Histogram generation circuit, optical distance measuring device, histogram generation method, and optical distance measuring method

Abstract

To provide a histogram generation circuit, an optical ranging device, a histogram generation method, and an optical ranging method that can reduce memory capacity by effectively utilizing a limited memory area.SOLUTION: A histogram generation circuit 60 for generating a histogram showing cumulative frequency of flight information distributed over a preset measurement range, based on each piece of the flight information which is flight time or flight distance from an output point of measurement light repeatedly output at a predetermined cycle to a detection point of reflected light for each measurement light, includes: a storage circuit 62 for storing the histogram; a storage processing circuit 61 which stores, in association with each other, a bin number for individually identifying each bin obtained by dividing the measurement range into a plurality of sections in sequence based on the flight information, and the flight information and the cumulative frequency included in each bin, in the storage circuit 62.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a histogram generation circuit, a light distance measuring device, a histogram generation method, and a light distance measuring method, and particularly relates to a histogram generation circuit, a light distance measuring device, a histogram generation method, and a light distance measuring method suitable for the case of using a photon counting type light receiving element.

Background Art

[0002] Patent Document 1 discloses a photodetector aimed at providing a technique capable of reducing the capacity of a memory for storing a histogram in a lidar that uses a histogram to calculate the flight time of light.

[0003] The photodetector has a plurality of light receiving parts configured to output a pulse signal in response to the incidence of photons, receives reflected light irradiated from an irradiation part and reflected by an object, and outputs in parallel the pulse signals respectively output from the plurality of light receiving parts. It includes a light receiving array part configured as such, a timing part configured to measure the elapsed time from the timing when the irradiation part irradiates light, a counting part configured to count, at every timing of a fixed period, the number of light receiving parts outputting the pulse signal among the plurality of light receiving parts as the response number, and output an adjusted response number obtained by subtracting or dividing a preset bias value from the response number, a memory whose address is associated with a timing value measured by the timing part, and a histogram generation part configured to generate a histogram by repeating, for a preset number of integration times, a process of integrating the adjusted response number as data of the address at an address in the memory specified from the timing value measured by the timing part.

[0004] According to the photodetector, when generating a histogram, since an adjusted response number that is a value smaller than the response number is integrated, the capacity of the memory can be reduced as compared with the case of integrating the response number as it is.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-169384 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, in the photodetector disclosed in Patent Document 1, the memory address used to generate the histogram is associated with the timing value measured by the timing unit. Specifically, an address space corresponding to the number of time bins set to cover the entire range of time that can be timed by the photodetector must be prepared, which leads to the problem that memory areas where no timing value exists are wasted.

[0007] Furthermore, because the total number of bins and the data bit width of each bin increase with the distance measurement resolution, there was a problem in that the longer the measurement distance and the higher the resolution of the distance measurement, the more unnecessarily the memory capacity increased.

[0008] The objective of the present invention is to provide a histogram generation circuit, an optical distance measuring device, a histogram generation method, and an optical distance measuring method that can reduce memory capacity by effectively utilizing a limited memory area. [Means for solving the problem]

[0009] To achieve the above objective, the first characteristic configuration of the histogram generation circuit according to the present invention is a histogram generation circuit that generates a histogram showing the cumulative frequency of flight information distributed in a preset measurement range based on each flight information, which is the flight time or flight distance from the time of output of the measurement light that is repeatedly output at a predetermined period to the time of detection of the reflected light for each measurement light, and comprises a storage circuit for storing the histogram, a bin number that identifies each bin, which is divided into a plurality of sections of the measurement range, in order based on the flight information, and the flight information and cumulative frequency contained in each bin, associating these with the storage circuit. The memory circuit comprises a reference memory circuit for locating already stored bin numbers when flight information is generated, and a storage memory circuit for storing bin numbers including the new bin number and the cumulative frequency. The memory processing circuit is configured to switch the functions of the reference memory circuit and the storage memory circuit each time flight information is generated, read the bin number stored in the reference memory circuit, sort the flight information in ascending or descending order, including the bin number of the bin to which the current flight time information belongs, and store the bin number, the flight information, and the cumulative frequency in the storage memory circuit. It lies in the fact that...

[0010] By associating the generated flight information with the bin number that individually identifies the bin to which the flight information belongs, based on the flight time or flight distance, and storing them in the memory processing circuit, it is not necessary to pre-allocate memory areas corresponding to bins to which no flight information is generated, thereby reducing the memory capacity. Furthermore, if multiple flight information entries are generated for the same bin number, the memory processing circuit associates the cumulative frequency with that bin number, thereby generating a histogram showing the cumulative frequency of the flight information distributed within a pre-set measurement range. It is preferable to use the flight time from the time of output of the measurement light to the time of detection of the reflected light for each measurement light as the flight information, but it is also possible to use the flight distance corresponding to that flight time.

[0011] The system includes two memory circuits: a reference memory circuit for accessing already stored bin numbers, and a storage memory circuit for storing bin numbers, including new bin numbers, and their cumulative frequencies. The memory processing circuit generates a histogram of the bin numbers and cumulative frequencies stored in the storage memory circuit. When new flight information is generated, the memory processing circuit activates the storage memory circuit as a reference memory circuit and compares the bin number read from the reference memory circuit with the bin number to which the new flight information belongs. The system then sorts the flight information so that each bin number is in ascending or descending order, and stores the bin number, flight information, and cumulative frequency in the storage memory circuit, which has been switched from the reference memory circuit. By separating the functions of the reference memory circuit and the storage memory circuit, reading from one memory circuit and writing to the other memory circuit can be performed simultaneously, increasing the processing speed compared to using a single memory circuit.

[0012] The second characteristic configuration is that, in addition to the first characteristic configuration described above, when flight information is generated, the memory processing circuit reads the bin number stored in the memory circuit, sorts the flight information in ascending or descending order, including the bin number of the bin to which the current flight information belongs, and stores the bin number, the flight information, and the cumulative frequency in the memory circuit.

[0013] The memory processing circuit finds the bin number to which the newly generated flight information belongs, reads each previously stored bin number from the memory circuit, and compares the read bin number with the bin number to which the new flight information belongs. Then, it rearranges each bin number in ascending or descending order and updates the memory circuit. At this time, if a bin number identical to the bin number to which the new flight information belongs exists, the memory processing circuit can update the cumulative frequency by adding 1 to the cumulative frequency of that bin number and then updating the memory circuit.

[0014] Same number three The characteristic configuration is as described above. oneIn addition to the characteristic configuration, the memory processing circuit stores the bin number, the flight information, and the cumulative frequency in the storage memory circuit without switching the functions of the reference memory circuit and the storage memory circuit when the flight information is first generated.

[0015] When the initial flight information is generated, there is no need to compare it with the bin number, so either memory circuit can be used as a storage memory circuit to store the bin number, flight information, and cumulative frequency (which will be "1" in this case).

[0016] Same number four The characteristic configuration is as described above, from the first to the second three In addition to any of the above characteristic configurations, the device further includes a FIFO-type memory circuit, the FIFO-type memory circuit comprising a write circuit that stores the flight information in synchronization with the generation of the flight information, and a read circuit that reads the flight information asynchronously with the generation of the flight information and outputs it to the memory processing circuit.

[0017] The memory processing circuit stores flight information in the FIFO-type memory circuit in sync with the generation of flight information, via a write circuit provided in the FIFO-type memory circuit. The memory processing circuit also reads the flight information stored in the FIFO-type memory circuit asynchronously with the generation of flight information, via a read circuit provided in the FIFO-type memory circuit, calculates the corresponding bin number based on the read flight information, and updates the histogram based on one of the first to fourth feature configurations described above. With this configuration, even if the interval between the generation of flight information is short, each piece of flight information can be buffered in the FIFO-type memory circuit, and time can be secured to update the histogram based on each piece of flight information read via the read circuit, thus preventing the inconvenience of data loss.

[0018] Same number Five The characteristic configuration is as described above. fourIn addition to the characteristic configuration, the writing circuit is configured to store dummy data indicating the end of the output of the flight information for one cycle in the FIFO-type storage circuit each time the output of the flight information ends at the predetermined cycle, and the reading circuit is configured to output a signal indicating the end of the output of the flight information for one cycle each time the dummy data is read.

[0019] When the generated flight information is sequentially written into the FIFO-type storage circuit via the writing circuit, there is a possibility that it may not be easily determined which cycle of the measurement light repeatedly output at a predetermined cycle the flight information read out via the reading circuit is. Even in such a case, if dummy data indicating the end of the output of the flight information for one cycle is stored in the storage circuit, the boundary of the flight information for one cycle can be easily determined.

[0020] A first characteristic configuration of the optical distance measuring apparatus according to the present invention is a light emitting element that outputs pulsed measurement light, a plurality of photon counting type light receiving elements that detect reflected light from an object with respect to the measurement light, an adding circuit that adds voltage pulses output from each light receiving element, a flight information calculation circuit that calculates, as flight information, the time from the output timing of the measurement light until the added value of the adding circuit reaches a predetermined addition threshold value or the distance corresponding to the time, and a histogram generation circuit that generates a histogram showing the cumulative frequency of the flight information distributed within a preset measurement range based on the flight information calculated by the flight information calculation circuit when the measurement light is repeatedly output at a predetermined cycle, from claim 1 to 5 any one of the cumulative A representative value calculation circuit that calculates a representative value of the flight information with respect to the object from the flight information distributed in the bin of the bin number at which the frequency reaches a predetermined histogram threshold value based on the histogram, and a distance calculation circuit that calculates the distance to the object based on the representative value of the flight information calculated by the representative value calculation circuit.

[0021] First to FiveBased on the histogram generated by a histogram generation circuit having any of the characteristic configurations, a representative value calculation circuit calculates a representative value of the flight time for an object, that is, highly probable flight information, in a region where the frequency reaches a predetermined histogram threshold value. Based on such flight information, a distance calculation circuit appropriately calculates the distance to the object.

[0022] The first characteristic configuration of the histogram generation method according to the present invention is a histogram generation method for generating a histogram showing the cumulative frequency of the flight information distributed in a preset measurement range based on each flight information which is the flight time or flight distance from the output time point of the measurement light repeatedly output at a predetermined period to the detection time point of the reflected light for each measurement light, the method comprising: a storage step of storing the histogram in a storage circuit; and a storage processing step of associating and storing in the storage circuit a bin number for individually identifying each bin obtained by dividing the measurement range into a plurality of sections in order based on the flight information, the flight information and the cumulative frequency included in each bin. picture in that respect.

[0023] The first characteristic configuration of the optical distance measurement method according to the present invention is a reflected light detection step of detecting the reflected light from an object with respect to the pulsed measurement light output from a light emitting element by a plurality of photon counting type light receiving elements, a flight information calculation step of calculating, as flight information, the time until the added value of the voltage pulses output from each light receiving element reaches a predetermined addition threshold value from the output time point of the measurement light or the distance corresponding to the time, and a histogram generation step of generating a histogram showing the cumulative frequency of the flight information distributed in a preset measurement range based on the flight information calculated in the flight information calculation step when the measurement light is repeatedly output at a predetermined period. 7 performing the histogram generation method described above; and based on the histogram, the cumulativeThe method comprises a representative value calculation step for calculating a representative value of the flight information for an object from the flight information distributed in the bins of the bin number where the frequency reaches a predetermined histogram threshold, and a distance calculation step for calculating the distance to the object based on the representative value of the flight information calculated in the representative value calculation step. [Effects of the Invention]

[0024] As described above, the present invention makes it possible to provide a histogram generation circuit, an optical distance measuring device, a histogram generation method, and an optical distance measuring method that can reduce memory capacity by effectively utilizing a limited memory area. [Brief explanation of the drawing]

[0025] [Figure 1] This is an explanatory diagram of the configuration of the optical distance measuring device according to the present invention. [Figure 2] This is an explanatory diagram of the light receiving circuit. [Figure 3] This is a diagram illustrating the histogram generation circuit. [Figure 4] This is an explanatory diagram of the operation of the histogram generation circuit and the representative value calculation circuit. [Figure 5] This is an explanatory diagram illustrating the operation of a histogram generation circuit and a representative value calculation circuit, showing other embodiments. [Figure 6] This is an explanatory diagram of the data writing and reading processes to the FIFO-type storage unit. [Figure 7] This is a diagram illustrating the histogram generation procedure when using a single memory circuit. [Figure 8] This is a diagram illustrating the histogram generation procedure when using two memory circuits. [Figure 9] This is a diagram illustrating the histogram generation procedure when using two memory circuits. [Modes for carrying out the invention]

[0026] The following describes the optical distance measuring device, optical distance measuring method, optical distance measuring device, histogram generation circuit, and histogram generation method used therein according to the present invention. As shown in Figure 1, the optical distance measuring device 200 is a TOF (Time-of-Flight) type optical distance measuring device housed in a casing C with a light-transmitting window and comprising: a light-emitting element 2 that outputs pulsed measurement light; a plurality of photon-count type photodetectors 3 that detect reflected light from an object in response to the measurement light; an optical scanning device 10 that scans the measurement light output from the light-emitting element 2 toward the measurement space and guides the reflected light diffusely reflected from the surface of an object present in the measurement space to the photodetectors 3; and a control circuit 100 that operates the optical scanning device 10 while driving the light-emitting element 2 and processes the signal for the reflected light detected by the photodetectors 3 to calculate the distance to the object.

[0027] The optical scanning device 10 includes a deflection mirror 11 that deflects the measurement light output from the light-emitting element 2 toward the measurement space and guides the reflected light from an object to the light-receiving element 3, a motor 13 that rotates the deflection mirror 11 around the rotation axis P, and an encoder 14 that detects the rotation speed and rotation position of the motor 13.

[0028] The encoder 14 consists of a disc 14A that rotates around a rotation axis P and has slits formed on its outer circumference at predetermined intervals, and a transmissive photointerrupter 14B that detects light passing through the slits formed in the disc 14A.

[0029] The deflection mirror 11 is fixed in a position tilted at 45 degrees with respect to the rotation axis P, and the light-receiving element 3, focusing lens 12, light-emitting element 2, and light-emitting lens 15 are respectively arranged on the axis of the rotation axis P. The pulsed measurement light output from the light-emitting element 2 passes through the light-emitting lens 15, is shaped into parallel light, propagates along the light guide 16, is deflected at a right angle by the deflection mirror 11, and is deflected and scanned as the deflection mirror 11 rotates before being output to the monitoring area.

[0030] Reflected light from an object propagates through the space surrounding the optical guide 16 and enters the deflection mirror 11. After being deflected in the axial direction of the rotation axis P, it passes through the focusing lens 12 and enters the photodetector 3. The optical scanning device 10 described above is merely an example, and is not limited to this configuration, as long as it is an optical scanning device capable of scanning or deflecting the measurement light output from the light-emitting element 2 in a predetermined direction and guiding the reflected light to the photodetector 3. For example, a configuration with a polygon mirror that rotates at a constant speed, a deflection mirror that oscillates with a driving force such as a piezoelectric element, a configuration in which the entire optical system is rotationally driven, or a configuration in which the entire optical system including the light-emitting element 2 and the photodetector 3 is rotationally driven can be employed.

[0031] A laser diode that outputs a laser in the near-infrared region is used as the light-emitting element 2, and a silicon photomultiplier (hereinafter referred to as "SiPM," which is an abbreviation for Silicon Photo Multipliers) is used, which consists of multiple single-photon avalanche diodes (hereinafter referred to as "SPAD," SPAD is an abbreviation for Single Photon Avalanche Diode) arranged in a matrix, as the photodetector element 3.

[0032] When a photon is incident on an avalanche photodiode (hereinafter referred to as "APD," ​​APD being an abbreviation for Avalanche Photo Diode), electron-hole pairs are generated. These electrons and holes are then accelerated by a high electric field, causing a chain reaction of collisions and ionization, which generates new electron-hole pairs.

[0033] APDs (Advanced Photon Detectors) have two operating modes: linear mode, which operates with a reverse bias voltage below the breakdown voltage, and Geiger mode, which operates with a reverse bias voltage above the breakdown voltage. In linear mode, the amplification factor can be variably controlled by applying and controlling a reverse bias voltage below the breakdown voltage. The output current is approximately proportional to the amount of incident light, and the amplification factor, i.e., the sensitivity, can be changed by the reverse bias voltage value, making it useful for measuring the amount of incident light. In Geiger mode, applying a reverse bias voltage above the breakdown voltage causes the avalanche phenomenon to occur with the incidence of a single photon.

[0034] In SPAD, the avalanche can be stopped non-destructively by lowering the applied voltage to the APD to the breakdown voltage. This process of stopping the avalanche phenomenon by lowering the applied voltage is called quenching. The simplest quenching circuit is achieved by connecting a quenching resistor in series with the APD. When an avalanche current is generated, the voltage across the terminals of the quenching resistor rises, causing the bias voltage of the APD to drop, and when it falls below the breakdown voltage, the avalanche current stops. Subsequently, when the applied voltage to the APD exceeds the breakdown voltage again, it becomes photodetectable, but there is a dead period during which the SPAD does not react.

[0035] When using a photon-count type photodetector, such as a SPAD, which outputs a voltage pulse in response to photon incidence, the influence of ambient light can be eliminated by repeatedly measuring the arrival time of the voltage pulse, creating a histogram, and extracting its maximum value.

[0036] The control circuit 100 includes a motor drive circuit 20, a light emission control circuit 30, a light receiving circuit 40, a time-of-flight measurement circuit 50, a histogram generation circuit 60, a representative value calculation circuit 70, and a distance calculation circuit 80.

[0037] The motor drive circuit 20 drives the motor 13 so that the deflection mirror 11 rotates at a predetermined speed based on the pulse signal output from the encoder 14. A brushless DC motor or a stepping motor is preferably used as the motor 13.

[0038] The light emission control circuit 30 controls the light-emitting element 2 to output pulsed measurement light at a predetermined period based on the pulse signal output from the encoder 14. For example, if the rotation speed of the deflection mirror 11 (motor 13) is 1200 rpm (50 msec per rotation) and the resolution of the scanning angle of the measurement light, i.e., the unit scanning angle, is 0.25°, then by outputting pulsed measurement light at 28.8 kHz, the measurement light will be output in units of 0.25°. Note that the above values ​​are merely examples, and the present invention is not limited to these values. The same applies to the values ​​exemplified below.

[0039] In practice, as will be described later, within a unit scanning angle of 0.25°, pulses of light with a pulse width of 1 nsec. are output 16 times at intervals of approximately 2 μsec. Based on the output from each photodetector 3 that detects the reflected light for each pulse, the time-of-flight measurement circuit 50 calculates the time of flight, that is, the time from when the measurement light is output until it is reflected by an object and returns, and the histogram generation circuit 60 generates a histogram.

[0040] As shown in Figures 1 and 2, the light-receiving circuit 40 includes a light-receiving element (SiPM) 3 and a comparison circuit 44 that outputs a pulse when the output of the light-receiving element (SiPM) reaches a predetermined summing threshold. The light-receiving element (SiPM) 3 includes multiple SPADs 3B, each consisting of a Geiger-mode avalanche photodiode 3A and a quenching resistor 41, and multiple pulse shaping circuits 42, which are differentiating circuits including a capacitor that shapes the output into a short pulse wave. Furthermore, it includes an adder circuit 43 that adds the short pulse waves output from each pulse shaping circuit 42. Generally, the number of SPADs constituting the SiPM is approximately 130 to 6000 elements per pixel, and the light-receiving element (SiPM) 3 generates a SiPM output with an output waveform corresponding to the number of SPADs that detected photons.

[0041] The time-to-flight measurement circuit 50 is composed of a TDC (Time-to-Digital Converter) that calculates the time from the rising edge of the light emission control signal output from the light emission control circuit 30 to the rising edge of the output signal of the comparison circuit 44 (specifically, from the time of output of the measurement light to the time when the sum value (SiPM output) of the summing circuit 43 reaches a predetermined summing threshold), that is, the time from the time of output of the measurement light to the time of detection of the reflected light, as the time-to-flight.

[0042] Figure 3 shows an example of the histogram generation circuit 60. The histogram generation circuit 60 generates a histogram showing the frequency at which the time of flight calculated by the time of flight measurement circuit 50 is distributed in each region on a time axis divided into multiple regions at predetermined time intervals, when the measurement light is repeatedly output at a predetermined period.

[0043] As will be described in detail later, the histogram generation circuit 60 includes a memory circuit 62 for storing histograms, a memory processing circuit 61 that generates histograms based on the output from the time-of-flight measurement circuit 50 and stores them in the memory circuit 62, and a FIFO-type memory circuit 63 that relays the time of flight between the time-of-flight measurement circuit 50 and the memory processing circuit 61.

[0044] The representative value calculation circuit 70 calculates a representative value of the flight time for an object based on the histogram, from the time at which the frequency is distributed in a region that reaches a predetermined histogram threshold. The distance calculation circuit 80 calculates the distance to the object based on the representative value of the flight time calculated by the representative value calculation circuit 70.

[0045] The aforementioned time-of-flight measurement circuit 50, histogram generation circuit 60, representative value calculation circuit 70, and distance calculation circuit 80 can be integrated using, for example, an FPGA (Field Programmable Gate Array).

[0046] The reflected light from the pulsed measurement light output from the light-emitting element 2 toward the object is detected by multiple SPADs provided in the SiPM, which is a photodetector 3. Voltage pulses output from each SPAD and waveform-shaped by the pulse shaping circuit 42 are added together by the summing circuit 43. The flight time from the time the measurement light is output until the summed value from the summing circuit 43 reaches a predetermined summing threshold is calculated by the flight time measurement circuit 50, and flight information is generated.

[0047] As shown in Figure 4, based on the time-of-flight measurement circuit 50's calculation of each flight time (in Figure 4, denoted as "TDC count value Tm,n, where m is the emission number and n is the edge detection number") for measurement light that is repeatedly output at a predetermined period (approximately 500 kHz in this embodiment) during a unit scanning angle (0.25°), the histogram generation circuit 60 generates a histogram showing the frequency (maximum value of 16) of the flight time corresponding to each region on the time axis, which is divided into multiple regions (in Figure 4, each region is denoted as a "bin" in Figure 4) at predetermined time intervals (0.5 nsec.). A bin is defined as a division of the flight time corresponding to the shortest to longest distance in the measurable range of the optical distance measuring device 200, or a division of the distance from the shortest to the longest distance, which is divided at predetermined distance intervals.

[0048] Specifically, the time of flight is defined as the period from the time of output of the measurement light to the time when the output value of the summing circuit 43 reaches a predetermined summing threshold. The TDC edge histogram, or frequency Hn, is obtained by adding the number of times the time of flight exists for each bin, and the TDCSUM histogram, or total time of flight Sn, is obtained by adding the time of flight existing for each bin.

[0049] Then, a representative value (Sn / Hn) of the flight time for an object is calculated by the representative value calculation circuit 70 from the flight times distributed in the bins where the frequency Hn reaches a predetermined histogram threshold. Furthermore, based on the representative value of flight time calculated by the representative value calculation circuit 70, the distance calculation circuit 80 calculates the distance to the object.

[0050] The representative value calculation circuit 70 calculates a representative value of the flight time for an object by dividing the total flight time Sn, which is the sum of all flight times distributed in the region where the frequency Hn is a predetermined histogram threshold, by the value of the frequency Hn. In the example in Figure 4, Sn = T 1,1 +T 2,1 +T 3,1 +···+T *,* The value obtained by dividing this value by the frequency Hn is the representative value.

[0051] In the example in Figure 4, representative values ​​are calculated only for bins where the frequency reaches a predetermined histogram threshold. However, the representative value calculation circuit 70 may calculate a representative value of the flight time for an object by dividing the sum of the total flight times distributed in bins where the frequency reaches a predetermined histogram threshold and in bins adjacent to that region by the sum of the frequencies of each corresponding bin, based on the histogram.

[0052] It is conceivable that histogram frequencies exist across different regions on a time axis divided into multiple bins. In such cases, a more accurate representative value can be obtained by calculating a representative value of the flight time for an object by dividing the sum of the flight times distributed in the bins where the frequency reaches a predetermined histogram threshold and the bins adjacent to that bin by the sum of the frequencies in the corresponding bins. In this case, the bins adjacent to the bin where the frequency reaches a predetermined histogram threshold may refer to the region with the larger frequency among the bins adjacent to either the left or right of the bin with the maximum frequency, or it may refer to the regions adjacent to both.

[0053] Furthermore, even if the frequency in a single bin does not reach a predetermined histogram threshold, if the sum of the frequencies distributed across multiple bins reaches the histogram threshold, the sum of the flight times distributed across multiple bins may be divided by the sum of the frequencies in the corresponding bins to calculate a representative value of the flight time for the object. As a result, a more accurate representative value can be obtained. A more accurate representative value can be obtained by reducing the effect of chattering, where frequencies are dispersed across adjacent bins in the histogram.

[0054] Furthermore, as shown in Figure 5, the time-of-flight measurement circuit 50 may be configured to calculate the time of flight as the time from the time of output of the measurement light to each point in time when the output value of the summing circuit 43 reaches a predetermined summing threshold (first summing threshold), and the time until the summing threshold higher than the predetermined summing threshold (second summing threshold) reaches that point.

[0055] The histogram generation circuit 60 is configured to generate a histogram showing the cumulative frequency of each flight time calculated by the time-of-flight measurement circuit 50, which is distributed in each region (bin) on a time axis divided into multiple regions (bins) at predetermined time intervals, when the measurement light is repeatedly output at a predetermined period.

[0056] Furthermore, the representative value calculation circuit 70 may be configured to calculate a representative value of the flight time for an object by dividing the sum of the flight times corresponding to a predetermined sum threshold, from the flight times distributed in the region (bin) where the frequency reaches a predetermined histogram threshold, by the frequency corresponding to the predetermined sum threshold.

[0057] In this case, as shown in Figure 5, the maximum value (Max) of the edge detection number n, indicated by "TDC count value Tm,n", becomes 32, which is equivalent to expanding the dynamic range of the histogram.

[0058] The histogram generation circuit 60 will be described in detail below, based on Figure 3. The histogram generation circuit 60 is a circuit block that generates a histogram showing the cumulative frequency of flight information distributed within a preset measurement range, based on each flight piece of information which is the flight time or flight distance from the time of output of the measurement light, which is repeatedly output at a predetermined period, to the time of detection of the reflected light for each measurement light.

[0059] In this embodiment, as described above, an example is described in which the flight time output from the flight time measurement circuit 50 is used as the flight information. However, the flight distance may also be used as the flight information, calculated from the flight time from when the measurement light emitted from the light-emitting element 2 is reflected from an object and detected by the multiple photoreceiving elements 3. In addition, the flight information may include other information such as the received light level or the time width of the received light signal, as long as it includes at least the flight time or flight distance.

[0060] The histogram generation circuit 60 includes a FIFO-type memory circuit 63, a memory circuit 62 for storing the histogram, a memory processing circuit 61 that stores in the memory circuit 62 the bin numbers that individually identify each bin, which is divided into multiple sections of the measurement range, in order based on flight information, and the flight time and cumulative frequency included in each bin, in association with each other. The bin numbers only need to be information that can individually identify the order of each bin. For example, if the bin numbers are set so that the numbers increase or decrease sequentially in accordance with the flight time or flight distance from the time of measurement light output, it becomes easier to use them for sorting in ascending or descending order.

[0061] The memory processing circuit 61 associates flight information, including the generated flight time, with the bin number of the bin to which that flight time belongs, and stores this information in the memory circuit 62. This eliminates the need to pre-allocate memory areas in the memory circuit 62 for bins to which no flight time occurs, thereby reducing the memory capacity of the memory circuit 62. Furthermore, if multiple flight times occur for the same bin number, the memory processing circuit 61 associates the cumulative frequency with that bin number, thereby generating a histogram representing the distribution of flight times within a pre-set measurement range.

[0062] The FIFO-type memory circuit 63 includes a memory block 67, a write circuit 64 that controls writing data to the memory block 67, a read circuit 65 that controls reading data from the memory block 67, and a state management circuit 66, among others.

[0063] The write circuit 64 controls the timing and write address for writing the flight time to the memory block 67 in synchronization with the flight time measurement circuit 50. The read circuit 65 controls the timing and read address for reading the flight time written to the memory block 67 by the write circuit 64. The state management circuit 66 manages the free capacity of the memory block 67 and also manages to prevent conflicts in access to the memory block 67 by the write circuit 64 and the read circuit 65.

[0064] The write circuit 64 manages the write address of the memory block 67 based on the signal from the state management circuit 66, controls the writing of the flight time from the flight time measurement circuit 50 to the memory block 67, and updates the address counter each time to manage the address to be written next. The read circuit 65 controls the reading of the flight time from the memory block 67 to the dummy determination circuit 68 based on the signal from the state management circuit 66, and updates the address counter each time to manage the address to be read next.

[0065] The state management circuit 66 manages the remaining capacity of the memory block 67 and outputs a signal corresponding to the write circuit 64 when it is full to suppress further writing, and outputs a signal corresponding to the read circuit 65 when it is empty to suppress further reading.

[0066] The time-of-flight measurement circuit 50, the memory block 67, and the write circuit 64 operate in synchronization with a predetermined periodic measurement clock that controls the light emission control circuit 30 and the like, to execute a sequence for writing the time of flight to the memory block 67. The memory processing circuit 61, the memory circuit 62, the memory block 67, and the read circuit 65 operate in synchronization with the histogram generation clock, to execute a sequence for reading the time of flight from the memory block 67.

[0067] In other words, the writing circuit 64 operates to store flight information in synchronization with the measurement clock that defines when flight time occurs, as determined by the flight time measurement circuit 50, and the reading circuit 65 outputs the flight information to the dummy detection circuit 68, and subsequently to the storage processing circuit 61, in synchronization with the histogram generation clock, which is asynchronous with the occurrence of flight time.

[0068] The writing circuit 64 includes a dummy data addition circuit 69 that stores dummy data in the memory block 67 each time the flight time output from the flight time measurement circuit 50 ends at a predetermined period, i.e., the pulse light emission period. This dummy data indicates the end of one cycle of flight information output.

[0069] The read circuit 65 outputs the flight time to the memory processing circuit 61 each time it reads the flight time from the memory block 67, and includes a dummy determination circuit 68 that outputs a signal to the memory processing circuit 61 indicating the end of the flight time output that occurs in one cycle of the pulse light emission period when dummy data is read from the memory block 67. The memory processing circuit 61 determines, based on the dummy data, whether or not it is flight information within one cycle of the pulse light emission period.

[0070] Even if the flight time measurement circuit 50 detects multiple flight times within a single cycle, and the interval between each flight time is short, buffering each flight time in the FIFO-type memory circuit 63 allows time to be secured to update the histogram based on the flight information read from the FIFO-type memory circuit 63, thus preventing the inconvenience of data loss.

[0071] The operation of the memory processing circuit 61 is described in detail below. As shown in the upper part of Figure 6, each time the time-of-flight measurement circuit 50 outputs the time of flight (labeled "detected echo" / "TDC count value" in Figure 6) for each emission period of the pulsed light, the TDC count value is stored (PUSHed) in the memory block 67 by the writing circuit 64.

[0072] During the pulse light emission period labeled "Measurement 1," the writing circuit 64 stores (PUSH) three "detection echoes" output from the time-of-flight measurement circuit 50 into the memory block 67, and the dummy data addition circuit 69 then stores (PUSH) dummy data indicating the end of the measurement. During the pulse light emission period labeled "Measurement 2," the writing circuit 64 stores (PUSH) two "detection echoes" output from the time-of-flight measurement circuit 50 into the memory block 67, and the dummy data addition circuit 69 then stores (PUSH) dummy data indicating the end of the measurement.

[0073] The flight time stored (PUSH) in memory block 67 is read (POP) by the read circuit 65 to the dummy determination circuit 68 when the memory processing circuit 61 is not busy and data is stored in memory block 67. If the dummy determination circuit 68 determines that the data is dummy data, it determines that the measurement for one emission period of pulsed light has ended and outputs a termination signal to the memory processing circuit 61 to indicate this. The memory processing circuit 61 generates a histogram based on the flight time input from the dummy determination circuit 68 up to the time the termination signal is detected.

[0074] The lower part of Figure 6 shows the sequence in which each flight time is stored (PUSH) in the memory block 67 by the write circuit 64 in the order shown in the upper part, and each flight time is read (POP) from the memory block 67 by the read circuit 65 asynchronously with the writing by the write circuit 64.

[0075] Figure 7 shows an example where the memory circuit 62 is composed of a single memory circuit. The example is explained when the memory processing circuit 61 calculates that the bin number to which the flight time input from the dummy determination circuit 68 belongs is "35". The leftmost column shows the bin number stored in the memory circuit 62 at that time. The memory circuit 62 stores data related to the bin number, such as the bin number, frequency, and flight time.

[0076] The memory processing circuit 61 compares the new bin number "35" with the bin numbers already stored in the memory circuit 62. If it determines that the bin numbers stored in the memory circuit 62 are smaller than the new bin number "35", it maintains that storage state. If it determines that the bin numbers stored in the memory circuit 62 are larger than the new bin number "35", it shifts the storage addresses of the larger bin numbers one by one and stores the new bin number "35" in the freed-up addresses. In Figure 7, the storage addresses of bin numbers "60", "50", and "40" are changed, and bin number "35" is stored in the storage address where bin number "40" was previously stored. If bin number "35" is already stored in the memory circuit 62, its frequency is increased by "1".

[0077] In other words, when flight information is generated, the memory processing circuit 61 reads the bin number stored in the memory circuit 62, sorts it in ascending or descending order including the bin number to which the current flight information belongs, and stores the bin number, flight information, and cumulative frequency in the memory circuit 62. Switching between ascending and descending order is done by sequentially incrementing or decrementing the write address.

[0078] Figures 8 and 9 show an embodiment in which the memory circuit 62 is composed of two memory circuits (memory circuit A and memory circuit B). When flight information is generated, one of memory circuits A and B functions as a reference memory circuit to retrieve already stored bin numbers, while the other functions as a storage memory circuit to store bin numbers including new bin numbers and cumulative frequencies.

[0079] Each time flight information is generated, the memory processing circuit 61 switches the functions of the reference memory circuit and the storage memory circuit, reads the bin number stored in the reference memory circuit, sorts it in ascending or descending order including the bin number to which the current flight time information belongs, and stores the bin number, flight information, and cumulative frequency in the storage memory circuit.

[0080] In other words, the memory circuit 62 comprises two parts: a reference memory circuit for locating already stored bin numbers, and a storage memory circuit for storing bin numbers including new bin numbers and cumulative frequencies. The memory processing circuit 61 then uses the bin numbers and cumulative frequencies stored in the storage memory circuit to create the most recent histogram.

[0081] When new flight information is generated, the memory processing circuit 61 activates the storage memory circuit as a reference memory circuit and compares the bin number read from the reference memory circuit with the bin number to which the new flight information belongs. The bin numbers are then sorted in ascending or descending order, and the bin number, flight information, and cumulative frequency are stored in the storage memory circuit, which has been switched from the reference memory circuit. By separating the functions of the reference memory circuit and the storage memory circuit, the read operation from one memory circuit and the write operation to the other memory circuit can be performed simultaneously, increasing the processing speed compared to using a single memory circuit.

[0082] In the example shown in Figure 8, memory circuit A functions as a reference memory circuit, and memory circuit B functions as a storage memory circuit. As described above, we will explain using the example where the memory processing circuit 61 calculates that the bin number to which the flight time input from the dummy determination circuit 68 belongs is "35". The leftmost figure shows the bin number stored at that time in memory circuit A, which functions as a reference memory circuit. As described above, memory circuit A stores data such as bin number, frequency, and flight time.

[0083] The memory processing circuit 61 compares the new bin number "35" with the bin number stored in the reference memory circuit (memory circuit A). If it determines that the bin number stored in the reference memory circuit (memory circuit A) is smaller than the new bin number "35", it stores the data related to the bin number stored in the reference memory circuit (memory circuit A) in the storage memory circuit (memory circuit B) in that order. If the memory processing circuit 61 determines that the bin number stored in the reference memory circuit (memory circuit A) is larger than the new bin number "35", it stores the data related to the new bin number "35" in the storage memory circuit (memory circuit B) at the next address, and then stores the data related to the bin number larger than the new bin number "35" stored in the reference memory circuit (memory circuit A) in the storage memory circuit (memory circuit B) at the next address. If the bin number "35" is already stored in the reference memory circuit (memory circuit A), it increments the frequency of that bin number by "1" and stores it in the storage memory circuit (memory circuit B).

[0084] Furthermore, when flight information is first generated, the memory processing circuit 61 stores the bin number, flight time, and cumulative frequency in the storage memory circuit without switching the functions of the reference memory circuit and the storage memory circuit. When flight information is first generated, there is no need to compare the bin numbers, so either memory circuit can be made to function as the storage memory circuit and store the bin number, flight information, and cumulative frequency (which will be "1" in this case).

[0085] Figure 9 shows the procedure by which, after each measurement until dummy data is input to the dummy detection circuit 68, the memory processing circuit 61 switches the functions of the reference memory circuit and the storage memory circuit to update the histogram data. After the last measurement, the data related to the bin number stored in the storage memory circuit is output to the representative value calculation circuit 70 to calculate the representative value, and then the distance is calculated by the distance calculation circuit.

[0086] In the embodiment described above, a configuration in which the histogram generation circuit 60 is equipped with a FIFO-type storage circuit 63 was explained. However, the histogram generation circuit 60 may be configured without the FIFO-type storage circuit 63. In this case, the measurement information from the time-of-flight measurement circuit 50 is directly output to the storage processing circuit 61, and each time, the storage processing circuit 61 updates and stores the histogram data in a single storage circuit 62 as described in Figure 7, or in a pair of storage circuits 62 as shown in Figures 8 and 9. However, it should be noted that if the output interval of the measurement information from the time-of-flight measurement circuit 50 is very short, there is a risk that the updating and storage of histogram data in the storage circuit 62 may be impaired.

[0087] As described above, the histogram generation method according to the present invention is a histogram generation method that generates a histogram showing the cumulative frequency of flight information distributed in a preset measurement range based on each flight information, which is the flight time or flight distance from the time of output of the measurement light that is repeatedly output at a predetermined period to the time of detection of the reflected light for each measurement light, and comprises a storage step of storing the histogram in a memory circuit, and a storage processing step of storing in the memory circuit, in association with a bin number that identifies each bin, which divides the measurement range into multiple sections, in order based on the flight information, and the flight information and cumulative frequency contained in each bin.

[0088] Furthermore, the optical distance measuring device according to the present invention comprises: a light-emitting element that outputs pulsed measurement light; a plurality of photon-count type photodetectors that detect reflected light from an object to the measurement light; an addition circuit that adds up the voltage pulses output from each photodetector; a flight information calculation circuit that calculates the time from the time the measurement light is output until the time the sum of the addition circuit reaches a predetermined addition threshold, or the distance corresponding to that time, as flight information; a histogram generation circuit that generates a histogram showing the cumulative frequency of the flight information distributed in a preset measurement range based on the flight information calculated by the flight information calculation circuit when the measurement light is repeatedly output at a predetermined period; a representative value calculation circuit that calculates a representative value of the flight information for an object from the flight information distributed in the bins of the bin numbers where the frequency reaches a predetermined histogram threshold based on the histogram; and a distance calculation circuit that calculates the distance to the object based on the representative value of the flight information calculated by the representative value calculation circuit.

[0089] Furthermore, the optical distance measurement method according to the present invention includes: a reflected light detection step in which a plurality of photon-count type photodetectors detect reflected light from an object to pulsed measurement light output from a light-emitting element; a flight information calculation step in which the time from the time of output of the measurement light to the time when the sum of the voltage pulses output from each photodetector reaches a predetermined sum threshold, or the distance corresponding to the time, is calculated as flight information; a histogram generation step in which a histogram generation method is executed to generate a histogram showing the cumulative frequency of flight information distributed in a preset measurement range based on the flight information calculated in the flight information calculation step when the measurement light is repeatedly output at a predetermined period; a representative value calculation step in which a representative value of flight information for an object is calculated from the flight information distributed in the bin of the bin number where the frequency reaches a predetermined histogram threshold based on the histogram; and a distance calculation step in which the distance to the object is calculated based on the representative value of flight information calculated in the representative value calculation step.

[0090] The embodiments described above are examples of the present invention, and the scope of the present invention is not limited by the description of the embodiments. [Explanation of symbols]

[0091] 2: Light-emitting element 3: Photodetector 10: Optical scanning device 20: Motor drive circuit 30: Light emission control circuit 40: Light receiving circuit 50: Flight time measurement circuit 60: Histogram generation circuit 61: Memory Processing Circuit 62:Memory circuit 63: FIFO-type memory circuit 64: Programming circuit 65: Readout Circuit 66: State Management Circuit 67: Memory Block 68: Dummy detection circuit 69: Dummy data addition circuit 70: Typical Value Calculation Circuit 80: Distance calculation circuit 100: Control circuit 200: Optical ranging device

Claims

1. A histogram generation circuit generates a histogram showing the cumulative frequency of the flight information distributed within a preset measurement range, based on each flight information which is the flight time or flight distance from the time of output of the measurement light, which is repeatedly output at a predetermined period, to the time of detection of the reflected light for each measurement light, A memory circuit for storing the aforementioned histogram, A memory processing circuit stores in the memory circuit a bin number that individually identifies each bin, which is divided into multiple sections of the measurement range, in order based on the flight information, and the flight information and cumulative frequency contained in each bin, in association with each bin. Equipped with, The memory circuit includes a reference memory circuit for locating already stored bin numbers when the flight information is generated, and a storage memory circuit for storing bin numbers including the new bin number and the cumulative frequency. The memory processing circuit is configured to switch the functions of the reference memory circuit and the storage memory circuit each time the flight information is generated, read the bin number stored in the reference memory circuit, sort the flight information in ascending or descending order, including the bin number of the bin to which the current flight time information belongs, and store the bin number, the flight information, and the cumulative frequency in the storage memory circuit.

2. The histogram generation circuit according to claim 1, wherein when the flight information is generated, the memory processing circuit reads the bin number stored in the memory circuit, sorts the flight information in ascending or descending order, including the bin number of the bin to which the current flight information belongs, and stores the bin number, the flight information, and the cumulative frequency in the memory circuit.

3. The histogram generation circuit according to claim 1, wherein the memory processing circuit stores the bin number, the flight information, and the cumulative frequency in the storage memory circuit without switching the functions of the reference memory circuit and the storage memory circuit when the flight information is first generated.

4. It also features a FIFO-type memory circuit, The histogram generation circuit according to any one of claims 1 to 3, further comprising: a write circuit that stores the flight information in synchronization with the generation of the flight information; and a read circuit that reads the flight information asynchronously with the generation of the flight information and outputs it to the storage processing circuit.

5. The histogram generation circuit according to claim 4, wherein the writing circuit is configured to store dummy data indicating the end of one cycle of the flight information output in the FIFO-type storage circuit each time the output of the flight information ends at a predetermined period, and the reading circuit is configured to output a signal indicating the end of one cycle of the flight information output each time the dummy data is read.

6. A light-emitting element that outputs pulsed measurement light, A plurality of photon-count type light-receiving elements for detecting reflected light from an object relative to the measurement light, An adder circuit that adds up the voltage pulses output from each photodetector, A flight information calculation circuit that calculates flight information the time from the time of output of the measurement light to the time when the sum value of the summing circuit reaches a predetermined summing threshold, or the distance corresponding to that time. A histogram generation circuit according to any one of claims 1 to 5, which generates a histogram showing the cumulative frequency of the flight information distributed within a preset measurement range based on the flight information calculated by the flight information calculation circuit when the measurement light is repeatedly output at a predetermined period, A representative value calculation circuit calculates a representative value of the flight information for the object from the flight information distributed in the bins of the bin number where the cumulative frequency reaches a predetermined histogram threshold, based on the histogram; A distance calculation circuit that calculates the distance to the object based on the representative value of the flight information calculated by the representative value calculation circuit, An optical distance measuring device equipped with the following features.

7. A histogram generation method that generates a histogram showing the cumulative frequency of flight information distributed within a preset measurement range, based on each flight information which is the flight time or flight distance from the time of output of the measurement light, which is repeatedly output at a predetermined period, to the time of detection of the reflected light for each measurement light, A storage step of storing the histogram in a memory circuit, A storage processing step involves associating a bin number that individually identifies each bin, which is divided into multiple sections of the measurement range, based on the flight information, with the flight information and cumulative frequency contained in each bin, and storing them in the storage circuit. Equipped with, The memory circuit includes a reference memory circuit for locating already stored bin numbers when the flight information is generated, and a storage memory circuit for storing bin numbers including the new bin number and the cumulative frequency. The memory processing step is configured to switch the functions of the reference memory circuit and the storage memory circuit each time the flight information is generated, read the bin number stored in the reference memory circuit, sort the flight information in ascending or descending order, including the bin number of the bin to which the current flight time information belongs, and store the bin number, the flight information, and the cumulative frequency in the storage memory circuit.

8. A reflected light detection step in which the reflected light from an object to pulsed measurement light output from a light-emitting element is detected by multiple photon-count type photodetectors, A flight information calculation step that calculates the time from the time of output of the measurement light to the time when the sum of the voltage pulses output from each photodetector reaches a predetermined sum threshold, or the distance corresponding to that time, as flight information. A histogram generation step of claim 7, which generates a histogram showing the cumulative frequency of the flight information distributed in a preset measurement range based on the flight information calculated in the flight information calculation step when the measurement light is repeatedly output at a predetermined period, A representative value calculation step, based on the histogram, calculates a representative value of the flight information for the object from the flight information distributed in the bins of the bin numbers where the cumulative frequency reaches a predetermined histogram threshold, A distance calculation step which calculates the distance to the object based on the representative value of the flight information calculated in the representative value calculation step, A light-based distance measuring method equipped with this feature.

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