring from the starting position of a certain time interval, which saves the power consumption and time of the TDC and reduces the amount of data to be transmitted.

[0039] For example, the specific circuit and timing diagram of the statistics module can be found in Figure 3 , consisting of two D flip-flop chains and a logic gate. The input of one of the D flip-flop chains is connected to the echo signal, and the output is connected to the counter, and the other D flip-flop chain is connected to the high level, and the output is connected to the logic gate. Exemplarily, the logic gate can be composed of an XOR gate.

[0040] According to this structure, the time interval of the input pulse signal in the statistical module can be counted. For example, if the pulse signal is a pulse signal reflected back from a distance of 3ps, the statistical module outputs a digital signal represented by 3ps.

[0041] Then, the measurement module starts to measure the time from 3 ps. Exemplarily, the measurement module may be formed by a vernier caliper TDC. It should be noted that the starting measurement position of the vernier caliper TDC is 3 ps.

[0042] Optionally, the statistics module can be composed of two submodules, such as Figure 4 As shown, the time-to-digital converter is composed of a first statistical module 401, a second statistical module 402, and a measurement module 403. The first statistical module 401 performs statistics based on the first clock, the second statistical module 402 performs statistics based on the third clock, and the measurement module 403 performs time measurement based on the second clock.

[0043] Exemplarily, the first statistical module can be pre-set to correspond to 5 time intervals based on the clock cycle of the first clock, for example, 1ps, 2ps, 3ps, 4ps and 5ps. And the second statistical module can be pre-set to correspond to 4 time intervals based on the clock cycle of the third clock, for example, 0.2ps, 0.4ps, 0.6ps, and 0.8ps. For example, when the echo signal returns from 3 meters, the first statistical module 401 counts the echo signal as a time interval of 3ps. The second statistical module 402 starts from 3ps and further counts the time interval of the echo signal. Exemplarily, the echo signal received by the transmission is counted as 0.6ps, and then the measurement module 403 starts to measure the specific time of the echo signal based on the second clock. For example, the measurement module 202 measures the echo signal as 0.08ps, and the digital time of the output of the TDC is 3.68ps. It can be seen that compared with the traditional TDC, the TDC provided in the present application does not need to start measuring from time 0, but directly starts measuring from the starting position of a certain time interval. This saves the power consumption and time of the TDC, reduces the amount of data that needs to be transmitted, and also improves the accuracy of time measurement.

[0044] In addition, the present application also provides a method for measuring flight time, such as Figure 5 As shown, the method specifically includes:

[0045] S1, dividing the distance to be measured into a plurality of first distances, wherein the first distances correspond to first time intervals one by one;

[0046] Exemplarily, in a DTOF detector with a ranging range of 5 meters, the detection distance is divided into 5 equal parts, representing the echo signal reflected back to the detector from 1 meter, 2 meters, 3 meters, 4 meters, and 5 meters away. The 5 detection distances correspond to 5 time intervals based on the clock period of the first clock, for example, 1ps, 2ps, 3ps, 4ps and 5ps respectively.

[0047] S2, counting a first time interval of the echo signal based on the first clock;

[0048] Exemplarily, counting the echo signal based on the first clock may mean that when the echo signal returns from 3 meters, the statistical module counts the echo signal as a time interval of 3ps. When the echo signal returns from 2.5 meters, the statistical module counts the echo signal as a time interval of 2ps. The clock period of the first clock may be 1ps.

[0049] S3, measuring a first moment in the first time interval where the echo signal is located based on a second clock;

[0050] Exemplarily, measuring the echo signal based on the second clock may refer to calculating the specific time of the echo signal based on the second clock starting from 3ps, for example, the echo signal measured based on the second clock may be 0.8ps. The clock period of the second clock may be 0.1ps.

[0051] S4, determining the flight time according to the first time interval and the first moment;

[0052] Exemplarily, based on the above three steps, according to the first time interval counted, for example, 3 ps, and the echo signal 0.8 ps measured by the second clock, it is determined that the digital time of this ranging is 3.8 ps.

[0053] The present application also provides another specific implementation for measuring flight time.

[0054] S1, dividing the distance to be measured into a plurality of first distances, where the first distances correspond to first time intervals one by one.

[0055] For example, in a DTOF detector with a ranging range of 5 meters, the detection distance is divided into 5 equal parts, representing the echo signal reflected back to the detector from 1 meter, 2 meters, 3 meters, 4 meters, and 5 meters away. The 5 detection distances correspond to 5 time intervals based on the clock period of the first clock, for example, 1ps, 2ps, 3ps, 4ps and 5ps respectively.

[0056] S2, counting the first time interval of the echo signal based on the first clock.

[0057] Exemplarily, counting the echo signal based on the first clock may mean that when the echo signal returns from 3 meters, the statistical module counts the echo signal as a time interval of 3ps. When the echo signal returns from 2.5 meters, the statistical module counts the echo signal as a time interval of 2ps. The clock period of the first clock may be 1ps.

[0058] S3: Divide each first distance into a plurality of first sub-distances, where the first sub-distances correspond to the first sub-time intervals in a one-to-one manner.

[0059] Exemplarily, 1ps, 2ps, 3ps, 4ps and 5ps can represent echo signals returned from distances of 1 meter, 2 meters, 3 meters, 4 meters and 5 meters. Further, each first distance can be divided into multiple sub-distances. For example, each distance can be divided into five equal parts, each representing a distance of 0.2. For example, the second sub-interval between 2 meters and 3 meters, 2.2 meters, 2.4 meters, 2.6 meters, 2.8 meters, and 3.0 meters can correspond to 2.2ps, 2.4ps, 2.6ps, 2.8ps and 3ps respectively.

[0060] S4, counting the first sub-time interval in the first time interval where the echo signal is located based on the third clock.

[0061] Exemplarily, based on the third clock statistics of the echo signal, when the echo signal returns from 2.58 meters, the flight time is counted as a time interval corresponding to 2 meters based on the statistical signal of the first clock, that is, 2ps, and then the flight time is counted as a time interval corresponding to 2.4 meters based on the statistical signal of the third clock, that is, 2.4ps.

[0062] S5: Measure a first moment in the first time interval where the echo signal is located based on a second clock.

[0063] Exemplarily, measuring the echo signal based on the second clock may refer to calculating the specific time of the echo signal based on the second clock starting from 2.4ps, for example, the echo signal measured based on the second clock may be 0.18ps. The clock period of the second clock may be 0.01ps.

[0064] S6, determining the flight time according to the first time interval and the first moment.

[0065] Exemplarily, based on the above five steps, according to the statistical first time interval, for example 2ps, and the sub-time interval of the first time interval, 2.4ps, and the echo signal 0.18ps measured by the second clock, it is determined that the digital time of this ranging is 2.58ps.

[0066] The above method is applied to the detection device provided in the aforementioned embodiment, and its implementation principle and technical effect are similar, which will not be repeated here.

[0067] It should be noted that, in this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0068] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application. It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings. The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A time-to-digital converter, characterized in that: include: A statistics module based on a first clock, used for counting a first time interval in which an input signal of the time-to-digital converter is located; and a measurement module based on a second clock, configured to measure a first moment of the input signal in the first time interval; The statistical module includes: a first D flip-flop chain, a second D flip-flop chain and a plurality of logic gates, wherein the logic gates are XOR gates; The input of each D flip-flop of the first D flip-flop chain is connected to the output of one of the XOR gates, the output of each chain of the first D flip-flops is connected to a counter, and the CP terminal is connected to an echo signal; The CP terminal of each D flip-flop in the second D flip-flop chain is connected to the clock signal, wherein the input of the first D flip-flop is connected to a high level, the input of each subsequent D flip-flop is connected to the output of the previous D flip-flop, and the input and output of each subsequent D flip-flop are connected to the input of the same XOR gate.

2. The time-to-digital converter according to claim 1, wherein: The statistics module includes: A first statistical module, for counting the first time interval of the input signal of the time-to-digital converter based on the first clock; a second statistical module, for counting a first sub-time interval in the first time interval where the input signal of the time-to-digital converter is located based on a third clock; The clock cycle of the third clock is smaller than the clock cycle of the first clock.

3. The time-to-digital converter according to claim 2, wherein: A clock cycle of the second clock is smaller than a clock cycle of the third clock.

4. The time-to-digital converter according to claim 3, wherein: The measurement module uses a vernier caliper type time-to-digital converter.

5. The time-to-digital converter according to claim 4, wherein: The number of time intervals of the first statistical module is smaller than the number of time intervals of the second statistical module.

6. A method for measuring flight time, characterized in that The method for measuring the flight time uses the time-to-digital converter according to any one of claims 1 to 5 to measure the flight time, and the method for measuring the flight time comprises: Dividing the distance to be measured into a plurality of first distances, wherein the first distances correspond to the first time intervals one by one; A first time interval in which the echo signal is located is counted based on the first clock; Measuring a first moment in the first time interval where the echo signal is located based on a second clock; The flight time is determined according to the first time interval and the first time.

7. The method for measuring flight time according to claim 6, characterized in that: The first time interval in which the statistical echo signal is located also includes: Dividing each first distance into a plurality of first sub-distances, wherein the first sub-distances correspond to the first sub-time intervals one by one; Counting a first sub-time interval in a first time interval where the echo signal is located based on a third clock; The clock period of the third clock is smaller than the clock period of the first clock.

8. The method for measuring flight time according to claim 7, characterized in that: A clock cycle of the second clock is smaller than a clock cycle of the third clock.

9. The method for measuring flight time according to claim 8, characterized in that: The number of the first time intervals is smaller than the number of the first sub-time intervals.

10. The method for measuring flight time according to claim 9, characterized in that: The method for measuring the echo signal is a vernier caliper method.

Images