Distance measuring device and method for calibrating distance measuring device

Abstract

A distance measuring device (10) comprises a light source (11), a light receiving unit (12), a drive control unit (13) that drives the light source (11) and the light receiving unit (12), a depth calculating unit (14) that calculates a depth on the basis of the amount of light received by the light receiving unit (12), a distance calculating unit (15) that includes a correction value storage unit (15a) for holding a correction value, and that calculates a distance by correcting the depth on the basis of the correction value, and a calibration unit (16) that executes calibration for updating the correction value, wherein the calibration unit (16) includes: a pulse phase sweeping unit (16a) that controls the drive control unit (13) to change a phase, which is the time difference from when a light emission pulse is output until an exposure pulse is output, such that a plurality of different phases are sequentially generated; and a correction value calculating unit (16b) that calculates the correction value from the plurality of phases and the corresponding plurality of depths obtained by the depth calculating unit (14), and causes the correction value storage unit (15a) to hold the calculated correction value.

Description

Distance measuring device and method for calibrating distance measuring device

[0001] The present disclosure relates to a distance measuring device and a method for calibrating a distance measuring device, and more particularly to a method for calibrating a time-of-flight (TOF) distance measuring device.

[0002] Conventionally, various techniques have been proposed for calibrating a TOF distance measuring device (see, for example, Patent Document 1).

[0003] Patent document 1 describes a method for calculating distance correction information in a distance measuring device that measures the distance to a measurement object by receiving reflected light of light irradiated onto the measurement object while a light-transmitting member is placed between the measurement object and the measurement object, and is characterized by having an actual measurement process in which the distance measuring device obtains an actual distance measured by measuring the distance to a calibration object placed at a predetermined specified distance while a light-transmitting member is placed between the measurement object and the calibration object, and a calculation process in which the distance correction information is calculated to correct the distance measured by the distance measuring device based on actual measurement error information between the actual distance obtained in the actual measurement process and the specified distance.

[0004] Japanese Patent Application Laid-Open No. 2021-148643

[0005] However, the technology of Patent Document 1 requires that an object be placed at a specified distance to obtain error information, and therefore requires a space with a large depth to place the object according to the distance measurement range (i.e., the range in which distance measurement is possible). Furthermore, in order to improve the accuracy of the error information, error information for a plurality of specified distances is required, which necessitates a step of moving the object in the depth direction (i.e., a method of moving an object).

[0006] Therefore, an object of the present disclosure is to provide a distance measuring device and a method for calibrating a distance measuring device that can calibrate distance measurement without moving the subject in the depth direction.

[0007] In order to achieve the above object, a distance measuring device according to one embodiment of the present disclosure is a distance measuring device for measuring a distance to a subject, the distance measuring device including: a light source that emits irradiation light toward the subject in accordance with an input light emission pulse; a light receiving unit that receives reflected light from the subject in an exposure interval determined by the input exposure pulse; a drive control unit that drives the light source by outputting the light emission pulse to the light source and drives the light receiving unit by outputting the exposure pulse to the light receiving unit; a depth calculation unit that calculates a depth corresponding to a time difference between when irradiation light is emitted from the light source and when the reflected light is received by the light receiving unit based on the amount of reflected light received by the light receiving unit; and a correction unit that holds a correction value. a distance calculation unit that calculates the distance to the subject by correcting the depth calculated by the depth calculation unit based on the correction value stored in the correction value storage unit, and a calibration unit that performs calibration to update the correction value, and the calibration unit has a pulse phase sweep unit that controls the drive control unit so that a plurality of different phases are generated in sequence by changing the phase, which is the time difference between when the light emission pulse is output and when the exposure pulse is output, and a correction value calculation unit that calculates correction values ​​from the plurality of phases and the corresponding plurality of depths obtained by the depth calculation unit, and stores the calculated correction values ​​in the correction value storage unit.

[0008] In order to achieve the above object, a method for calibrating a distance measuring device according to one embodiment of the present disclosure is a method for calibrating a distance measuring device that measures a distance to a subject, the distance measuring device including a light source that emits irradiation light toward the subject in accordance with an input light emission pulse, a light receiving unit that receives reflected light from the subject in an exposure interval determined by an input exposure pulse, a drive control unit that drives the light source by outputting the light emission pulse to the light source and drives the light receiving unit by outputting the exposure pulse to the light receiving unit, a depth calculation unit that calculates a depth corresponding to a time difference between when irradiation light is emitted from the light source and when the reflected light is received by the light receiving unit based on the amount of reflected light received by the light receiving unit, and a correction value calculation unit that holds a correction value. The calibration method includes a distance calculation unit having a memory unit and correcting the depth calculated by the depth calculation unit based on the correction value stored in the correction value memory unit to calculate the distance to the subject, and the calibration method includes a calibration step of performing calibration to update the correction value, and the calibration step includes a pulse phase sweep step of controlling the drive control unit so that a plurality of different phases are generated in sequence by changing the phase, which is the time difference between when the light emission pulse is output and when the exposure pulse is output, and a correction value calculation step of calculating correction values ​​from the plurality of phases and the corresponding plurality of depths obtained by the depth calculation unit and storing the calculated correction values ​​in the distance calculation unit.

[0009] The present disclosure provides a distance measuring device and a method for calibrating a distance measuring device that can calibrate distance measurement without moving the subject in the depth direction.

[0010] FIG. 1 is a block diagram illustrating a configuration of a distance measuring device according to an embodiment. FIG. 2 is a diagram illustrating a calibration method for a distance measuring device according to an embodiment. FIG. 3 is a diagram illustrating a nonlinear correction value calculated by a correction value calculation unit of the distance measuring device according to an embodiment. FIG. 4 is a diagram illustrating a tilt correction value and an offset correction value calculated by a correction value calculation unit of the distance measuring device according to an embodiment. FIG. 5 is a flowchart illustrating the overall procedure of a calibration method using the distance measuring device according to an embodiment. FIG. 6 is a diagram illustrating functions of the distance measuring device according to an embodiment that correspond to a case where the installation position of a subject used for calibration of the distance measuring device is changed. FIG. 7 is a diagram illustrating a case where high accuracy of calibration is required. FIG. 8 is a diagram illustrating a first function for improving the accuracy of calibration provided by the distance measuring device according to an embodiment. FIG. 9 is a diagram illustrating a second function for improving the accuracy of calibration provided by the distance measuring device according to an embodiment. FIG. 10 is a diagram illustrating one function for speeding up calibration provided by the distance measuring device according to an embodiment.

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that each embodiment described below represents a specific example of the present disclosure. The numerical values, realization means, components, arrangement and connection of components, steps, order of steps, etc. shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, each drawing is not necessarily an exact illustration. In each drawing, substantially identical components are assigned the same reference numerals, and duplicate explanations are omitted or simplified.

[0012] 1 is a block diagram showing the configuration of a distance measuring device 10 according to an embodiment. In addition to the distance measuring device 10, the diagram also shows an object 20, light 21 directed toward the object 20, and light 22 reflected from the object 20.

[0013] The distance measuring device 10 is a TOF distance measuring device with a calibration function, and includes a light source 11 , a light receiving unit 12 , a drive control unit 13 , a depth calculation unit 14 , a distance calculation unit 15 , and a calibration unit 16 .

[0014] The light source 11 is a light source that emits irradiation light 21 such as infrared (IR) light toward the subject 20 in accordance with an input light emission pulse, and is, for example, an LED (light emitting diode) or an LD (laser diode).

[0015] The light receiving unit 12 is a circuit that receives reflected light 22 from the subject 20 during an exposure period determined by the input exposure pulse and outputs an electrical signal (RAW data) indicating the amount of received light, and is, for example, a solid-state imaging element such as a CMOS sensor composed of pixels arranged two-dimensionally.

[0016] The drive control unit 13 is a circuit that drives the light source 11 by outputting an emission pulse to the light source 11 and drives the light receiving unit 12 by outputting an exposure pulse to the light receiving unit 12, and is composed of, for example, a timer circuit and a pulse generating circuit.

[0017] The depth calculation unit 14 is a circuit that calculates a depth (hereinafter also referred to as "pre-correction depth"), which is distance data (depth value) corresponding to the time difference between when the irradiated light 21 is emitted from the light source 11 and when the reflected light 22 is received by the light receiving unit 12, based on the amount of light (RAW data) of the reflected light 22 received by the light receiving unit 12, and is composed of, for example, a frame memory, an arithmetic circuit, etc.

[0018] The distance calculation unit 15 has a correction value storage unit 15a that stores correction values, and is a circuit that calculates the distance to the subject 20 by correcting the pre-correction depth calculated by the depth calculation unit 14 based on the correction value stored in the correction value storage unit 15a, and is configured, for example, by a processor with a built-in program. In this specification, the "distance to the subject 20" or "distance" means the distance from the distance measuring device 10 to the subject 20.

[0019] The calibration unit 16 is a circuit that performs calibration to update the correction value, and includes a pulse phase sweep unit 16a that controls the drive control unit 13 so that multiple different phases are generated in sequence by changing the phase, which is the time difference between the output of the light emission pulse and the output of the exposure pulse (i.e., performing a phase sweep), and a correction value calculation unit 16b that calculates a correction value from the multiple phases and multiple corresponding pre-correction depths obtained by the depth calculation unit 14 (these pre-correction depths for calibration are also called ``calibration data''), and stores the calculated correction value in the correction value memory unit 15a.

[0020] More specifically, the correction value calculation unit 16b calculates, as correction values, a nonlinear correction value for correcting the linearity between the plurality of phases and the plurality of pre-correction depths, and a slope correction value and an offset correction value for specifying a linear function for correcting the distance to the subject 20 indicated by the depth after linearity has been corrected (hereinafter also referred to as the "corrected depth"). The calibration unit 16 is configured, for example, by a processor or the like having a built-in program. The method of calibrating distance measurement by the calibration unit 16 is also called the "phase sweep method."

[0021] 2A and 2B are diagrams illustrating a calibration method (i.e., a phase sweep method) of the distance measuring device 10 according to the embodiment. More specifically, (a) of Fig. 2A is a diagram illustrating a conventional calibration method using a subject movement method, and (b) of Fig. 2B is a diagram illustrating a calibration method using the phase sweep method according to the embodiment (i.e., the function of the calibration unit 16).

[0022] 2 shows timing charts of signals when calibration is performed using three distances to the subject (i.e., calibration distances) of 0.3 m, 0.6 m, and 0.9 m for the conventional subject movement method and the phase sweep method of the embodiment. In Fig. 2, "irradiated light" indicates the timing at which the irradiated light is emitted (i.e., the timing of a high active light emission pulse), "reflected light" indicates the timing at which the reflected light reaches the distance measuring device 10 (i.e., the timing of a low active exposure pulse), and "exposure" indicates the timing at which the light receiving unit 12 receives the light (i.e., the timing of a low active exposure pulse).

[0023] Furthermore, for each of the three calibration distances, two types of exposure sequences (referred to as "A0 exposure" and "A1 exposure") with different phases are performed. Both the A0 exposure and the A1 exposure are packets having an exposure section with the same time width as the light emission pulse, and the combined exposure section of the A0 exposure and the A1 exposure forms one continuous time section in terms of the relative elapsed time from the light emission pulse. Furthermore, the hatching within the A0 exposure and the A1 exposure in the timing chart for "reflected light" corresponds to the amount of light received (i.e., charge) generated at the light receiving unit 12 by the A0 exposure and the A1 exposure, respectively.

[0024] In the conventional subject movement method, as shown in Figure 2(a), distance measurement is performed by placing the subject 0.3 m away from the distance measuring device, performing A0 exposure and A1 exposure multiple times (e.g., 1,000 times), and calculating the distance using RAW data indicating the amount of light received at each of the A0 exposure and A1 exposure accumulated in the light receiving unit 12. Next, the subject is placed 0.6 m away from the distance measuring device and similar distance measurement is performed, and then the subject is placed 0.9 m away from the distance measuring device and similar distance measurement is performed. From the pair of the measured distance obtained by such distance measurement involving subject movement and the actual distance, a correction value for correcting the error between them is calculated.

[0025] In the phase sweep method of the embodiment, as shown in (b) of Figure 2, the subject 20 is placed at a position an arbitrary reference distance (e.g., 0.3 m) away from the distance measuring device 10, and then, under the control of the pulse phase sweep unit 16a, A0 exposure and A1 exposure at the reference phase are first performed a number of times (e.g., 1,000 times) to measure the distance, and the distance is calculated using RAW data indicating the amount of light received in each of the A0 exposure and A1 exposure accumulated in the light receiving unit 12. Next, under the control of the pulse phase sweep unit 16a, the phases of the A0 exposure and the A1 exposure are shifted in the decreasing direction to a phase corresponding to when the distance between the subject 20 and the distance measuring device 10 is set to 0.6 m (i.e., the exposure timing is advanced by the time required for light to travel back and forth over the increased 0.3 m), and a similar distance measurement is performed.Furthermore, the phases of the A0 exposure and the A1 exposure are shifted in the decreasing direction to a phase corresponding to when the distance between the subject 20 and the distance measuring device 10 is set to 0.9 m (i.e., the exposure timing is further advanced by the time required for light to travel back and forth over the increased 0.3 m), and a similar distance measurement is performed.In this way, from the pair of the uncorrected depth obtained by distance measurement involving a sweep that shifts the phases of the exposure pulses (i.e., the A0 exposure and the A1 exposure) in small steps and the calculated distance calculated from the phases of the exposure pulses, the correction value calculation unit 16b calculates a correction value for correcting these errors.

[0026] The formula for calculating the distance from the amount of light received in the A0 exposure and the A1 exposure is the same whether using the conventional subject movement method or the phase sweep method of the embodiment. In other words, if the width of the light emission pulse (10 ns in FIG. 2) is Δt, the amount of light received in the A0 exposure is C0, the amount of light received in the A1 exposure is C1, and the speed of light is C, the distance L is calculated using the following formula 1.

[0027] L=Δt×C / 2×C1 / (C0+C1) ・・Formula 1

[0028] For example, if Δt = 10 ns and C0:C1 = 4:1, then C = 3 × 10 ８ From m, L = 10 x 10 －９ x3 x 10 ８ / 2 x 1 / (4 + 1) = 0.3 (m)

[0029] Therefore, in this example, in the phase sweep method of the embodiment, shifting the phase of the exposure pulses (i.e., A0 exposure and A1 exposure) in a decreasing direction in steps equivalent to 2 ns (= 10 ns × 1 / 5) corresponds to moving the subject 20 farther in steps of 0.3 m.

[0030] In the phase sweep method shown in FIG. 2(b), the phase of the exposure pulse is shifted in steps while the timing of the light emission pulse is fixed. However, conversely, the phase of the light emission pulse may be shifted in steps while the timing of the exposure pulse is fixed.

[0031] In the phase sweep method shown in FIG. 2B, the phase of the exposure pulse is shifted in steps in a direction in which the calibration distance increases from a small value to a large value. However, conversely, the phase of the exposure pulse may be shifted in steps in a direction in which the calibration distance decreases from a large value to a small value.

[0032] Furthermore, in the distance measurement and calibration, in addition to the A0 exposure and the A1 exposure, the A2 exposure and the A3 exposure may also be measured to measure the amount of light (i.e., background light) in the exposure sections corresponding to the A0 exposure and the A1 exposure when there is no reflected light. In this case, if the received light amounts of the background light obtained in the A2 exposure and the A3 exposure are C2 and C3, respectively, the distance L is calculated by the following equation 2 instead of the above equation 1.

[0033] L=Δt×C / 2×(C1-C3) / {(C0-C2)+(C1-C3)}...Formula 2

[0034] As described above, the phase sweep method of the embodiment does not require the movement of the object, which is required in the conventional object movement method, and this allows the calibration of distance measurement to be automated and completed in a short time. Furthermore, the problem of the calibration work being dependent on the manual movement of the object is avoided.

[0035] 3 is a diagram illustrating the nonlinear correction value calculated by the correction value calculation unit 16b of the distance measuring device 10 according to the embodiment. More specifically, (a) of FIG. 3 shows a plot illustrating the relationship between the pre-correction depth (vertical axis) calculated by the depth calculation unit 14 and the subject distance (horizontal axis), and an example of an approximate straight line passing through these plots. (b) of FIG. 3 shows an example of the relationship between the nonlinear correction value (vertical axis) calculated by the correction value calculation unit 16b and the pre-correction depth (horizontal axis). The subject distance on the horizontal axis in (a) of FIG. 3 corresponds to the phases of the A0 exposure and the A1 exposure (i.e., the time difference between the light emission pulse and the exposure pulse) that are shifted in steps corresponding to each calibration distance in (b) of FIG. 2.

[0036] To calculate the nonlinear correction value, the correction value calculation unit 16b first plots points determined by the pre-correction depth calculated by the depth calculation unit 14 and the subject distance for each phase (i.e., corresponding subject distance) swept stepwise under the control of the pulse phase sweep unit 16a, as shown in (a) of Fig. 3, and calculates an approximate line passing through these points using the least squares method or the like. Then, as shown in (b) of Fig. 3, the correction value calculation unit 16b calculates, for each pre-correction depth, the difference between the pre-correction depth and a point on the approximate line corresponding to the pre-correction depth (i.e., the post-correction depth), as a nonlinear correction value, and stores the nonlinear correction value in the correction value storage unit 15a of the distance calculation unit 15.

[0037] 4 is a diagram illustrating the tilt correction value and offset correction value calculated by the correction value calculation unit 16b of the distance measuring device 10 according to the embodiment. Shown here are examples of the approximate line shown in FIG. 3A (the "linearity approximate line" in the figure) and a linear function (the "correction linear function" in the figure) determined by the tilt correction value and offset correction value calculated by the correction value calculation unit 16b.

[0038] For each corrected depth after linearity correction using the method shown in Figure 3, the correction value calculation unit 16b calculates a linear function indicating the difference between the distance indicated by the corrected depth and the calculated distance calculated from the phase of the corresponding exposure pulse (i.e., the time difference between the light emission pulse and the exposure pulse), and stores the slope and offset that specify this linear function in the correction value memory unit 15a of the distance calculation unit 15 as the slope correction value and the offset correction value, respectively.

[0039] As described above (see FIGS. 3 and 4), the three correction values ​​(non-linear correction value, tilt correction value, and offset correction value) calculated by correction value calculation unit 16b during calibration and stored in correction value storage unit 15a are used by distance calculation unit 15 when calculating the distance from the pre-correction depth during distance measurement. That is, during distance measurement, distance calculation unit 15 first calculates the corrected depth by adding the non-linear correction value stored in correction value storage unit 15a to the pre-correction depth calculated by depth calculation unit 14, and then calculates the distance to subject 20 by adding the correction value to the distance indicated by the calculated post-correction depth in accordance with a linear function determined by the tilt correction value and offset correction value stored in correction value storage unit 15a.

[0040] 5 is a flowchart showing the overall procedure of the calibration method by the distance measuring device 10 according to the embodiment. First, the calibration unit 16 performs initial settings for the drive control unit 13 via the pulse phase sweep unit 16a (S10). More specifically, the calibration unit 16 instructs the drive control unit 13 via the pulse phase sweep unit 16a about initial values ​​for the phase sweep, i.e., the reference phases for the A0 exposure and the A1 exposure, and turns off the correction operation for the distance calculation unit 15.

[0041] Next, the calibration unit 16 adjusts the number of times of light emission (S11). More specifically, the calibration unit 16 controls the drive control unit 13 by using the pulse phase sweep unit 16a, so that the drive control unit 13 outputs a light emission pulse to the light source 11 and an exposure pulse to the light receiving unit 12 in accordance with the adjusted number of times of light emission.

[0042] The calibration unit 16 determines whether or not it has completed acquiring all calibration data (i.e., pre-correction depth) corresponding to the ranging range (S12). If it has not completed (No in S12), it acquires the pre-correction depth calculated by the depth calculation unit 14 (S13), shifts the phase of the exposure pulse to a phase corresponding to the next calibration distance, and then controls the drive control unit 13 using the pulse phase sweep unit 16a to cause the drive control unit 13 to output an emission pulse to the light source 11 and output an exposure pulse to the light receiving unit 12 in accordance with the adjusted number of emissions (pulse phase sweep step S14).

[0043] On the other hand, when the calibration unit 16 determines that acquisition of all calibration data (i.e., pre-correction depths) corresponding to the ranging range has been completed (Yes in S12), the correction value calculation unit 16b calculates three correction values ​​(non-linear correction value, tilt correction value, and offset correction value) using the pre-correction depths acquired up to that point in the calculation procedure described above (S15), and stores and updates the correction value in the correction value storage unit 15a of the distance calculation unit 15 (S16). Steps S15 and S16 correspond to correction value calculation steps in which correction values ​​are calculated from the phases of multiple exposure pulses and the corresponding multiple depths obtained by the depth calculation unit 14, and the calculated correction values ​​are stored in the correction value storage unit 15a.

[0044] As described above, the phase sweep calibration method using the distance measuring device 10 according to the embodiment eliminates the need to move the object, which is required in the conventional object movement method, and thus distance measurement calibration is automated and completed in a short time. Furthermore, the problem of the calibration work being dependent on manual movement of the object is avoided.

[0045] 6A and 6B are diagrams illustrating the function of the distance measuring device 10 in accordance with an embodiment when the installation position of the subject 20 used for calibrating the distance measuring device 10 is changed. More specifically, Fig. 6A shows a method for installing the distance measuring device 10 at a position 5.0 m away from the subject 20 as the reference distance and performing calibration for a distance measurement range of 5 to 10 m, and Fig. 6B shows a method for installing the distance measuring device 10 at a position 1.0 m away from the subject 20 and performing calibration for a distance measurement range of 1 to 6 m.

[0046] 6A, when the reference distance is 5 m and calibration is performed for a distance measurement range of 5 to 10 m, the pulse phase sweep unit 16a uses the phase corresponding to the reference distance (5 m) as the reference phase (i.e., initial phase) and performs calibration by shifting the phase in steps by an amount corresponding to a plurality of sections obtained by dividing the distance measurement range of 5 to 10 m. In this case, the level of the irradiated light 21 (IR) emitted from the light source 11 is I, the reflectance of the subject 20 is R (%), and the number of times the irradiated light 21 is emitted is a (times).

[0047] In contrast to this, as shown in FIG. 6B, if the reference distance is changed to 1.0 m (i.e., 1 / 5 times), and calibration is performed for the distance measurement range of 1 to 6 m, under the control of the calibration unit 16, the pulse phase sweep unit 16a changes the reference phase (i.e., initial phase) to be smaller by an amount equivalent to 4 m so as to correspond to the new reference distance (1 m), and further reduces the number of times of emission of the irradiated light 21 (IR) from the light source 11 to 1 / 5 so as to maintain the level of the irradiated light 21 at I. ２ The drive control unit 13 is controlled so that the number of times of light emission is reduced to 1 / 5. ２ Instead of reducing the reflectance of the object 20 to 1 / 5, ２ may be reduced to

[0048] The function of the distance measuring device 10 to accommodate such changes in the reference distance increases the degree of freedom in the position at which the subject 20 is placed for calibration of the distance measuring device 10 .

[0049] The distance measuring device 10 according to the embodiment further has, as more detailed functions, a function for improving the accuracy of calibration and a function for increasing the speed of calibration, as will be described below.

[0050] 7A and 7B are diagrams illustrating cases where high-precision calibration is required. The diagram illustrates the distance measurement principle of a distance measuring device according to a modified example of the embodiment, which performs exposure sequences with four different phases (A0 exposure, A1 exposure, B0 exposure, and B1 exposure). More specifically, FIG. 7A illustrates the timing (i.e., phases) of the four types of exposure (A0 exposure, A1 exposure, B0 exposure, and B1 exposure), and FIG. 7B illustrates the combination of two exposures used for each distance section to be measured.

[0051] 7A, in this modified example, in addition to the two types of A exposure (A0 exposure and A1 exposure) similar to those in the above embodiment, two types of B exposure (B0 exposure and B1 exposure) are used for measuring longer distances. Both the B0 exposure and the B1 exposure are packets having an exposure section with the same time width as the light emission pulse, and the combined exposure section of the B0 exposure and the B1 exposure forms one continuous time section following the A1 exposure in terms of the relative elapsed time from the light emission pulse.

[0052] With these four types of exposure, the present modification has a three times wider distance measurement range than the above embodiment, as shown in Fig. 7B. In other words, by using A0 and A1 exposures for short distances, A1 and B0 exposures for medium distances, and B0 and B1 exposures for long distances, the distance measurement range is three times that of the above embodiment.

[0053] However, in such a distance measuring device according to the modified example, the combination of the two exposure types used for distance measurement switches depending on the distance to be measured (i.e., at the break of a packet), and therefore the tendency of the obtained calibration data (i.e., the pre-correction depth) may change. Therefore, the calibration unit 16 of the distance measuring device 10 according to the present embodiment has a function of improving accuracy in response to such change in the tendency of the calibration data at the break of a packet.

[0054] 8 is a diagram illustrating one of the functions for improving the accuracy of calibration provided by the distance measuring device 10 according to the embodiment. This diagram shows an example of a plot of the relationship between the pre-correction depth (vertical axis) and each phase (phase shift amount [step]) obtained when the intervals of the phase shift by the pulse phase sweep unit 16a are made shorter in some sections of the distance measurement range compared to other sections. As shown in this example, the accuracy of calibration is improved by acquiring more detailed calibration data and calculating correction values ​​at points where the trend of the calibration data (i.e., the pre-correction depth) changes, such as at the break between packets.

[0055] For this purpose, in the phase sweep, the phase intervals for acquiring calibration data in detail and the phase shift intervals in those phase intervals are set in the calibration unit 16 by phase or by an increase rate relative to the normal phase shift interval, etc. This allows the pulse phase sweep unit 16a to determine the phase shift intervals during the phase sweep according to the parameters set in the calibration unit 16 so that correction values ​​are calculated for some intervals within the ranging range of the distance measuring device 10 at finer distance intervals than for other intervals within the ranging range.

[0056] 9 is a diagram illustrating a second function for improving the accuracy of calibration provided in the distance measuring device 10 according to the embodiment. This diagram shows a flowchart illustrating the recalibration function provided by the calibration unit 16.

[0057] The calibration unit 16 first determines whether the correction values ​​calculated by the correction value calculation unit 16b satisfy the reference characteristics (S20). For example, the calibration unit 16 determines whether each of the three correction values ​​(non-linear correction value, slope correction value, and offset correction value) is within a corresponding predetermined allowable range.

[0058] As a result, if it is determined that at least one correction value does not satisfy the reference characteristics (No in S20), the calibration unit 16 adjusts the pulse width or timing (i.e., phase) of at least one of the light emission pulse and exposure pulse output from the drive control unit 13 via the pulse phase sweep unit 16a (S21), and then performs calibration again (S22). For example, if it is determined that at least one correction value does not satisfy the reference characteristics, the calibration unit 16 increases the pulse width of the light emission pulse and exposure pulse or shortens the interval of the phase shift during phase sweep, and then performs calibration again, until the correction value satisfies the reference characteristics or until a predetermined number of repetitions is reached. This ensures that the correction value calculated by calibration satisfies the reference characteristics, and high accuracy of the calibration can be ensured.

[0059] In the above recalibration, the method of determining whether the correction value satisfies the standard characteristics is based on the correction value itself, but the method is not limited to this, and it may be determined whether the correction value satisfies the standard characteristics using the corrected distance. For example, using the correction value determined in the first calibration (i.e., with the correction ON), phase sweep data (i.e., linearity data of the corrected distance) is acquired, and the R in the linearity data of the corrected distance is calculated. ２ If the value is less than the reference value, or if the maximum distance error of the linearity data of the corrected distance is greater than the reference value (for example, if the reference value of the distance error is 3% and the corrected distance when the phase is set to 300 mm is 312 mm (i.e., an error of 4%)), the drive pulse may be adjusted and calibration may be performed again.

[0060] Note that the function of improving the accuracy of calibration provided by the distance measuring device 10 according to the embodiment is not limited to the two methods described above. For example, the correction value calculation unit 16b may store multipath component estimation data, and calculate the correction value after removing the multipath component estimation data from the pre-correction depth obtained by the depth calculation unit 14. Specifically, a component ratio caused by multipath components in the pre-correction depth obtained by the depth calculation unit 14 is calculated in advance by actual measurement or the like, and the calculated ratio is stored in the correction value calculation unit 16b as multipath component estimation data. Then, during calibration, the correction value calculation unit 16b calculates three correction values ​​(a non-linear correction value, a slope correction value, and an offset correction value) after removing the multipath component estimation data from the pre-correction depth obtained from the depth calculation unit 14. This allows calibration to be performed taking multipath components into consideration, thereby ensuring high accuracy of the calibration.

[0061] 10 is a diagram illustrating one of the calibration speed-up functions of the distance measuring device 10 according to the embodiment. A graph is shown showing the change in temperature of the distance measuring device 10 (vertical axis) over time (horizontal axis) since the distance measuring device 10 was started. The graph also includes an explanation of how to change the number of times the light source 11 emits light for calibration.

[0062] In general, pre-correction depth is temperature dependent, so calibration should be performed after the temperature of the distance measuring device 10 has stabilized. If the driving conditions and specifications for light emission and exposure are the same, the saturation temperature of the distance measuring device 10 (i.e., the operating temperature of the distance measuring device 10) is determined by the number of light emissions. More specifically, the time required to reach the saturation temperature is inversely proportional to the number of light emissions.

[0063] Therefore, in the distance measuring device 10 according to the embodiment, after startup, the calibration unit 16 increases the number of light emissions for calibration by a predetermined number of times or by a predetermined rate of increase from a predetermined standard number of light emissions until a predetermined temperature is reached or until a predetermined time has elapsed, thereby shortening the time until the distance measuring device 10 reaches its saturation temperature.

[0064] The function of speeding up calibration provided in the distance measuring device 10 according to the embodiment is not limited to the above-described method. For example, the three correction values ​​(non-linear correction value, tilt correction value, and offset correction value) stored in the correction value storage unit 15 a may include a predetermined base correction value as a part thereof, and the calibration unit 16 may, in a predetermined case, update only the base correction value among the correction values ​​stored in the correction value storage unit 15 a during calibration.

[0065] For example, when calibrating a plurality of distance measuring devices 10 that have the same light emission and exposure operating conditions and specifications, the calibration unit 16 may update only the nonlinear correction value among the three correction values ​​stored in the correction value storage unit 15 a during calibration, thereby reducing the time required for calibration compared to updating all three correction values.

[0066] Furthermore, the calibration unit 16 may perform calibration to update the correction value only for a portion of the distance range of the distance measuring device 10. For example, the calibration unit 16 may perform calibration to update the nonlinear correction value only for a predetermined phase range in the phase sweep, a range of N steps before and after a predetermined phase in the phase sweep, a predetermined range of pre-correction depth, or a pre-correction depth within a range of M bits of the pre-correction depth (or M [%] of the pre-correction depth). This reduces the time required for calibration compared to updating the correction value for the entire distance measuring range.

[0067] Furthermore, the calibration unit 16 performs calibration for the base correction value among the three correction values ​​held in the correction value memory unit 15a using all of the pre-correction depths obtained corresponding to each phase of the exposure pulse in the calibration (i.e., each calibration distance), while for the correction values ​​other than the base correction value among the three correction values ​​held in the correction value memory unit 15a, calibration may be performed using some of the pre-correction depths obtained corresponding to each calibration distance.

[0068] For example, when calibrating multiple distance measuring devices 10 with the same light emission and exposure driving conditions and specifications, the nonlinear correction value (i.e., the base correction value) among the three correction values ​​is made common to the multiple distance measuring devices 10, and calibration is performed by updating only the tilt correction value and the offset correction value. At this time, the calibration unit 16 calibrates the tilt correction value and the offset correction value using calibration data (i.e., thinned pre-correction depth) acquired by a phase sweep at an interval larger than the phase shift interval used when calculating the base correction value (e.g., a phase shift interval at which data from two or three points can be acquired within the distance measurement range). As a result, the base correction value is calibrated using detailed calibration data, and the non-base correction values ​​are calibrated using coarse calibration data, and the detailed and coarse calibrations are merged, thereby shortening the time required for calibration compared to when only detailed calibration is performed.

[0069] In addition, in cases where the calibration unit 16 performs calibration on the two-dimensional pixels that make up the light receiving unit 12 and averages across multiple frames to obtain one pre-correction depth, in order to reduce the data measurement time during calibration, the calibration is performed with a small number of frames used to obtain one pre-correction depth and a wide measurement area (i.e., the pixel area to be calibrated), and then, if the accuracy of the distance output in the corrected ranging is worse than the standard, the number of frames used to obtain one pre-correction depth can be increased, the measurement area can be narrowed, and calibration can be performed again.

[0070] For example, in the first calibration, the average frame number is 5 and the measurement area is 20 x 20 pixels. If the output distance value (i.e., the corrected depth) [mm] subsequently deviates by 3% or more from the actual distance, the average frame number is changed to 20 and the measurement area is changed to 10 x 10 pixels for the second calibration, and then calibration is performed again. In this method, the total average pixel number subject to calibration is 2000 pixels in both the first and second cases. However, since the larger the average frame number, the less susceptible to random noise. Therefore, the narrower the measurement area, the less susceptible to differences in depth characteristics within the light-receiving surface of the light-receiving unit 12, and calibration can be performed with higher accuracy. However, the time required for calibration increases in proportion to the average frame number.

[0071] In addition, the calibration unit 16 may store temperature characteristic correction data in advance to correct the temperature dependency of the pre-correction depth, and perform calibration after correcting errors due to temperature dependency on the pre-correction depth calculated by the depth calculation unit 14 using the temperature characteristic correction data, thereby reducing the temperature waiting time required when starting calibration.

[0072] Furthermore, the calibration unit 16 may simultaneously calibrate a plurality of distance measuring devices 10 using the same subject by shifting the timing of light emission and exposure to prevent interference.

[0073] As described above, the distance measuring device 10 according to the embodiment is a distance measuring device 10 for measuring the distance to the subject 20, and includes a light source 11 that emits irradiation light 21 toward the subject 20 in accordance with an input light emission pulse, a light receiving unit 12 that receives reflected light 22 from the subject 20 in an exposure interval determined by the input exposure pulse, a drive control unit 13 that drives the light source 11 by outputting a light emission pulse to the light source 11 and drives the light receiving unit 12 by outputting an exposure pulse to the light receiving unit 12, a depth calculation unit 14 that calculates a depth corresponding to the time difference between when the irradiation light 21 is emitted from the light source 11 and when the reflected light 22 is received by the light receiving unit 12, based on the amount of reflected light 22 received by the light receiving unit 12, and a correction value calculation unit 15 that stores a correction value. The image processing device includes a correction value memory unit 15a that stores a correction value stored in the correction value memory unit 15a, a distance calculation unit 15 that calculates the distance to the subject 20 by correcting the depth calculated by the depth calculation unit 14 based on the correction value stored in the correction value memory unit 15a, and a calibration unit 16 that performs calibration to update the correction value. The calibration unit 16 includes a pulse phase sweep unit 16a that controls the drive control unit 13 so that a plurality of different phases are generated in sequence by changing the phase, which is the time difference between when a light emission pulse is output and when a light exposure pulse is output, and a correction value calculation unit 16b that calculates correction values ​​from the plurality of phases and the corresponding plurality of depths obtained by the depth calculation unit 14, and stores the calculated correction values ​​in the correction value memory unit 15a.

[0074] As a result, the phase sweep method according to the embodiment eliminates the need to move the object, which is required in conventional object movement methods, and distance measurement calibration is automated and completed in a short time. Furthermore, the problem of the calibration work being dependent on manual movement of the object is avoided.

[0075] Here, the correction value calculation unit 16b calculates, as correction values, a non-linear correction value for correcting linearity between the plurality of phases and the plurality of depths, and a tilt correction value and an offset correction value that specify a linear function for correcting the distance indicated by the depth after the linearity has been corrected, and the distance calculation unit 15 corrects the linearity of the depth calculated by the depth calculation unit 14 using the non-linear correction value, and calculates the distance to the subject 20 using the linear function specified by the tilt correction value and the offset correction value for the depth after the linearity has been corrected. In this way, the three correction values ​​are updated by calibration.

[0076] Furthermore, the pulse phase sweep unit 16a controls the drive control unit 13 so that a plurality of exposure pulses having the above-mentioned phases are output for one light emission pulse, thereby realizing calibration using a phase sweep method in which the timing of the exposure pulse is shifted in steps while the timing of the light emission pulse is fixed.

[0077] Furthermore, the subject 20 is placed at a position different from a reference distance, which is a distance to the subject 20 predetermined for calibration, and the pulse phase sweep unit 16a controls the drive control unit 13 to output a light emission pulse and an exposure pulse for each of the plurality of phases after shifting the phases by a time corresponding to the difference between the distance to the subject 20 and the reference distance, and the correction value calculation unit 16b adjusts the calculated correction value in accordance with the difference. This provides the distance measuring device 10 with a function that can accommodate changes in the reference distance, increasing the degree of freedom in the position at which the subject 20 is placed for calibration of the distance measuring device 10.

[0078] Furthermore, the pulse phase sweep unit 16a calculates the reflectance of the object 20 from the difference, and controls the drive control unit 13 to output a light emission pulse and an exposure pulse according to the calculated reflectance. This keeps the amount of received light at the same level even when the reference distance is changed, and increases the degree of freedom in the position where the object 20 is placed for calibration.

[0079] Furthermore, the pulse phase sweep unit 16a calculates the number of times that the irradiation light 21 is emitted for calibration from the difference for each of the plurality of phases, and controls the drive control unit 13 so that the irradiation light 21 is emitted for the calculated number of times from the light source 11. The calibration unit 16 then performs calibration for each of the plurality of phases using the irradiation light 21 for the calculated number of times. This maintains the amount of received light at the same level even when the reference distance is changed, and increases the degree of freedom in the position at which the subject 20 is placed for calibration.

[0080] Furthermore, the pulse phase sweep unit 16a determines the above-mentioned multiple phases so that correction values ​​are calculated at finer distance intervals for some sections within the ranging range of the ranging device 10 than for other sections within the ranging range. As a result, at locations where the tendency of the calibration data changes, such as at breaks in packets, more detailed calibration data is acquired and correction values ​​are calculated, thereby improving the accuracy of calibration.

[0081] Furthermore, if the correction value calculated by the correction value calculation unit 16b does not satisfy the reference characteristics, the calibration unit 16 performs calibration again after adjusting the pulse width or timing of at least one of the light emission pulse and the exposure pulse output from the drive control unit 13. This maintains a highly accurate correction value and achieves highly accurate distance measurement.

[0082] The correction value calculation unit 16b also holds multipath component estimation data, and calculates the correction value after removing the multipath component estimation data from the depth obtained by the depth calculation unit 14. This allows calibration to be performed taking multipath components into consideration, ensuring high accuracy of the calibration.

[0083] Furthermore, the correction values ​​stored in the correction value storage unit 15 a include a predetermined base correction value as a part thereof, and the calibration unit 16 updates only the base correction value among the correction values ​​stored in the correction value storage unit 15 a through calibration. This allows calibration to be performed taking multipath components into consideration, and high accuracy of the calibration can be ensured.

[0084] Furthermore, the calibration unit 16 performs calibration to update the correction values ​​for only some distance sections within the ranging range of the ranging device 10. This reduces the time required for calibration compared to updating the correction values ​​for all sections of the ranging range.

[0085] Furthermore, the calibration unit 16 performs calibration for the base correction value among the correction values ​​held in the correction value storage unit 15a using all of the depths calculated by the depth calculation unit 14 corresponding to the multiple time differences, and performs calibration for the correction values ​​held in the correction value storage unit 15a other than the base correction value using some of the depths calculated by the depth calculation unit 14 corresponding to the multiple time differences. This results in merging the detailed calibration and the coarse calibration, and reduces the time required for calibration compared to when only the detailed calibration is performed.

[0086] Furthermore, the pulse phase sweep unit 16a controls the drive control unit 13 so that the light source 11 emits the irradiation light 21 a plurality of times in each of the plurality of phases, and controls the drive control unit 13 so that the number of times of emission is greater in a first predetermined time period after startup of the distance measuring device 10 than in a second predetermined time period following the first predetermined time period. This shortens the time required for the distance measuring device 10 to reach a saturation temperature, and shortens the time required for calibration.

[0087] Furthermore, a calibration method using a distance measuring device according to the embodiment is a method for calibrating a distance measuring device 10 that measures the distance to an object 20, and the distance measuring device 10 includes a light source 11 that emits irradiation light 21 toward the object 20 in accordance with an input light emission pulse, a light receiving unit 12 that receives reflected light 22 from the object 20 in an exposure section determined by the input exposure pulse, a drive control unit 13 that drives the light source 11 by outputting a light emission pulse to the light source 11 and drives the light receiving unit 12 by outputting an exposure pulse to the light receiving unit 12, a depth calculation unit 14 that calculates a depth corresponding to the time difference between when the irradiation light 21 is emitted from the light source 11 and when the reflected light 22 is received by the light receiving unit 12, based on the amount of reflected light 22 received by the light receiving unit 12, and a correction value storage unit 15 that stores a correction value. The calibration method includes a calibration step of performing calibration to update the correction value, and the calibration step includes a pulse phase sweep step of controlling the drive control unit 13 so that a plurality of different phases are generated in sequence by changing the phase, which is the time difference between the output of a light emission pulse and the output of an exposure pulse, and a correction value calculation step of calculating a correction value from the plurality of phases and a plurality of corresponding depths obtained by the depth calculation unit 14, and storing the calculated correction value in the correction value memory unit 15a.

[0088] As a result, the phase sweep method according to the embodiment eliminates the need to move the object, which is required in conventional object movement methods, and distance measurement calibration is automated and completed in a short time. Furthermore, the problem of the calibration work being dependent on manual movement of the object is avoided.

[0089] While the distance measuring device and the distance measuring device calibration method according to the present disclosure have been described above based on the embodiments, the present disclosure is not limited to these embodiments. As long as they do not deviate from the gist of the present disclosure, various modifications conceivable by those skilled in the art to the present embodiments and other forms constructed by combining some of the components of the embodiments are also included within the scope of the present disclosure.

[0090] For example, in the above embodiment, the functions for increasing the accuracy of calibration and the functions for increasing the speed of calibration have been described using the figures from FIG. 7 onward, but these functions for increasing the accuracy and the speed are not essential. Furthermore, one or more functions arbitrarily selected from these functions for increasing the accuracy and the speed may be implemented in the distance measuring device 10.

[0091] In addition, the calibration method for a distance measuring device according to the present disclosure may be realized as a program or program product that causes a computer to execute the steps included in the calibration method, or as a non-transitory computer-readable recording medium on which the program is recorded.

[0092] The present disclosure can be used as a TOF distance measuring device with a calibration function, in particular as a distance measuring device that can calibrate distance measurement without moving the subject in the depth direction, for example, as a distance measuring device mounted on a vehicle, a distance measuring device mounted on a mobile terminal, etc.

[0093] REFERENCE SIGNS LIST 10 Distance measuring device 11 Light source 12 Light receiving unit 13 Drive control unit 14 Depth calculation unit 15 Distance calculation unit 15a Correction value storage unit 16 Calibration unit 16a Pulse phase sweep unit 16b Correction value calculation unit 20 Object 21 Irradiation light 22 Reflected light

Claims

1. A distance measuring device for measuring the distance to an object, comprising: a light source that emits illumination light toward the object in accordance with an input illumination pulse; a light receiving unit that receives reflected light from the object in an exposure interval determined by the input exposure pulse; a drive control unit that drives the light source by outputting the illumination pulse to the light source and drives the light receiving unit by outputting the exposure pulse to the light receiving unit; a depth calculation unit that calculates a depth corresponding to the time difference between when illumination light is emitted from the light source and when the reflected light is received by the light receiving unit based on the amount of reflected light received by the light receiving unit; a distance calculation unit that has a correction value memory unit that stores correction values and calculates the distance to the object by correcting the depth calculated by the depth calculation unit based on the correction values stored in the correction value memory unit; and a calibration unit that performs calibration to update the correction values, wherein the calibration unit has: a pulse phase sweep unit that controls the drive control unit to change the phase, which is the time difference between when the illumination pulse is output and when the exposure pulse is output, so that a plurality of different phases are generated in sequence; a correction value calculation unit that calculates a correction value from the plurality of phases and the corresponding plurality of depths obtained by the depth calculation unit, and stores the calculated correction value in the correction value storage unit.

2. The distance measuring device of claim 1, wherein the correction value calculation unit calculates, as the correction values, a non-linear correction value for correcting linearity between the multiple phases and the multiple depths, a slope correction value that specifies a linear function for correcting the distance indicated by the depth after linearity has been corrected, and an offset correction value, and the distance calculation unit corrects linearity for the depth calculated by the depth calculation unit using the non-linear correction value, and calculates the distance to the subject using the linear function specified by the slope correction value and the offset correction value for the depth after linearity has been corrected.

3. The distance measuring device according to claim 1, wherein the pulse phase sweep unit controls the drive control unit so that a plurality of exposure pulses having a plurality of time differences are output in response to one light emission pulse.

4. The distance measuring device of claim 1, wherein the subject is placed at a position different from a reference distance, which is a distance to the subject that is predetermined for the calibration, the pulse phase sweep unit controls the drive control unit to output the light emission pulse and the exposure pulse for each of the multiple phases after shifting them by a time equivalent to the difference between the distance to the subject and the reference distance, and the correction value calculation unit adjusts the correction value it calculates in accordance with the difference.

5. The distance measuring device according to claim 4, wherein the pulse phase sweep unit calculates the reflectance of the subject from the difference, and controls the drive control unit to output the light emission pulse and the exposure pulse according to the calculated reflectance.

6. The distance measuring device of claim 4, wherein the pulse phase sweep unit calculates the number of times that the illumination light is emitted for the calibration from the difference for each of the plurality of phases, and controls the drive control unit so that the illumination light for the calculated number of times is emitted from the light source, and the calibration unit performs the calibration using the illumination light for the calculated number of times for each of the plurality of phases.

7. The distance measuring device according to claim 1, wherein the pulse phase sweep unit determines a plurality of the phases for a portion of the distance measuring range of the distance measuring device so that the correction value is calculated at finer distance intervals than for other portions of the distance measuring range.

8. The distance measuring device according to claim 2, wherein, if the correction value calculated by the correction value calculation unit does not satisfy the reference characteristics, the calibration unit adjusts the pulse width or timing of at least one of the light emission pulse and the exposure pulse output from the drive control unit and then performs calibration again.

9. The distance measuring device according to claim 2, wherein the correction value calculation unit holds multipath component estimation data, and in calculating the correction value, the multipath component estimation data is removed from the depth obtained by the depth calculation unit before calculating the correction value.

10. A distance measuring device as described in claim 1, wherein the correction values stored in the correction value memory unit include a predetermined base correction value as a part thereof, and the calibration unit updates only the base correction value among the correction values stored in the correction value memory unit by the calibration.

11. The distance measuring device according to claim 1, wherein the calibration unit performs calibration to update the correction value only for a part of the distance range of the distance measuring device.

12. The distance measuring device of claim 1, wherein the calibration unit performs calibration for a base correction value among the correction values stored in the correction value memory unit using all of the depths calculated by the depth calculation unit corresponding to the multiple phases, and performs calibration for correction values stored in the correction value memory unit other than the base correction value using some of the depths calculated by the depth calculation unit corresponding to the multiple phases.

13. The distance measuring device of claim 1, wherein the pulse phase sweep unit controls the drive control unit so that the light source emits irradiation light a plurality of times at each of the plurality of phases, and controls the drive control unit so that the number of times of emission is greater during a first predetermined time period after startup of the distance measuring device than during a second predetermined time period following the first predetermined time period.

14. A method for calibrating a distance measuring device that measures the distance to an object, the distance measuring device comprising: a light source that emits illumination light toward the object in accordance with an input light emission pulse; a light receiving unit that receives reflected light from the object in an exposure interval determined by the input exposure pulse; a drive control unit that drives the light source by outputting the light emission pulse to the light source and drives the light receiving unit by outputting the exposure pulse to the light receiving unit; a depth calculation unit that calculates a depth corresponding to the time difference between when the illumination light is emitted from the light source and when the reflected light is received by the light receiving unit based on the amount of reflected light received by the light receiving unit; and a distance calculation unit that has a correction value storage unit that stores a correction value and calculates the distance to the object by correcting the depth calculated by the depth calculation unit based on the correction value stored in the correction value storage unit; the calibration method includes a calibration step that performs calibration to update the correction value, the calibration step including: a pulse phase sweep step that controls the drive control unit to change the phase, which is the time difference between when the light emission pulse is output and when the exposure pulse is output, so that a plurality of different phases are generated in sequence; a correction value calculation step of calculating a correction value from the plurality of phases and the corresponding plurality of depths obtained by the depth calculation unit, and causing the distance calculation unit to store the calculated correction value.

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