Light detection device, light detection method and optical distance sensor

A light detection device detects incident light according to a detection start timing. The light detection device includes photosensors, a signal combining circuit, a detection circuit, a time measurement circuit, and a timing extraction circuit. The photosensors receive light to generate output signals indicating light reception results, respectively. The signal combining circuit sums output signals from the respective photosensors to generate a combined signal. The detection circuit detects a timing at which the combined signal reaches a first threshold or larger to generate a detection signal. The time measurement circuit measures a count period between the detection start timing and the detected timing based on the detection signal. The timing extraction circuit extracts timing information from a predetermined period defined by the detected timing as a reference, the timing information indicating a timing at which the combined signal increases.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. national stage of application No. PCT/JP2019/009228, filed on Mar. 8, 2019. Priority under 35 U.S.C. § 119(a) and 35 U.S.C. § 365(b) is claimed from Japanese Application No. 2018-048003, filed Mar. 15, 2020, the disclosure of which is also incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a light detection device, a light detection method, and an optical distance sensor including the light detection device.

BACKGROUND ART

There is known an optical distance sensor that uses time of flight (TOF) of light. The optical distance sensor irradiates an object with light and detects the light reflected by the object, thereby measuring a distance corresponding to the time of fight of light traveling back and forth to the object. For the optical distance sensor, a technique using a single photon avalanche photodiode (SPAD) for light detection is proposed (e.g., Patent Documents 1 and 2).

Patent Document 1 discloses a light detector including a plurality of SPADs in an optical distance measurement device. The light detector of Patent Document 1 sums rectangular pulses output from the plurality of SPADs, compares a summed output value with a predetermined reference value, and outputs a trigger signal according to a comparison result.

Patent Document 2 discloses a distance measurement device including a plurality of SPADs in a receiver unit. The distance measurement device of Patent Document 2 determines that a measurement pulse is detected when a summated signal indicating the sum of electrical pulses output from the plurality of SPAD crosses a predetermined threshold and a rising slope of the summated signal crosses a predetermined slope threshold.

CITATION LIST

Patent Document

Patent Document 1: JP 5644294 B

SUMMARY

Technical Problem

In the related art such as Patent Document 1, the detection target is one timing per each light detection by using the plurality of SPADs, with the aim of knowing the total number of photons simultaneously detected by the plurality of SPADs. Since the reaction of the SPAD to the photon is stochastic, it is desirable to obtain a plurality of timings when the plurality of SPADs are caused to receive light simultaneously, in order to accurately perform the light detection using statistical processing, for example.

An object of the present disclosure is to provide a light detection device, a light detection method, and an optical distance sensor capable of facilitating light detection accurately in an optical distance sensor.

Solution to Problem

A light detection device according to the present disclosure detects incident light according to a detection start timing. The light detection device includes a plurality of photosensors, a signal combining circuit, a detection circuit, a time measurement circuit, and a timing extraction circuit. The plurality of photosensors receive light to generate output signals indicating light reception results, respectively. The signal combining circuit sums a plurality of output signals from the respective photosensors to generate a combined signal. The detection circuit detects a timing at which the combined signal reaches a first threshold or larger to generate a detection signal indicating the detected timing. The time measurement circuit measures a count period between the detection start timing and the detected timing based on the detection signal. The timing extraction circuit extracts timing information from a predetermined period defined by the detected timing as a reference, the timing information indicating a timing at which the combined signal increases.

A light detection method according to the present disclosure provides a method by which a light detection device detects incident light according to a detection start timing.

An optical distance sensor according to the present disclosure includes a light projector that projects light, and a light detection device. The time measurement circuit in the light detection device measures the count period using a timing at which the light projector projects light as the detection start timing.

Advantageous Effect

With the light detection device, the light detection method, and the optical distance sensor according to the present disclosure, it is possible to facilitate the light detection accurately in the optical distance sensor.

DETAILED DESCRIPTION

Hereinafter, embodiments of a light detection device, a light detection method, and an optical distance sensor according to the present disclosure will be described with reference to the accompanying drawings. Note that, the same components are denoted by the same reference signs in each of the following embodiments.

Application Example

An example to which a light detection device according to the present disclosure can be applied will be described with reference toFIG.1.FIG.1is a view for describing an application example of a light detection device1according to the present disclosure.

The light detection device1according to the present disclosure is applied to a TOF-type optical distance sensor2. The optical distance sensor2includes a light projector20that projects pulsed light to the outside, for example, as illustrated inFIG.1. The light detection device1constitutes a receiver unit, which receives light from the outside, in the optical distance sensor2.

The optical distance sensor2according to the present disclosure can be applied to a photoelectric sensor for industrial automation applications, for example. The optical distance sensor2detects reflected light of the pulsed light projected from the light projector20using the light detection device1to measure a distance to an object that reflects the light based on time of flight of the light. The optical distance sensor2can detect whether the object is located at a specific position.

In the present application example, for improving the sensitivity of light detection or the like in the optical distance sensor2, an SPAD is used as a sensor element i.e. a photosensor in the light detection device1from the viewpoint of increasing the sensitivity of light detection in the optical distance sensor2. The SPAD is highly sensitive enough to respond to the incidence of one photon. However, the detection may have variations because the reaction is stochastic. In view of suppressing the detection variations of the SPAD in the present application example, a plurality of timings are acquired when the light detection device1performs one light detection using a plurality of SPADs simultaneously, so as to facilitate highly accurate light detection.

Configuration Example

Hereinafter, embodiments as configuration examples of the light detection device1and the optical distance sensor2will be described.

First Embodiment

In a first embodiment, the optical distance sensor2and the light detection device1that detects local timings near a timing at which the number of SPADs having received light reaches a detection threshold or more will be described.

The configurations of the optical distance sensor2and the light detection device1according to the first embodiment will be described hereinafter.

1-1. Configuration of Optical Distance Sensor

The configuration of the optical distance sensor2according to the present embodiment will be described with reference toFIG.2.FIG.2is a block diagram illustrating the configuration of the optical distance sensor2.

The optical distance sensor2includes the light projector20, a controller25, and the light detection device1, for example, as illustrated inFIG.2. The light projector20includes, for example, a light source21and a light source driver22.

In the light projector20, the light source21includes, for example, an LD (laser diode) or an LED. The light source21emits light such as infrared light. The light source driver22is a circuit that drives the light emission of the light source21. The light source driver22causes the light source21to emit light of a pulse shape, that is, pulsed light, at a timing controlled by the controller25. The pulsed light has a pulse width of, for example, several nanoseconds to several tens of nanoseconds.

The controller25includes, for example, a CPU, a RAM, a ROM, and the like, and controls each component. For example, the controller25generates various control signals so as to control the whole operation of the optical distance sensor2.

As illustrated inFIG.2, the light detection device1includes a SPAD array10, a signal processor11, and a distance measurer12, for example. The light detection device1includes, for example, an amplifier that amplifies an electric signal generated by the SPAD in response to incident light, a drive circuit for the SPAD, and the like, in the SPAD array10or the signal processor11.

The SPAD array10is configured by arranging a plurality of SPADs in an array form. Each SPAD of the SPAD array10is implemented by operating an avalanche photodiode (APD) in a Geiger mode.

The signal processor11performs signal processing to detect a timing at which light as a detection target of the light detection device1arrives, based on the output signal output from the SPAD array10. The distance measurer12calculates a distance value indicating a distance according to time of flight of light based on a signal processing result of the signal processor11. Details of the configuration of the light detection device1will be described hereinafter.

1-2. Configuration of Light Detection Device

A configuration example of the light detection device1according to the first embodiment will be described with reference toFIG.3andFIG.4.FIG.3is a block diagram illustrating the configuration of the light detection device1according to the present embodiment.

As illustrated inFIG.3, the light detection device1of the present embodiment includes a plurality of SPADs10ato10cthat constitute the SPAD array10, and a signal combining circuit13, a detection circuit14, and a timing extraction circuit3that constitute a signal processor11. In addition, the light detection device1includes, for example, a TDC (time/digital converter)4and a calculator5which constitute the distance measurer12.

The SPADs10ato10care examples of photosensors that stochastically respond to photons incident in the light detection device1. Hereinafter, an example in which the number of the SPADs10ato10cin the SPAD array10is three will be described.

Each of the SPADs10a,10b, and10creceives light and generates output signals Sa, Sb, and Sc, indicating a light reception result, respectively. For example, a waveform shaping circuit, which shapes signal waveforms of the output signals Sa to Sc of the SPADs10ato10cinto rectangular pulse shapes, is appropriately incorporated in the light detection device1. The respective output signals Sa to Sc from the SPAD array10are input to the signal combining circuit13of the signal processor11.

The signal combining circuit13sums up the plurality of output signals Sa to Sc thus input therein to generate a combined signal S1. The signal combining circuit13outputs the combined signal S1thus generated to the detection circuit14and the timing extraction circuit3. The signal combining circuit13can be configured by applying a known technique (for example, see Patent Document 1).

Based on the combined signal S1from the signal combining circuit13, the detection circuit14detects the timing of the light obtained as a detection target of the light detection device1, to generate a detection signal S2indicating a detection result. As illustrated inFIG.3, the detection circuit14includes a detection threshold setting circuit15and a determination circuit16, for example.

The detection threshold setting circuit15sets a predetermined detection threshold V1to the determination circuit16. The detection threshold V1indicates a reference to determine a timing of simultaneous detection in the plurality of SPADs10ato10. The detection threshold V1is an example of a first threshold in the present embodiment.

The determination circuit16includes, for example, a comparator and a logic circuit. The determination circuit16compares and determines the combined signal S1and the detection threshold V1, and generates the detection signal S2according to a determination result. The determination circuit16of the present embodiment generates the detection signal S2indicating a determination result on whether or not the combined signal S1reaches the detection threshold V1or larger. The detection signal S2is input to the TDC4and the timing extraction circuit3.

The timing extraction circuit3of the present embodiment includes one or more binarization threshold setting circuits31aand31b(generically referred to as a “binarization threshold setting circuit31”), one or more comparison circuits32aand32b(generically referred to as a “comparison circuit32”), and one or more holding circuits33aand33b(generically referred to as a “holding circuit33”). Hereinafter, an example in which the timing extraction circuit3includes two sets of the binarization threshold setting circuit31, the comparison circuit32, and the holding circuit33will be described.

As to the binarization threshold setting circuit31, the first binarization threshold setting circuit31asets, for example, a first binarization threshold V3asmaller than the detection threshold V1to the first comparison circuit32a. In addition, the second binarization threshold setting circuit31bsets a second binarization threshold V3bto the second comparison circuit32b, the second binarization threshold V3bbeing larger than the first binarization threshold V3aand smaller than the detection threshold V1, for example.

Each of the binarization thresholds V3aand V3bindicates a reference to determine a timing at which the reaction of the SPADs10ato10cchanges near the timing detected by the detection threshold V1, for example. Each of the binarization thresholds V3aand V3b(generically referred to as a “binarization threshold V3”) is an example of a second threshold in the present embodiment.

The combined signal S1from the signal combining circuit13is input to the comparison circuit32of the timing extraction circuit3. The first comparison circuit32acompares the combined signal S1with the first binarization threshold V3a, and generates, for example, a first binary signal S3aindicating whether or not the combined signal S1is equal to or larger than the first binarization threshold V3a. The first comparison circuit32aoutputs the generated first binary signal S3ato the first holding circuit33a.

Similarly to the first binary signal S3a, the second comparison circuit32bgenerates a second binary signal S3baccording to a result of the comparison of the combined signal S1with the second binarization threshold V3b, and outputs the second binary signal S3bto the second holding circuit33b. Hereinafter, the first and second binary signals S3aand S3bare generically referred to as a “binary signal S3”.

The detection signal S2from the detection circuit14is input to the holding circuit33of the timing extraction circuit3. The first and second holding circuits33a,33brespectively hold first and second timing information D3aand D3b(generically referred to as “timing information D3”) indicating signal states of the respective binary signals S3aand S3bfor a predetermined holding period with the detection signal S2as a reference. The holding period is, for example, several nanoseconds to several tens of nanoseconds. A configuration example of the holding circuit33is illustrated inFIG.4.

In the configuration example ofFIG.4, the holding circuit33includes a plurality of storage elements34and a plurality of delay elements35connected in series with each other. Each of the storage elements34is connected to one delay element35at an input terminal and holds a binary value of “0” or “1”. It is desirable that each of the delay elements35have a common delay period (for example, several tens of picoseconds to several hundreds of picoseconds).

In the holding circuit33in the example ofFIG.4, the binary signal S3from the comparison circuit32is input to a series circuit of the delay elements35. The detection signal S2from the detection circuit14is input to a control terminal of each of the storage elements34. Each of the storage elements34takes in and holds the binary signal S3output to the corresponding delay element35in accordance with the timing indicated by the detection signal S2. As a result, the holding circuit33can hold the timing information D3indicating a signal state of the binary signal S3for each delay period in the holding period terminated at the timing indicated by the detection signal S2.

Returning toFIG.3, a detection start timing signal S0is input to the TDC4from the controller25. The detection start timing signal S0is an example of a control signal indicating a timing at which the operation of the TDC4is started.

The TDC4is an example of a time measurement circuit that generates time information as a digital value (time/digital conversion) to measure time. The TDC4measures a count period that is from the timing indicated by the detection start timing signal S0to the timing indicated by the detection signal S2based on the detection start timing signal S0and the detection signal S2, and generates time information D1indicating the count period as a measurement result.

The calculator5includes, for example, a CPU that executes various arithmetic processes in cooperation with software, a RAM, a ROM, and the like. The calculator5operates as the distance measurer12together with the TDC4. The calculator5acquires the time information D1from the TDC4and also acquires the respective timing information D3aand D3bfrom the respective holding circuits33aand33bof the timing extraction circuit3. The calculator5executes arithmetic processing to calculate a distance according to the time of flight of light based on the acquired time information D1and timing information D3aand D3b. The arithmetic processing includes, for example, various types of statistical processing and the like.

Note that hardware resources such as the CPU constituting the calculator5may be shared by the controller25of the optical distance sensor2or may be separately provided. In addition, the calculator5, the controller25, and the like may be configured by various hardware circuits such as an ASIC and an FPGA.

The operations of the optical distance sensor2and the light detection device1configured as described above will be described hereinafter.

In the optical distance sensor2, the controller25(FIG.2) controls the light source driver22of the light projector20to cause the light source21to emit pulsed light at predetermined time intervals, for example. When the projected pulsed light is reflected by an object which is a distance measurement target of the optical distance sensor2, the projected pulsed light can be incident onto the optical distance sensor2as the reflected light.

At the time of controlling the light projector20, the controller25generates the detection start timing signal S0indicating the timing for light projection, and outputs the detection start timing signal S0to the TDC4(FIG.3) of the distance measurer12.

In synchronization with the light projection of the light projector20, the light detection device1in the optical distance sensor2performs light detection to detect the reflected light of the pulsed light during a predetermined light reception period from the timing at which light is projected. The light reception period is set to a period shorter than the time interval of the pulsed light, for example, and may be set in view of the time of flight of light corresponding to an upper limit of a distance to be measured (for example, the light reception period of 200 ns for the distance upper limit of 30 m).

In the light detection of the light detection device1, the SPAD array10receives light, and the signal processor11performs signal processing on a signal as a light reception result, thereby generating the detection signal S2indicating the timing when the reflected light arrives. Based on the detection signal S2, the distance measurer12measures the time of flight, taken until the projected pulsed light is reflected by the object and received by the TDC4, as a count period. The distance measurer12can calculate a distance value by multiplying, for example, half of the measured count period by the speed of light.

By using the SPADs10ato10cin the light detection device1in the above optical distance sensor2, it is possible to increase the sensitivity of light detection and improve the accuracy of the distance measurement. However, since the detection of photons by the SPADs10ato10cis stochastic, stochastic detection variations is conceivable in the situation where the plurality of SPADs10ato10cdetect photons simultaneously for one light projection.

To solve this, the light detection device1of the present embodiment acquires a plurality of timings for one detection start timing by comparison and determination using the detection threshold V1of the detection circuit14and each of the binarization thresholds V3aand V3bof the timing extraction circuit3, and performs the statistical processing in the distance measurer12. Hereinafter, details of the operation of the light detection device1of the present embodiment will be described.

2-1. Operation of Light Detection Device

Details of the operation of the light detection device1according to the present embodiment will be described with reference toFIG.5andFIG.6.

FIG.5AtoFIG.5Dare timing charts for describing a method of combining the combined signal S1in the light detection device1.FIG.6AtoFIG.6Fare timing charts illustrating the operation of the light detection device1.

In the light detection device1(FIG.3) of the present embodiment, the SPADs10ato10creceive light in the respective stochastic operations and generate the output signals Sa, Sb, and Sc, respectively. The signal waveforms of the output signals Sa, Sb, and Sc are illustrated inFIG.5A,FIG.5B, andFIG.5C, respectively.

In the example ofFIG.5AtoFIG.5C, each of the output signals Sa to Sc is a rectangular pulse P1having a predetermined pulse width. Each of the SPADs10ato10cstochastically reacts to incident photons, so that the rectangular pulse P1is generated in each of the output signals Sa to Sc.

In the example ofFIG.5AtoFIG.5C, the output signal Sa of the first SPAD10arises at time t1(FIG.5A), and the output signal Sb of the second SPAD10brises at time t3after time t1(FIG.5B). In addition, the output signal Sa of the third SPAD10arises at time t2between time t1and time t3(FIG.5C).

The signal combining circuit13sums up the output signals Sa to Sc from the SPADs10ato10cto generate the combined signal S1. The combined signal S1based on the output signals Sa to Sc inFIG.5AtoFIG.5Cis illustrated inFIG.5D.

The combined signal S1illustrated inFIG.5Dis the sum of the three output signals Sa to Sc (FIG.5AtoFIG.5C) at the same time. For example, the sum of the combined signal S1is one according to the output signal Sa ofFIG.5Afrom time t1to time t2.

In addition, the combined signal S1in the example ofFIG.5Dincreases from “1” to “2” at time t2by the sum of the two rectangular pulses P1(FIG.5AandFIG.5C). The combined signal S1further increases to “3” at time t3by the sum of the three rectangular pulses P1(FIG.5A toFIG.5C). In this manner, a signal level of the combined signal S1changes according to the number of the SPADs10ato10cwith light received.

The relationship between the combined signal S1combined as described above and the various thresholds V1, V3a, and V3bis illustrated inFIG.6A. Hereinafter, an example will be described in which the light projector20projects light at time t0and the light detection device1performs light detection during a light reception period T1from time t0as illustrated inFIG.6AtoFIG.6D. In this case, the detection start timing signal S0indicating time t0is input from the controller25to the TDC4.

The combined signal S1in the example ofFIG.6Aincreases after time t0, reaches the first binarization threshold V3aat time t11, reaches the second binarization threshold V3bat time t12, and reaches the detection threshold V1at time t13. In the light detection device1, the combined signal S1is input to the detection circuit14and the timing extraction circuit3.

The detection circuit14compares the combined signal S1and the detection threshold V1to generate the detection signal S2. At the same time, the comparison circuits32aand32bof the timing extraction circuit3compare the combined signal S1and the binarization thresholds V3aand V3b, respectively, and generate the respective binary signals S3aand S3b.FIG.6B,FIG.6C, andFIG.6Dillustrate the detection signal S2, the first binary signal S3a, and the second binary signal S3b, respectively, based on the combined signal S1of the example ofFIG.6A.

At time t11, the combined signal S1reaches the first binarization threshold V3a, so that the first binary signal S3aswitches from “0” to “1” as illustrated inFIG.6C. Similarly, at time t12, the second binary signal S3bswitches from “0” to “1” as illustrated inFIG.6D. The binary signals S3aand S3bare sequentially input to the holding circuits33aand33b, respectively.

At time t13after time t12, the combined signal S1reaches the detection threshold V1, so that the detection signal S2rises as illustrated inFIG.6B. Then, the TDC4performs measurement (time/digital conversion) of a period T2from time t0indicated by the detection start timing signal S0to time t13at which the detection signal S2rises as a count period. The TDC4holds the time information D1indicating the measured count period T2.

In addition, in response to the rising of the detection signal S2at time t13, the respective holding circuits33aand33bof the timing extraction circuit3hold the timing information D3aand D3bon the binary signals S3aand S3binput after time t10, which is earlier than time t13by a holding period T3. The timing information D3aand D3bholding the binary signals S3aand S3binFIG.6CandFIG.6Dare illustrated inFIG.6EandFIG.6F, respectively.

InFIG.6EandFIG.6F, a newer signal state is illustrated on the left side of the drawing in accordance with the arrangement order of the storage elements34ofFIG.4. InFIG.6E, “1” is recorded on the left side of the drawing in accordance with the binary signal S3aofFIG.6Cbeing “1” from time t11to time t13at the end. InFIG.6F, a section where “1” is recorded is shorter than that inFIG.6Ein accordance with the binary signal S3binFIG.6Dbeing “1” from time t12, which is after time t11.

According to the timing information D3a, D3bofFIG.6EandFIG.6F, it is possible to identify timings t11and t12, at which the combined signal S1reaches the respective binarization thresholds V3aand V3bor larger, relative to time t13at the end. Each piece of the timing information D3aand D3bis input to the calculator5.

The calculator5performs an operation for distance measurement based on the time information D1indicating the count period T2and the timing information D3aand D3b. For example, first, the calculator5converts the relative time information based on the timing information D3aand D3binto absolute time information with the detection start timing time t0as a reference. Specifically, the calculator5calculates periods T31and T32from time t0to times t11and t12, respectively, as illustrated inFIG.6CandFIG.6Dbased on, for example, the count period T2and the length of the section of “1” in the timing information D3aand D3b.

Further, the calculator5executes predetermined statistical processing based on the count period T2of the TDC4and the periods T31and T32calculated from the timing information D3. For example, the calculator5calculates an average value of the plurality of periods T2, T31, and T32for one-time projection and reception of light to calculate the time of flight of light or a corresponding distance value.

In addition, the calculator5may perform statistical processing using a histogram or the like by accumulating the time information D1and the timing information D3obtained by a plurality of times of projection and reception of light in a RAM or the like, and may calculate a distance value according to a peak position of the histogram, for example. It is possible to efficiently increase the number of histogram samples using the time information D1and the timing information D3.

According to the above operation of the light detection device1, when time t13at which simultaneous light reception is performed by the plurality of SPADs10ato10bis detected based on the combined signal S1obtained by combining the output signals Sa to Sc of the SPADs10ato10c, the timing information D31and D32indicating times t11and t12at which the combined signal S1has increased immediately before time t13can be further acquired. As a result, the plurality of timings t11to t13at which the plurality of SPADs10ato10creact in one-time projection and reception of light are obtained. Thus, it is possible to facilitate the light detection accurately in the optical distance sensor2.

As described above, the light detection device1according to the present embodiment detects incident light according to the predetermined detection start timing. The light detection device1includes the plurality of SPADs10ato10c, the signal combining circuit13, the detection circuit14, the TDC4, and the timing extraction circuit3. The plurality of SPADs10ato10creceive light and generate the respective output signals Sa to Sc indicating the light reception results. The signal combining circuit13sums up the plurality of output signals Sa to Sc from the SPADs10ato10cto generate the combined signal S1. The detection circuit14detects the timing at which the combined signal S1is equal to or larger than the detection threshold V1, which is an example of the first threshold, and generates the detection signal S2indicating the detected timing. The TDC4measures the count period T2that is a period between the detection start timing and the detected timing based on the detection signal S2. The timing extraction circuit3extracts the timing information D3aand D3bindicating whether or not the combined signal S1is equal to or larger than the binarization thresholds V3aand V3b, which are examples of the second threshold, in the predetermined holding period T3defined by the detected timing as a reference.

According to the light detection device1described above, it is possible to facilitate the light detection accurately in the optical distance sensor2by acquiring the timing information D3aand D3bin addition to the count period T2.

In the present embodiment, the photosensors of the light detection device1are the SPADs10ato10cthat stochastically react to the incident photons. Even if each of the SPADs10ato10coperates stochastically, it is possible to accurately perform the light detection using the timing information D3.

In the present embodiment, the light detection device1further includes the calculator5. The calculator5calculates the periods T31and T32from the detection start timing to the timing indicated by the timing information, based on the extracted timing information D3aand D3band the measured count period T2. The calculator5executes the statistical processing based on the calculated periods T31and T32and the count period T2. With the statistical processing, it is possible to implement highly accurate distance measurement.

In the present embodiment, the timing extraction circuit3includes the comparison circuit32and the holding circuit33. The comparison circuit32is provided for each binarization threshold V3and generates the binary signal S3indicating whether or not the combined signal S2is equal to or larger than the binarization threshold V3. The holding circuit33is provided for each comparison circuit32, and holds the timing information D3which is based on the binary signal S3obtained from the comparison circuit32for the holding period T3until the timing indicated by the detection signal S2. As a result, the timing information D3based on each of the binary signals S3can be acquired.

In addition, the optical distance sensor2according to the present embodiment includes the light projector20that projects light, and the light detection device1. The TDC4of the light detection device1measures the count period T2using the timing at which the light projector20projects light as the detection start timing. According to the optical distance sensor2of the present embodiment, it is possible to accurately perform the light detection in the light detection device1and to improve the accuracy of the distance measurement.

In addition, the light detection method according to the present embodiment is a method by which the light detection device1including the plurality of SPADs10ato10cdetects incident light according to a predetermined detection start timing. The present method includes: receiving light at the plurality of SPADs10ato10cto generate each of the output signals Sa to Sc indicating the light reception result; and summing up the plurality of output signals Sa to Sc to generate the combined signal S1. Further, the present method includes: detecting a timing at which the combined signal S1is equal to or larger than a predetermined first threshold to generate the detection signal S2indicating the detected timing; and measuring the count period T2between the detection start timing and the detected timing based on the detection signal S2. Further, the present method includes acquiring the timing information D3, which indicates the timing indicating whether the combined signal S1is equal to or larger than at least one second threshold in the holding period T3based on the detected timing. According to the present method, it is possible to facilitate the light detection accurately in the optical distance sensor2.

In the above description, the example in which the number of the SAPDs10ato10cincluded in the light detection device1is three has been described. The number of SAPDs10ato10cincluded in the light detection device1may be four or more, or may be two.

In addition, the example in which the timing extraction circuit3of the light detection device1includes two sets of the respective circuits31,32, and33has been described in the above description. The timing extraction circuit3of the light detection device1may include three or more sets of the respective circuits31to33, or may include one set of the respective circuits31to33. In addition, there may be a plurality of sets of the detection circuit and the timing extraction circuit.

Second Embodiment

In the first embodiment, the timing information D3indicates a timing in the holding period that is terminated at the timing indicated by the detection signal S2, but timing information acquired by a light detection device is not limited thereto. In a second embodiment, a configuration example of the above light detection device will be described with reference toFIG.7andFIG.8.

FIG.7is a block diagram illustrating a configuration of a light detection device1A according to the second embodiment. In a light detection device1A according to the present embodiment, a detection circuit14A further includes a delay circuit17in addition to a configuration similar to that of the light detection device1of the first embodiment (FIG.3) as illustrated inFIG.7.

In the detection circuit14A of the present embodiment, the delay circuit17delays the detection signal S2output from the determination circuit16by a predetermined delay period (for example, several hundred picoseconds to several nanoseconds) to generate a delayed detection signal S2A. In the present embodiment, instead of the detection signal S2from the determination circuit16, the delayed detection signal S2A from the delay circuit17is output to the TDC4and the holding circuits33aand33bof the timing extraction circuit3.

FIG.8AtoFIG.8Eare timing charts illustrating an operation of the light detection device1A according to the second embodiment.FIG.8Ais an example of a timing chart of the combined signal S1.FIG.8Billustrates the detection signal S2based on the combined signal S1ofFIG.8A.FIG.8Cillustrates the delayed detection signal S2A based on the detection signal S2ofFIG.8B.FIG.8DandFIG.8Erespectively illustrate first and second binarization signals S3aand S3bbased on the combined signal S1ofFIG.8A.

In the present configuration example, the first binarization threshold V3ais set to a value larger than the detection threshold V1as illustrated inFIG.8A. In other words, the detection threshold V1is set to a value smaller than one of the plurality of binarization thresholds V3aand V3b. For example, setting the detection threshold V1to be small may be advantageous from the viewpoint of enabling detection even when the number of photons during light reception is small in the light detection device1A.

The combined signal S1in the example ofFIG.8Aincreases after time t0, reaches the detection threshold V1at time t21, and then, reaches the first binarization threshold V3aat time t22. Therefore, the detection signal S2inFIG.8Brises at time t21before time t22, at which the binary signal S3ainFIG.8Drises. On the other hand, the delayed detection signal S2A, which is delayed by a delay period T4of the delay circuit17from time t21, rises at time t23after time t22as illustrated inFIG.8C.

According to the light detection device1A of the present embodiment, the respective holding circuits33aand33bof the timing extraction circuit3hold the timing information D3aand D3bfor the holding period T3terminated at time t23indicated by the delayed detection signal S2A. As a result, it is possible to acquire the timing information D3aindicating the timing at which the combined signal S1reaches the binarization threshold V3aor larger, after time t21at which the combined signal S1reaches the detection threshold V1or larger.

As described above, in the light detection device1A according to the present embodiment, the detection circuit14A delays the detection signal S2by the predetermined delay period to output the delayed signal to the holding circuit33as the delayed detection signal S2A. One of the binarization thresholds V3is larger than the detection threshold V1. As a result, it is possible to acquire the timing information D3indicating the timing at which the signal increases after the timing indicated by the detection signal S2. The number of the binarization thresholds V3larger than the detection threshold V1is not limited to one and may be plural. In addition, every binarization threshold V3may be larger than V1.

Third Embodiment

In the first embodiment, the configuration example has been described that the light detection device1is provided with the holding circuits33aand33bfor each binarization threshold V3to hold the plurality of pieces of timing information D3aand D3b. In the present embodiment, a light detection device that integrally holds timing information of the plurality of binarization thresholds V3aand V3bwill be described with reference toFIG.9toFIG.11.

FIG.9is a block diagram illustrating a configuration of a light detection device1B according to the third embodiment. The light detection device1B according to the present embodiment is obtained by changing a configuration of a timing extraction circuit3A as illustrated inFIG.9from a configuration similar to that of the light detection device1of the first embodiment (FIG.3). The timing extraction circuit3A of the present embodiment includes an integration circuit37, which is configured to integrate the plurality of binary signals S3aand S3b, and one holding circuit33, instead of the plurality of holding circuits33aand33bin the configuration similar to that of the first embodiment.

The integration circuit37generates an integrated signal S30that indicates a timing of each comparison result in an integrated manner based on the plurality of binary signals S3aand S3bfrom the comparison circuits32aand32b. A configuration example of the integration circuit37is illustrated inFIG.10.

In the configuration example ofFIG.10, the integration circuit37includes a plurality of logic gates71aand71band a plurality of delay elements72aand72b, provided for each of the comparison circuits32, and one OR gate70. It is desirable that each of the delay elements72aand72bhave a common delay period.

The first logic gate71acalculates a logical product between the first binary signal S3aand an inverted signal of a delayed result of the same binary signal S3aby the delay element72a, to generate a first logic signal S31a. Similarly, the second logic gate71bgenerates a second logic signal S31bbased on the second binary signal S3b. The logic signals S31aand S31bare input to the OR gate70. The OR gate70calculates a logical sum of the plurality of logic signals S31aand S31bto generate the integrated signal S30indicating a calculation result.

FIG.11AtoFIG.11Fare timing charts illustrating an operation of the light detection device1B according to the third embodiment.FIG.11Ais an example of a timing chart of the combined signal S1.FIG.11Billustrates the detection signal S2based on the combined signal S1ofFIG.11A.FIG.11CandFIG.11Drespectively illustrate first and second logic signals S31aand S31bbased on the combined signal S1ofFIG.11A.FIG.11Eillustrates the integrated signal S30based on the logic signals S31aand S31bofFIG.11CandFIG.11D.FIG.11Fillustrates the timing information D3based on the integrated signal S30ofFIG.11E.

According to the integration circuit37, the logic signals S31aand S31bform rectangular pulses as illustrated inFIG.11CandFIG.11D, in response to timings t11and t12at which the combined signal S1reaches the binarization thresholds V3aand V3bor larger with the binary signals S3aand S3brising. The rectangular pulse of each of the logic signals S31aand S31bhas a pulse width corresponding to the delay period of the delay elements72aand72b.

The integration circuit37obtains the logical sum of the logic signals S31aand S31bofFIG.11CandFIG.11Dbased on the binary signals S3aand S3bto generate the integrated signal S30. According to the integrated signal S30, each rectangular pulse of the logic signals S31aand S31bofFIG.11CandFIG.11Dis included as illustrated inFIG.11E.

As the integrated signal S30is held as the timing information D3, “1” is recorded in accordance with the timings t11and t12at which the logic signals S31aand S31brise as illustrated inFIG.11F. As a result, the information amount equivalent to the plurality of pieces of timing information D3aand D3bin the first embodiment can be obtained with one piece of the timing information D3. In addition, the circuit area corresponding to the holding circuit33can be reduced.

As described above, the timing extraction circuit3A includes the plurality of comparison circuits32, the integration circuit37, and the holding circuit33in the light detection device1B according to the present embodiment. The comparison circuit32is provided for each second threshold and generates the binary signal S3indicating whether or not the combined signal S1is equal to or larger than the second threshold. The integration circuit37generates the integrated signal S30that indicates the timings of the respective comparison results in an integrated manner based on the plurality of binary signals S3from the respective comparison circuits32. The holding circuit33holds the timing information D3in the holding period T3until the timing indicated by the detection signal S2based on the integrated signal S30. As a result, it is possible to acquire the timing information D3integrating the plurality of comparison results.

Other Embodiments

The example in which the detection threshold V1is a constant value has been described in each of the above embodiments. In the light detection device of the present embodiment, the detection threshold V1may be set based on the maximum value of the combined signal S1. A modification of the detection threshold setting circuit15when the detection threshold V1is set based on the maximum value of the combined signal S1will be described with reference toFIG.12.

As illustrated inFIG.12, the detection threshold setting circuit15of the present modification includes a comparator61and two multiplexers60and62, for example. The detection threshold setting circuit15holds the maximum value of the input combined signal S1as the detection threshold V1, and outputs the held detection threshold V1.

The combined signal S1from the signal combining circuit13is input to the comparator61and the multiplexer62in the detection threshold setting circuit15. The multiplexer60outputs an initial value signal Si indicating an initial value or the detection threshold V1to the comparator61and the multiplexer62.

The comparator61compares the combined signal S1with the signal output from the multiplexer60. The comparator61outputs a signal indicating a comparison result to a control terminal of the multiplexer62.

The multiplexer62switches a signal to be output to an input terminal of the multiplexer60between the combined signal S1and the initial value signal Si or the detection threshold V1output from the multiplexer60, according to the comparison result of the comparator61.

According to the detection threshold setting circuit15of the above configuration example, every time the combined signal S1updates the maximum value, the updated detection threshold V1can be generated. The detection circuit14in the light detection device1of the present modification compares and determines, for example, the detection threshold V1output from the detection threshold setting circuit15ofFIG.12with the combined signal S1delayed appropriately using the determination circuit16to generate the detection signal S2. The controller25may output a reset signal Sr, for example, immediately before light projection or the like to set the detection threshold V1to the initial value.

The configuration example in which the photosensors of the light detection devices1to1B are the SPADs10ato10chas been described in each of the above embodiments. In the present embodiment, the photosensor of the light detection device is not necessarily the SPAD.

In addition, the configuration examples of the light detection devices1to1B that extract the timing information D3by binarization using the binarization threshold V2has been described in each of the above embodiments. The light detection device of the present embodiment may acquire timing information on an increase of the combined signal S1in the holding period or the like without using the binarization threshold V2.

In addition, the application example of the optical distance sensor2for industrial automation applications has been illustrated in the above description. The application of the optical distance sensor2and the light detection devices1to1B according to the present disclosure is not limited thereto, and may be, for example, in-vehicle applications. The optical distance sensor2may be, for example, a LiDAR or a distance image sensor.

As described above, various embodiments of the present disclosure have been described, but the present disclosure is not limited to the above contents, and various modifications can be made within a range where the technical idea is substantially the same. Hereinafter, various aspects according to the present disclosure will be additionally described.

A first aspect according to the present disclosure is a light detection device (1) for detecting incident light according to a detection start timing. The light detection device1includes a plurality of photosensors (10ato10c), a signal combining circuit (13), a detection circuit (14), a time measurement circuit (4), and a timing extraction circuit (3). The plurality of photosensors receive light to generate output signals (Sa to Sc) indicating light reception results, respectively. The signal combining circuit sums a plurality of output signals from the respective photosensors to generate a combined signal (S1). The detection circuit detects a timing at which the combined signal reaches a first threshold or larger (V1) to generate a detection signal (S2) indicating the detected timing. The time measurement circuit measures a count period between the detection start timing and the detected timing based on the detection signal. The timing extraction circuit extracts timing information (D3) from a predetermined period (T3) defined by the detected timing as a reference, the timing information indicating a timing at which the combined signal increases.

As a second aspect, in the light detection device of the first aspect, the photosensor is a single photon avalanche photodiode (SPAD) configured by an avalanche photodiode operated in a Geiger mode.

As a third aspect, the light detection device according to the first or second aspect further includes a calculator (5). The calculator calculates a period from the detection start timing to the timing indicated by the timing information based on the extracted timing information and the measured count period.

As a fourth aspect, in the light detection device according to the third aspect, the calculator executes statistical processing based on the calculated period and the count period.

As a fifth aspect, in the light detection device according to any one of the first to fourth aspects, the timing extraction circuit includes at least one comparison circuit (32) and a holding circuit (33). The comparison circuit corresponds to a second threshold and generates a binary signal (S3) indicating whether or not the combined signal is the second threshold or larger. The holding circuit corresponds to each comparison circuit, and holds timing information which is based on the binary signal obtained from a corresponding comparison circuit for predetermined period until the timing indicated by the detection signal.

As a sixth aspect, in the light detection device according to any one of the first to fourth aspects, the timing extraction circuit (3A) includes a plurality of comparison circuits (32), an integration circuit (37), and a holding circuit (33). The comparison circuit corresponds each of second thresholds and generates a binary signal (S3) indicating whether or not the combined signal is a corresponding second threshold or larger. The integration circuit generates, based on a plurality of binary signals from the respective comparison circuits, an integrated signal (S30) that indicates timings of respective comparison results in an integrated manner. The holding circuit holds the timing information (D3) for a predetermined period until the timing indicated by the detection signal based on the integrated signal.

As a seventh aspect, in the light detection device of the fifth or sixth aspect, the detection circuit includes a delay circuit (17) that delays the detection signal by a predetermined delay period to output the delayed detection signal to the holding circuit. At least one of the second thresholds is larger than the first threshold.

As an eighth aspect, in the light detection device according to any one of the first to seventh aspects, the first threshold is set based on a maximum value of the combined signal.

A ninth aspect includes a light projector (20) that projects light, and the light detection device according to any one of the first to eighth aspects. The time measurement circuit in the light detection device measures the count period using a timing at which the light projector projects light as the detection start timing.

A tenth aspect is a light detection method by which a light detection device (1) including a plurality of photosensors (10ato10c) detects incident light according to a detection start timing. The present method includes: receiving light at the plurality of photosensors to generate each of output signals (Sa to Sc) indicating light reception results; and summing the plurality of output signals (Sa to Sc) from the respective photosensors to generate a combined signal (S1). The present method includes: detecting a timing at which the combined signal reaches a first threshold (V1) or larger to generate a detection signal (S2) indicating the detected timing; and measuring a count period (T2) between the detection start timing and the detected timing based on the detection signal. The present method includes acquiring timing information indicating a timing at which the combined signal increases in a predetermined period defined by the detected timing as a reference.

REFERENCE SIGNS LIST

1,1A,1B light detection device

14,14A detection circuit

2optical distance sensor

3,3A timing extraction circuit