Distance measuring device

Abstract

A distance measurement device is provided with: a projection light unit that projects a projection light pulse; a light receiving unit including a predetermined number of light receiving elements each receiving reflected light as the projection light pulse reflected by an object, and a signal processing unit; and a measurement unit that measures the distance on the basis of the time difference between the light projection timing of the projection light pulse and the light reception timing of the reflected light. The signal processing unit detects a falling timing at which a value of a response waveform generated on the basis of the number of light receiving elements that respond to the reflected light among the predetermined number of light receiving elements exceeds a threshold value and then becomes equal to or less than the threshold value. The measuring unit determines, as the light receiving timing, a timing that is earlier than the detected falling timing by the response time amount of the predetermined number of light receiving elements, the response time being a set value that is a non-sensing time of the predetermined number of light receiving elements or a time based on the set value.

Description

Technical Field

[0001] The present disclosure relates to a distance measuring device. Background Art

[0002] Conventionally, there are devices that irradiate light to a measurement target area and measure the distance from the device to the object using reflected light from an object in the measurement target area. For example, there are optical distance measuring devices that use the time from irradiation of light to detection of reflected light, that is, the time of flight (TOF) of light traveling back and forth between the device and the object.

[0003] As a technology related to such an optical distance measuring device, for example, an optical distance measuring device disclosed in the following patent document 1 is known. In this optical distance measuring device, a plurality of SiPMs (Silicon Photo Multipliers) are arranged side by side in the vertical direction. Each of the plurality of SiPMs is a TOF measurement photodetector and a noise measurement photodetector as a photodetector. In each of the plurality of SiPMs, a specified number of SPADs (Single Photon Avalanche Diodes) are arranged in an array. In the TOF measurement photodetector, when the number of SPADs in the High state is greater than a specified number, the time elapsed after the laser light emission moment is measured, and a histogram (i.e., a response waveform) about the measurement time is generated. When the peak value in the histogram is greater than a specified threshold value, the time corresponding to the peak position is output from the histogram circuit as TOF. At this time, with reference to the noise average-threshold conversion table created in advance, a boundary level corresponding to the noise average value output by the detection circuit of the noise measurement photodetector is set as the above-mentioned specified threshold value of the histogram circuit. As a result, regardless of the noise level, the false detection rate of the reflected light signal is maintained at a predetermined value.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2020-134224 Summary of the invention

[0007] However, when there are airborne particles in the measurement target area, erroneous measurements may occur in the distance measurement device. For example, in the device disclosed in Patent Document 1, when there are airborne particles such as fog in the measurement target area, reflected light as irradiation light reflected by the airborne particles may be received by the SPAD. When the peak value of the response waveform generated by the reflected light is above a predetermined threshold, the time corresponding to the peak position is output as TOF. As a result, the measured distance is not the distance between the object in the measurement target area and the device.

[0008] The present disclosure has been made to solve the above-mentioned technical problems, and an object thereof is to provide a distance measuring device capable of suppressing erroneous measurement of the distance from the distance measuring device to an object even when there are airborne particles in a measurement target area.

[0009] A distance measuring device according to an embodiment of the present disclosure includes: a light projecting unit that projects a projection light pulse to a measurement target area; a light receiving unit that includes a plurality of light receiving element groups and a signal processing unit, each of the plurality of light receiving element groups corresponding to each pixel and including a predetermined number of light receiving elements, each of the predetermined number of light receiving elements receiving reflected light as the projection light pulse reflected by an object in the measurement target area; and a measuring unit that performs distance measurement for each pixel based on a time difference between a light projection time of the projection light pulse and a light reception time of the reflected light, the signal processing unit detecting a falling time at which a value of a response waveform generated based on the number of light receiving elements among the predetermined number of light receiving elements that respond to the reflected light exceeds a threshold value and then becomes less than the threshold value, the measuring unit determining a time that is earlier than the detected falling time by a response time of the predetermined number of light receiving elements as the light reception time, the response time being a set value that is a non-sensing time of the predetermined number of light receiving elements or a time based on the set value.

[0010] When a projection light pulse is projected onto a measurement target area where there are airborne particles such as fog and an object, the projection light pulse is reflected by the airborne particles immediately after the light is projected, and the reflected light (hereinafter, also referred to as the first reflected light) is received by the receiving light element. Therefore, the response waveform rises immediately after the light is projected. Thereafter, the projection light pulse that has passed through the airborne particles is reflected by the object, and the reflected light (hereinafter, also referred to as the second reflected light) is received by the receiving light element. The projection light pulse does not pass through the object, so no reflected light occurs at a position farther than the object. Therefore, the response waveform rises immediately after the light is projected according to the reception of the first reflected light, and then falls in accordance with the reception of the second reflected light. That is, the rise of the response waveform is affected by the airborne particles, but the fall of the response waveform is less likely to be affected by the airborne particles. Here, in the response waveform generated based on the reception of reflected light reflected by an object in the absence of spatially suspended particles, the time difference between the rising moment when the value of the response waveform exceeds the threshold and the falling moment when the value of the response waveform becomes below the threshold after the rising moment is equivalent to the response time of the receiving light element (that is, a set value as the insensitivity time of a specified number of receiving light elements, or a time based on the set value). This is because each receiving light element responds almost simultaneously to the reflected light reflected by the object. Therefore, in one embodiment of the present disclosure, the moment estimated based on the falling moment and the response time is determined as the moment of receiving light. That is, the distance measurement is performed using the falling moment that is less likely to be affected by spatially suspended particles. Therefore, even when there are spatially suspended particles in the measurement object area, it is possible to suppress erroneous measurement of the distance from the distance measuring device to the object. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 : is a block diagram showing a schematic structure of a distance measurement device according to the first embodiment.

[0012] Figure 2 This is a diagram for explaining distance measurement using the TOF measurement method.

[0013] Figure 3 This is a diagram for explaining changes in the voltage applied to the SPAD using an active quenching circuit.

[0014] Figure 4A This is a diagram for explaining a response waveform generated according to the number of response elements when there are no particles suspended in space between the distance measurement device and the object.

[0015] Figure 4B This is a diagram for explaining a response waveform generated according to the number of response elements when there are fine particles floating in the space between the distance measurement device and the object.

[0016] Figure 5 is a flowchart illustrating the flow of processing of the distance measurement device according to the first embodiment.

[0017] Fig. 6A is used to explain the concentration ratio of the suspended particles in space in the modified example of the first embodiment. Figure 4B A diagram of the response waveforms that produce multiple falling moments under a low-medium environment.

[0018] Figure 6B is used to explain the concentration ratio of the suspended particles in space in the modified example of the first embodiment. Fig. 6A A diagram of the response waveforms that produce multiple falling moments under a low-medium environment.

[0019] Figure 7 This is a diagram for explaining changes in the voltage applied to the SPAD using the passive quenching circuit included in the distance measurement device according to the second embodiment.

[0020] Figure 8 This is a diagram for explaining a response waveform generated when no object exists in a measurement target region where airborne particles exist.

[0021] Fig. 9 is a flowchart illustrating the flow of processing of the distance measurement device according to the third embodiment.

[0022] Fig. 10A This is a diagram for explaining an example in which the distance measurement device according to the fourth embodiment linearly increases the supply current supplied to each projection light element for projection of a projection light pulse.

[0023] Fig. 10B This is a diagram for explaining an example in which the distance measurement device according to the modification of the fourth embodiment nonlinearly increases the supply current supplied to each projection light element for projection of a projection light pulse.

[0024] Fig.11 FIG. 1 is a hardware structure diagram related to processing of a distance measurement device according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0025] <First Embodiment>

[0026] Hereinafter, a distance measuring device according to a first embodiment of the present disclosure will be described with reference to the accompanying drawings. According to this embodiment, the distance measuring device 10 measures the distance from the distance measuring device 10 to an object in a measurement target area by a TOF measurement method. The distance measuring device 10 is a measuring device such as LiDAR (Light Detection And Ranging). Figure 1As shown in FIG. 1 , the distance measuring device 10 includes a control unit 11, a light projecting unit 20, a light receiving unit 30, and a measuring unit 40. The control unit 11 controls the operation of the distance measuring device 10. The light projecting unit 20 projects a projection light pulse to the measurement target area. The light receiving unit 30 receives reflected light, which is the projection light pulse reflected by the object in the measurement target area. The measuring unit 40 measures the distance from the distance measuring device 10 to the object in the measurement target area based on the received light pulse as the reflected light received by the light receiving unit 30.

[0027] like Figure 2 As shown, in the distance measuring device 10 using the TOF measurement method, the distance measurement is performed based on the time difference ΔT between the projection light timing of the projection light pulse and the reception light timing of the reception light pulse. The projection light timing is the time when the projection light unit 20 projects the projection light pulse. The reception light timing is the time when the reflected light of the projection light pulse reflected by the object 50 is received as the reception light pulse by the reception light unit 30. The distance measurement is performed in units of pixels in the reception light unit 30.

[0028] like Fig.11 As shown in the figure, as a hardware structure, the distance measuring device 10 includes a processor 1001 and a memory 1002. For example, the distance measuring device 10 may also include a microcomputer. The microcomputer may also include a CPU (Central Processing Unit), a system bus, an input / output interface, a ROM (Read Only Memory), a RAM (Random Access Memory), a non-volatile memory, etc. The memory 1002 stores in advance programs related to the control of the light projection unit 20 and the measuring unit 40 and programs related to the control process using the measurement results of the measuring unit 40. The functions of the control unit 11 and the measuring unit 40 may also be realized by the above hardware structure.

[0029] The projection light unit 20 includes a VCSEL (Vertical Cavity Surface Emitting Laser) array 21 and a light source control unit 22 for controlling each projection light element of the VCSEL array 21 according to a projection light instruction from the control unit 11. The VCSEL array 21 is a plurality of projection light elements arranged in an array. The plurality of projection light elements may also be a plurality of vertical cavity surface emitting lasers. The VCSEL array 21 is controlled by the light source control unit 22 and each of the plurality of projection light elements projects a rectangular projection light pulse at a predetermined time.

[0030] The light receiving unit 30 includes a SPAD array 31 and a signal processing unit 32 that processes an output signal from the SPAD array 31 and outputs it to the measuring unit 40. In the SPAD array 31, a plurality of SPADs are arranged in an array, similarly to the individual projection light elements of the VCSEL array 21. Each SPAD is a light receiving element having a high response characteristic and capable of reducing the pixel size. In the SPAD array 31, a predetermined number (for example, 18) of SPADs constitute a pixel as a group of SPADs (i.e., a group of light receiving elements). That is, a group of multiple SPADs is arranged in the SPAD array 31. Each SPAD receives, as a received light pulse, a reflected light of a projection light pulse projected from the projection light unit 20 and reflected by an object in the measurement object area.

[0031] In addition, the light receiving unit 30 includes an active quenching circuit for each SPAD in order to restore each SPAD to a state in which it can respond to a received light pulse (ie, reflected light) in a short time. Figure 3 As shown, the active quenching circuit applies a reverse bias voltage Va to each SPAD, and after a specified time from the interruption, even if light is being received, the applied voltage V of each SPAD, which has been reduced from the voltage Va in accordance with the interruption (i.e., sag) caused by light reception, is forcibly restored to the voltage Va. The main difference between the active quenching circuit and the passive quenching circuit to be described later is that in the active quenching circuit, a transistor or the like is used as an active element, instead of a quenching resistor as a passive element. The active quenching circuit does not restore the applied voltage to each SPAD until a certain delay time has passed since the occurrence of the interruption, and immediately after the delay time, the applied voltage is forcibly restored to the voltage Va quickly. In addition, a hybrid quenching circuit equipped with both a transistor and a quenching resistor can be used instead of the active quenching circuit. Regardless of the projection light pulse width (i.e., the time when the projection light pulse is projected), the active quenching circuit restores each SPAD to a state where it can respond within a fixed setting value ΔTd of the insensitivity time (i.e., Dead Time) of each SPAD. The set value ΔTd of the insensitivity time may also be pre-stored in the distance measuring device 10. The state in which the response is possible may be, for example, a state in which the applied voltage is the voltage Va. As such an active quenching circuit, for example, there is an active circuit. In addition, the reverse bias voltage Va may correspond to an example of a predetermined voltage value.

[0032] The signal processing unit 32 processes the signals outputted by each SPAD according to the reception of the received light pulse, and outputs the information obtained by the processing to the measuring unit 40. Specifically, the signal processing unit 32 generates a response waveform (i.e., a histogram) corresponding to the number of SPADs (hereinafter, also referred to as the number of response elements) that respond to the received light pulse among the specified number of SPADs constituting one pixel for each sampling time. For example, the value of the response waveform is the total value of the output values ​​(e.g., binary signals of 0 and 1) of each SPAD constituting one pixel. The signal processing unit 32 calculates the information related to the received light moment for TOF measurement (hereinafter, also referred to as received light moment information) based on the generated response waveform for each pixel, and outputs the calculated received light moment information to the measuring unit 40. Hereinafter, the above series of processing may also be referred to as received light moment information output processing.

[0033] In the light reception timing information output process in this embodiment, first, for each pixel, the applied voltage of the SPAD after the projection of the projected light pulse is generated. Figure 3 The response waveform corresponding to the number of SPADs (i.e., SPADs in response) below the voltage threshold Vth shown (i.e., the number of response elements). For example, when the applied voltage is below the voltage threshold Vth, the SPAD outputs 1 (i.e., a high signal), and when the applied voltage is higher than the voltage threshold Vth, the SPAD outputs 0 (i.e., a low signal). The sum of the output signals of the SPAD is the number of SPADs whose applied voltage is below the voltage threshold Vth. A response waveform can also be generated for each pixel at the moment when the applied voltage is below the voltage threshold Vth (i.e., the response moment of the SPAD). In the case where there are no floating particles in the space such as fog between the distance measuring device 10 and the object 50, for example, a response waveform as shown in 4A is generated. Then, for each pixel, the rising moment Ta1 at which the value of the response waveform exceeds the threshold (i.e., the determination threshold) Nth and the falling moment Tb1 at which the value of the response waveform becomes below the determination threshold Nth after the rising moment Ta1 are detected. The detected rising moment Ta1 and falling moment Tb1 are output to the measuring unit 40. In addition, when there are particles floating in the space between the distance measuring device 10 and the object 50, for example, Figure 4B The response waveform of the example is then detected for each pixel at the rising time Ta2 when the value of the response waveform exceeds the determination threshold Nth and the falling time Tb2 when the value of the response waveform becomes less than the determination threshold Nth after the rising time Ta2. The rising time Ta3 is estimated by subtracting the setting value ΔTd of the insensitivity time common to each SPAD from the detected falling time Tb2. The estimated rising time Ta3 and the detected falling time Tb2 are output to the measuring unit 40.

[0034] The reason for estimating the rising time Ta3 from the falling time Tb2 as described above is explained below. In an environment where airborne particles such as fog, pollen, PM2.5 (i.e., tiny particulate matter) exist in the measurement object area, reflected light is generated due to the reflection of the projected light pulse by the airborne particles around the distance measuring device 10 immediately after the projected light pulse is projected. This reflection also includes continuous scattering caused by the particles. When the reflected light is received by the light receiving unit 30, as shown in FIG. Figure 4B As shown in the example, the number of SPADs in the response increases rapidly and reaches the maximum value Nmax immediately after the light is projected. That is, the response waveform is saturated. This is because the projected light pulse is reflected by the floating particles in the space near the distance measuring device 10, so almost no attenuated reflected light is received immediately after the light is projected. In addition, Figure 4A as well as Figure 4B The horizontal axis is the time elapsed after the projection of the projected light pulse. Figure 4B In FIG. 1 , the dotted line shows the response waveform generated when the reflected light from the same object 50 is received in an environment where there are no floating particles in the measurement target area (i.e., equivalent to Figure 4A Response waveform of the example in ).

[0035] Here, it is assumed that the rising time when the value of the response waveform exceeds the judgment threshold value Nth is used as the reception time of the reflected light from the object 50 to measure the distance to the object 50. However, in the case of the presence of airborne particles, the rising time Ta2 mainly caused by the reflected light from the airborne particles is used for distance measurement. In other words, the rising time of the response waveform is affected by the airborne particles existing in the measurement object area. As a result, since the rising time Ta2 is earlier than the rising time Ta3 mainly caused by the reflected light from the object 50, the distance from the distance measuring device 10 to the object 50 is measured as shorter than the actual distance.

[0036] On the other hand, since reflected light does not occur at a point farther away than the object 50, the falling portion of the response waveform mainly becomes a waveform corresponding to the reflected light from the object 50. For example, Figure 4B As shown in the example, the response waveform rises and reaches the maximum value Nmax and then drops steeply. In other words, the falling portion of the response waveform decreases rapidly because no additional reflected light is generated, and further, the decreasing speed corresponds to the distance to the object 50. In this way, the rising moment of the response waveform is affected by the floating particles in the space, but the falling moment of the response waveform is hardly affected by the floating particles in the space.

[0037] Furthermore, in the response waveform generated by receiving the reflected light from the object 50 in the absence of the spatially suspended particles, the time difference between the rising time Tal exceeding the determination threshold Nth and the falling time Tb1 becoming less than the determination threshold Nth after the rising time Ta1 is equivalent to the set value ΔTd of the insensitivity time of the SPAD. In detail, since the size of a pixel is small, each SPAD constituting a pixel receives the reflected light and responds almost at the same time. Each SPAD that responds at the same time is estimated to be in a response state between the set value ΔTd of the insensitivity time. Further, the response waveform is generated according to the total value of the output value (i.e., the binary signal (0, 1)) of each SPAD element constituting a pixel. In other words, the response waveform is generated according to the number of SPADs in the response state of a pixel. Therefore, the response start and response end of each SPAD roughly correspond to the rise and fall of the response waveform. When the determination threshold Nth is set to the half value between the peak value of the response waveform and the noise level, it can be regarded that the half-value width (Tb1-Ta1) of the response waveform = the set value ΔTd of the insensitivity time. In addition, there are cases where the relationship is not strictly true depending on the value of the judgment threshold Nth of the response waveform, but the impact is not significant. In addition, the above relationship can also be modified according to the level of the judgment threshold Nth. For example, when the judgment threshold Nth is lower than the half value of the response waveform, Tb1-Ta1>ΔTd. In addition, when the judgment threshold Nth is higher than the half value of the response waveform, Tb-Ta1<ΔTd. Therefore, the above relationship can also be established by multiplying the prescribed coefficient corresponding to the level of the judgment threshold Nth by the set value ΔTd of the insensitivity time. Thus, for the judgment threshold Nth that is not the half value, the rising time of the response waveform can be accurately estimated. In addition, in the present embodiment, the median of the insensitivity time of each SPAD is used as the set value ΔTd of the insensitivity time of the SPAD.

[0038] Therefore, in an environment where airborne particles exist in the measurement target area, the rise time Ta3 is estimated by subtracting the set value ΔTd of the SPAD insensitivity time from the fall time Tb2 (see Figure 4B ) By using the estimated rising time Ta3 as the light receiving time for distance measurement, it is possible to perform distance measurement using the falling time Tb1 which is not affected by the particles floating in space.

[0039] The measuring unit 40 receives the received light timing information (i.e., the detected rising timing, the detected falling timing, and the estimated rising timing) calculated for each pixel as described above from the signal processing unit 32 of the light receiving unit 30. The measuring unit 40 receives information related to the timing of the projected light (hereinafter also referred to as the projected light timing information) instructed to the projecting light unit 20 from the control unit 11. The projected light timing information indicates the projected light timing of the projected light pulse. The measuring unit 40 performs a measurement process for measuring the distance from the distance measuring device 10 to the object in the measurement target area based on the received light timing information and the projected light timing information.

[0040] In the present embodiment, reference is made to Figure 5 The flowchart shown in FIG. 1 is used to explain the process of the distance measuring device 10 in detail. When the process of the distance measuring device 10 is started, the VCSEL array 21 of the light projection unit 20 is in a state capable of projecting light, and the SPAD array 31 of the light receiving unit 30 is in a state capable of receiving light. Then, according to the light projection instruction of the control unit 11, the projection light pulse is projected from the light projection unit 20 (step S101).

[0041] Next, the measuring unit 40 obtains the received light time information (step S103). For example, the signal processing unit 32 of the light receiving unit 30 performs the received light time information output processing as described above. The received light time information (i.e., the detected rising time, the detected falling time Tb1, and the estimated rising time) calculated for each pixel is obtained from the signal processing unit 32.

[0042] Next, the measuring unit 40 determines whether the reflected light from the object 50 is received as a received light pulse for each pixel (step S105). Specifically, the first pixel is first taken as the target pixel, and it is determined whether the received light pulse is received at the target pixel. When the value of the response waveform generated for the target pixel does not exceed the above-mentioned determination threshold value Nth, it is determined that the received light pulse is not received in the target pixel (No in step S105), and the process of step S107 is performed. In the process of step S107, the target pixel is changed to the next pixel, and the process of step S105 is performed for the changed target pixel.

[0043] When the value of the response waveform generated for the target pixel exceeds the above-mentioned determination threshold value Nth, it is determined that the received light pulse is received at the target pixel (Yes in step S105), and the process of step S109 is performed. In the process of step S109, it is determined whether the detected rising time is affected by the floating particles in space.

[0044] Specifically, the measuring unit 40 determines whether the time difference between the estimated rising time and the detected rising time is greater than a time threshold. Figure 4AIn the example, since the number of response elements does not increase rapidly immediately after the light projection, the time difference between the detected rising time Ta1 and the estimated rising time is less than the time threshold. That is, it is determined that the detected rising time Ta1 is not affected by the floating particles in space (No in step S109).

[0045] In this way, when it is determined that the detected rising time is not affected by the floating particles in space, the first distance calculation process is performed (step S111). Specifically, the distance calculation process is performed in which the detected rising time is used as the light receiving time. Figure 4A In the example of FIG. 1 , the distance from the distance measurement device 10 to the object for the target pixel is calculated based on the time difference ΔT between the detected rising time Ta1 and the projection light timing indicated by the projection light timing information obtained from the control unit 11 .

[0046] On the other hand, Figure 4B In the example of , as described above, in an environment where the floating particles in space exist in the measurement target area, the number of response elements increases rapidly immediately after the projection light, so the difference between the detected rise time Ta2 and the estimated rise time Ta3 is greater than the above time threshold. That is, the detected rise time Ta1 is determined to be affected by the floating particles in space (Yes in S109).

[0047] In this way, if it is determined that the light is affected by the floating particles in space, the second distance calculation process is performed (step S113). Specifically, the distance calculation process is performed using the estimated rising time (i.e., the time obtained by subtracting the set value ΔTd of the SPAD insensitivity time from the detected falling time Tb2) as the time of receiving light. Figure 4B In the example of FIG. 1 , the distance from the distance measurement device 10 to the object for the target pixel is calculated based on the time difference ΔT between the estimated rising time Ta3 and the projection light timing indicated by the projection light timing information obtained from the control unit 11 .

[0048] After calculating the distance to the object pixel in the above-mentioned step S111 or S113, if the processing of step S105 is not performed on all pixels (No in step S115), the processing after the above-mentioned step S107 is performed. When the processing of step S105 is performed on all pixels (Yes in S115), the measurement result is output (step S117). The measurement result is output to the host device or the control unit 11. The control unit 11 stores the measurement result in the storage unit. The processing ends after step S117. In addition, the processing is performed in units of one sampling (i.e., one projection light). Figure 5 Flowchart of the process.

[0049] As described above, in the distance measuring device 10 of the present embodiment, the signal processing unit 32 of the light receiving unit 30 detects, for each pixel, the falling time when the response waveform generated according to the number of SPADs (i.e., the number of response elements) that respond to the reflected light among the predetermined number of SPADs exceeds the judgment threshold Nth and becomes less than the judgment threshold Nth. Then, the measuring unit 40 performs distance measurement in the measurement target area according to the time difference ΔT between the light projection time of the light projection unit 20 and the light reception time obtained for each pixel in the light receiving unit 30. When the rise of the response waveform is affected by airborne particles such as fog, the measuring unit 40 determines the time obtained by subtracting the set value ΔTd of the SPAD insensitivity time from the falling time detected by the signal processing unit 32 as the above-mentioned light reception time.

[0050] In this way, when the distance measuring device 10 is affected by the floating particles in space, the setting value ΔTd of the SPAD's insensitivity time is subtracted from the falling time to estimate the rising time. By determining the estimated rising time as the light receiving time, the falling time that is less likely to be affected by the floating particles in space can be used for distance measurement. Therefore, even in an environment where there are floating particles in space, the distance from the distance measuring device 10 to the object can be accurately measured. In addition, the setting value ΔTd of the insensitivity time is an example of the response time of the light receiving element.

[0051] In addition, the signal processing unit 32 detects the rising time (i.e., the detected rising time) and the falling time when the value of the response waveform exceeds the determination threshold value Nth for each pixel. When the time difference between the estimated rising time (i.e., the time obtained by subtracting the setting value ΔTd of the SPAD insensitivity time from the falling time) and the detected rising time is greater than the time threshold (Yes in step S109), the measuring unit 40 determines the estimated rising time as the light receiving time. When the time difference between the estimated rising time and the detected rising time is less than the time threshold (No in step S109), the detected rising time is determined as the light receiving time.

[0052] In the case where the space floating particles do not exist in the measurement object area, the time difference between the estimated rise time and the detected rise time is small. On the other hand, in the case where the space floating particles exist in the measurement object area, the time difference between the estimated rise time and the detected rise time is large. Therefore, it is possible to determine whether the space floating particles exist based on the above-mentioned time difference. Furthermore, in the absence of space floating particles, the ranging using the detected rise time as the light receiving time is more accurate than the ranging using the estimated rise time as the light receiving time. This is because the detected value is more accurate than the estimated value. In addition, with respect to the projected light pulse and the received light pulse, the rise of the pulse is steeper than the fall. Therefore, compared with the rise time estimated based on the fall time, the detected rise time is accurate as the light receiving time. Therefore, the detected rise time can improve the ranging accuracy more than the estimated rise time.

[0053] Therefore, when the time difference is greater than the time threshold, the measuring unit 40 determines that there are floating particles in space, and determines the estimated rising time as the light receiving time. In addition, when the time difference is less than the time threshold, the measuring unit 40 determines that there are no floating particles in space, and determines the detected rising time as the light receiving time. Thus, by selecting the detected rising time or the estimated rising time as the light receiving time according to the presence or absence of floating particles in space, the accuracy of distance measurement can be improved.

[0054] In addition, as a variation of this embodiment, when two or more falling times are detected for one pixel in an environment where there are floating particles in space, the light receiving time can be determined based on the latest falling time. Fig. 6A In the environment, Figure 4B Compared with the environment, the concentration of suspended particles in space is low. Fig. 6A In the example shown, the number of response elements is below the determination threshold value Nth before the response waveform for the reflected light from the object 50 is generated. After that, the number of response elements again becomes below the determination threshold value Nth in the response waveform for the reflected light from the object 50. That is, there are two times Tb3 and Tb4 when the number of response elements becomes below the determination threshold value Nth. Figure 6B In the environment, Fig. 6A Compared with the environment, the concentration of suspended particles in space is even thinner. Figure 6B There are also two times Tb5 and Tb6 when the value becomes equal to or less than the determination threshold value Nth.

[0055] In this way, when more than two falling times are detected, the received light time is determined based on the latest falling time among the two or more falling times. This is because since the projected light pulse does not pass through the object 50, no reflected light is generated at a location farther than the object 50, and therefore the latest falling time corresponds to the reflected light from the object 50. Thus, even if more than two falling times are detected, the falling time corresponding to the reflected light from the object 50 can be used for distance measurement. As a result, erroneous measurement can be suppressed.

[0056] <Second Embodiment>

[0057] Next, a distance measuring device according to a second embodiment of the present disclosure will be described with reference to the accompanying drawings. In the second embodiment, the main difference from the first embodiment is that a passive quenching circuit is used instead of an active quenching circuit in the light receiving unit 30. For substantially the same structure as the first embodiment, the same reference numerals are given and their descriptions are omitted.

[0058] In the distance measuring device 10 according to the present embodiment, in the light receiving unit 30, as a circuit for restoring each SPAD to a state capable of responding, a passive quenching circuit is used instead of the above-mentioned active quenching circuit. The passive quenching circuit is a simpler circuit than the active quenching circuit. The passive quenching circuit applies a reverse bias voltage Va to each SPAD, and gradually restores the applied voltage of the SPAD, which is reduced from the voltage Va according to the interruption phenomenon caused by light reception, to the voltage Va by using the quenching function of the quenching resistor as a passive element. Therefore, immediately after the interruption phenomenon caused by light reception occurs, the applied voltage to the SPAD is gradually restored. However, when the intensity of the received light pulse is also strong during the recovery of the applied voltage, the probability of the interruption phenomenon occurring continuously is high. That is, during the period of the projection light pulse width ΔTv of the projection light pulse (i.e., the time when the projection light pulse is projected), the applied voltage to the SPAD is not restored. After the projection light pulse drops (i.e., the projection light pulse is no longer received), the applied voltage begins to be restored. Then, after the set value ΔTd of the induction time has elapsed from the start of recovery, the recovery of the applied voltage is completely completed.

[0059] That is, Figure 7 As shown, in the passive quenching circuit, the applied voltage is not restored during the period when the received light pulse is received. After the reception time of the received light pulse (i.e., the time when the projected light pulse is projected: the projected light pulse width ΔTv) has passed, the SPAD recovers to a state where it can respond during the fixed SPAD insensitivity time setting value ΔTd.

[0060] In other words, the longer the projection light pulse width ΔTv is, the more delayed the recovery start time of the applied voltage is. The time taken for recovery is the set value ΔTd of the insensitivity time, which is considered fixed. Therefore, the time until each SPAD recovers to a state where it can respond becomes longer. That is, the time when the output value of each SPAD changes from 1 to 0 is delayed. As a result, for the response waveform, the time difference between the estimated rising time and the detected falling time becomes longer according to the projection light pulse width ΔTv.

[0061] Therefore, in this embodiment, the signal processing unit 32 subtracts the set value ΔTd of the SPAD insensitivity time and the projection light pulse width ΔTv from the fall time to estimate the rise time. Thus, the rise time can be estimated by taking into account the delay in the recovery start time of the applied voltage caused by the projection light pulse width ΔTv in the passive quenching circuit.

[0062] Therefore, in the processing of the distance measurement device 10 of this embodiment, when the influence of the floating particles in the space is applied, the distance measurement is performed using the rise time estimated by taking into account the projection light pulse width ΔTv as described above. Figure 5 The processing of the distance measuring device 10 of the present embodiment is described. In addition, only the differences from the first embodiment are described. In step 103, the signal processing unit 32 subtracts the set value ΔTd of the SPAD insensitivity time and the projection light pulse width ΔTv from the detected falling time to estimate the rising time. The measuring unit 40 obtains the received light time information including the estimated rising time from the signal processing unit 32. In the second distance calculation processing of step S113, the measuring unit 40 calculates the distance from the distance measuring device 10 to the object 50 for the target pixel based on the time difference ΔT between the received light time as the estimated rising time and the projection light time obtained from the control unit 11.

[0063] Thus, in the distance measuring device 10 according to the present embodiment, the light receiving unit 30 has a passive quenching circuit that starts to restore the respective applied voltages of the predetermined number of SPADs according to the occurrence of the interruption phenomenon caused by light reception. By the passive quenching circuit, the applied voltage of the SPAD that has been reduced from the reverse bias voltage Va according to the interruption phenomenon caused by light reception stops decreasing in the received light, and is restored to the above voltage Va after the light is no longer received. The measuring unit 40 calculates the distance using the estimated rising time (i.e., the time obtained by subtracting the set value ΔTd of the SPAD insensitivity time and the projection light pulse width ΔTv from the detected falling time) obtained from the signal processing unit 32 as the above-mentioned received light time.

[0064] As described above, in the light receiving unit 30 having a passive quenching circuit, the applied voltage is not restored during the period of receiving the reflected light of the projected light pulse. In other words, the longer the projected light pulse width ΔTv is, the more delayed the recovery start time of the applied voltage is. Therefore, the time difference between the estimated rise time and the detected fall time also becomes longer corresponding to the projected light pulse width ΔTv. Therefore, the distance measuring device 10 subtracts the set value ΔTd of the SPAD's insensitivity time and the projected light pulse width ΔTv from the fall time to estimate the rise time. Thus, the rise time can be estimated by taking into account the delay in the recovery start time of the applied voltage caused by the projected light pulse width ΔTv in the passive quenching circuit. Therefore, even in the case of adopting a passive quenching circuit, the erroneous estimation of the rise time caused by the deviation in the recovery start time of the applied voltage can be suppressed. In addition, the sum of the set value ΔTd of the insensitivity time and the projected light pulse width ΔTv is an example of the response time of the receiving light element.

[0065] <Third Embodiment>

[0066] Next, a distance measurement device according to a third embodiment of the present disclosure will be described with reference to the accompanying drawings. In the third embodiment, the main difference from the first embodiment is that distance measurement is not performed for pixels whose slope of the response waveform between before and after the falling moment becomes less than a predetermined slope threshold. For substantially the same structure as the first embodiment, the same reference numerals are given and their descriptions are omitted.

[0067] In an environment where there are floating particles in space, if there is an object 50 in front of the projection of the projection light pulse, as described above Figure 4B As shown in the example, the falling part of the response waveform decreases rapidly with a slope corresponding to the distance from the distance measuring device 10 to the object 50. This is because the reflection of the projection light pulse ends on the object 50 through which the projection light pulse does not pass, and not only the projection light pulse and the reflected light decay due to the inverse square law of the distance, but also due to the passage of the floating particles in the space. After the reflected light from the object 50 is received, the SPAD does not respond again. Therefore, the time taken for the response waveform to drop from the peak value (for example, the maximum value Nmax) to the value before receiving the light (for example, the noise level) is approximately the set value ΔTd of the SPAD's insensitivity time. In other words, as long as the response waveform decreases from the drop from the maximum value Nmax until the set value ΔTd of the insensitivity time has passed, it can be determined that there is an object 50 in front of the projection of the projection light pulse.

[0068] On the other hand, when there is no object 50 in front of the projection of the projection light pulse, Figure 8As shown in the example, the response waveform gradually decreases from the maximum value Nmax. This is because the projected light pulse and the reflected light attenuate due to the inverse square law of the distance and due to the passage of the floating particles in space, while the reflected light is generated when the floating particles in space pass through.

[0069] That is, in an environment where there are suspended particles in space, the slope of the response waveform before and after the falling moment when there is no object 50 in front of the projection of the projection light pulse is smaller than the slope of the response waveform before and after the falling moment when there is an object 50. The slope of the response waveform may be, for example, the slope of a line segment connecting a point on the response waveform before the falling moment in an interval after a peak value (for example, maximum value Nmax) of the response waveform and a point on the response waveform after the falling moment in the interval.

[0070] Reference Fig. 9 , the processing of the distance measurement device 10 according to the present embodiment will be described. Figure 5 and Fig. 9 The difference is step S121. Fig. 9 In the above process, if it is determined that the detected rising moment is affected by the floating particles in space (Yes in step S109), the process of step S121 is performed. In step S121, it is determined whether the slope of the response waveform before and after the falling moment is below the slope threshold. For example, if Figure 8 Is the slope θ of the response waveform corresponding to the time from the maximum value Nmax before the falling time Tb7 to the end of the falling time after the falling time Tb7 (for example, the reduction of the noise level) less than the slope threshold corresponding to the set value ΔTd of the SPAD insensitivity time?

[0071] When the slope of the response waveform before and after the falling moment is less than the slope threshold (yes in step S121), it is estimated that there is no object 50 in front of the projection of the projection light pulse. Therefore, the measuring unit 40 does not perform distance measurement for the target pixel. Thus, it is possible to suppress the distance measurement for the non-existent object 50.

[0072] In addition, in the present embodiment, an example is described in which the slope of the response waveform before and after the falling moment is the slope of the response waveform corresponding to the time from the maximum value Nmax until the falling of the response waveform ends. However, for example, the slope of the response waveform before and after the falling moment may be the slope of the response waveform corresponding to the difference between the number of response elements before a predetermined time from the falling moment (i.e., the value of the response waveform) and the number of response elements after the predetermined time from the falling moment.

[0073] In this way, the measuring unit 40 does not perform distance measurement for pixels in which the slope of the response waveform in a section that includes the falling point and is located after the peak of the response waveform is less than or equal to the slope threshold.

[0074] In an environment where there are floating particles in space, when there is an object 50 in front of the projection of the projection light pulse, the response waveform drops steeply. On the other hand, when there is no object 50 in front of the projection of the projection light pulse, the response waveform drops gradually. Therefore, when the slope of the above-mentioned response waveform is below the slope threshold, the measuring unit 40 infers that there is no object 50 in front of the projection of the projection light pulse, and therefore does not perform distance measurement for the target pixel. Thus, it is possible to suppress the distance measurement for the non-existent object 50. In other words, it is possible to suppress erroneous measurements caused by floating particles in space.

[0075] <Fourth Embodiment>

[0076] Next, with reference to the accompanying drawings, a distance measuring device according to a fourth embodiment of the present disclosure is described. In the fourth embodiment, the main difference from the first embodiment is that the projection light pulse is projected in a manner that gradually increases the amount of light. For structures that are substantially the same as those in the first embodiment, the same reference numerals are given and their descriptions are omitted.

[0077] In an active quenching circuit that also recovers the applied voltage during light reception, each SPAD recovers to a state capable of responding to reflected light in a shorter time than a passive quenching circuit. Here, when the concentration of particles floating in space is high, the intensity of reflected light from the particles floating in space becomes high. In this case, there is a possibility that almost all SPADs will repeatedly produce an oscillation phenomenon in which the applied voltage drops from the voltage Va to below the voltage threshold Vth.

[0078] Therefore, in this embodiment, the light projection unit 20 projects light pulses with gradually increasing light amounts. Fig. 10A As shown in the example, when the projection light pulse emits light, the projection light unit 20 increases the supply current I supplied to each projection light element in a gentle straight line (in other words, linearly). Next, after a predetermined time from the start of light projection, the projection light unit 20 sharply reduces the supply current I. Thus, the projection light unit 20 can project a projection light pulse whose light amount gradually increases and then rapidly decreases.

[0079] In this way, by projecting a projection light pulse whose light amount gradually increases, the moment when the applied voltage is lower than the voltage threshold Vth can be dispersed in each SPAD, compared with the case of projecting a rectangular projection light pulse. When the light amount of the projection light pulse is small, the incidence of response is low, so only a part of the specified number of SPADs responds. When the light amount of the projection light pulse increases, the incidence of response increases, so the SPADs that have not responded also respond. That is, the response time of each SPAD is dispersed. When the SPAD with delayed response time recovers to a state where it can respond, the SPAD does not respond again when the light reception period of the projection light pulse ends. That is, no oscillation phenomenon occurs in the SPAD. In this way, the number of SPADs that produce the above-mentioned oscillation phenomenon can be reduced.

[0080] In the above description, an example is described in which the light projection unit 20 projects a projection light pulse in which the supply current I increases in a gentle linear manner and the light quantity gradually increases. Fig. 10B As shown in the example, the light projection unit 20 may project a projection light pulse in which the supply current I increases in a gentle curve shape (in other words, nonlinearly) and the light intensity gradually increases.

[0081] Thus, the light receiving unit 30 includes an active quenching circuit that restores the applied voltage to the voltage Va after a predetermined time from the occurrence of the interruption phenomenon due to light reception. The light projecting unit 20 increases the light intensity of the projected light pulse in accordance with the elapse of time when projecting the projected light pulse.

[0082] In an active quenching circuit, each SPAD recovers to a state capable of responding to reflected light in a shorter time than a passive quenching circuit. Here, when the concentration of airborne particles is high, stronger reflected light is received from airborne particles located at a close distance. In this case, almost all SPADs may simultaneously produce an oscillation phenomenon in which the response to an applied voltage lower than the voltage threshold Vth is repeatedly generated. In contrast, by increasing the amount of light of the projected light pulse in accordance with the passage of time, the response time of each SPAD is dispersed compared to the case where the amount of light of the projected light pulse is increased once (i.e., the case of projecting a rectangular-shaped projected light pulse). Therefore, as described above, the number of SPADs that produce oscillation phenomena can be reduced.

[0083] In addition, the present disclosure is not limited to the above-mentioned embodiments, and for example, the embodiments described below may also be used. (1) The projection light unit 20 of the distance measuring device 10 is a SPAD-LiDAR facing a short-range range (for example, a measurement range of about 20 m), and the measurement direction rotates at a predetermined angle for each sampling. However, the distance measuring device 10 may also be other types of distance measuring devices using a TOF measurement method.

[0084] (2) The distance measuring device 10 measures the distance from the distance measuring device 10 to an object in a measurement target area. The measurement target area may be a fixed area or a variable area. For example, the distance measuring device 10 is set on a moving body, and the measurement target area may also change according to the movement of the moving body.

[0085] (3) In the above-mentioned embodiments, an example in which the signal processing unit 32 estimates the rising time has been described. However, the rising time may be estimated by the measuring unit 40 .

Claims

1. A distance measuring device, comprising: a light projection unit that projects a light pulse toward a measurement target area; a light receiving unit including a plurality of light receiving element groups each corresponding to each pixel and including a predetermined number of light receiving elements each receiving reflected light as the projection light pulse reflected by an object within the measurement target area, and a signal processing unit; and a measuring unit, wherein the measuring unit performs distance measurement for each pixel based on a time difference between a light projection time of the projection light pulse and a light reception time of the reflected light, The signal processing unit detects, for each pixel, a falling point at which a value of a response waveform generated according to the number of light receiving elements among the predetermined number of light receiving elements that respond to the reflected light exceeds a threshold and then becomes less than the threshold. the measuring unit determines a time which is earlier than the detected falling time by the response time of the prescribed number of light receiving elements as the light receiving time, The response time is a set value that is a non-sensing time of the predetermined number of light receiving elements, or a time based on the set value.

2. The distance measuring device according to claim 1, wherein: The specified number of light receiving elements is a specified number of SPADs, i.e. single photon avalanche diodes. The light receiving unit includes a passive quenching circuit that starts to restore the applied voltages of the predetermined number of SPADs in response to the occurrence of an interruption phenomenon caused by light reception. The response time is the sum of the pulse width of the projection light pulse and a set value that is a non-sensing time of the predetermined number of SPADs.

3. The distance measuring device according to claim 1, wherein: When the measuring unit detects two or more of the falling time points for each pixel, the measuring unit determines the light reception time point based on the latest falling time point.

4. The distance measuring device according to claim 1, wherein: The signal processing unit further detects, for each pixel, a rising time when the value of the response waveform exceeds the threshold value, The measuring unit determines the moment that is earlier than the falling moment by the response time amount as the light receiving moment when the time difference between the moment that is earlier than the falling moment by the response time amount and the rising moment is greater than a prescribed time threshold, and determines the rising moment as the light receiving moment when the time difference between the moment that is earlier than the falling moment by the response time amount and the rising moment is less than the prescribed time threshold.

5. The distance measuring device according to claim 1, wherein: The measuring unit does not perform the distance measurement for a pixel in which the slope of the response waveform is equal to or smaller than a slope threshold value in a section that includes the falling time point and is located after a peak of the response waveform.

6. The distance measuring device according to claim 1, wherein: The specified number of light receiving elements is a specified number of SPADs, i.e. single photon avalanche diodes. The light receiving unit includes an active quenching circuit, which restores the applied voltage to a predetermined value after a predetermined time has passed since an interruption phenomenon caused by light reception occurs. The response time is a set value serving as a non-sensing time of the predetermined number of SPADs.

7. The distance measuring device according to claim 6, wherein: The projection light unit increases the light intensity of the projection light pulse in accordance with the passage of time during projection of the projection light pulse.

Images