Calibration tools and methods for 2D LiDAR sensors

Abstract

In a forklift equipped with a 2D LiDAR sensor for detecting the orientation of a pallet, the calibration of the 2D LiDAR sensor is performed appropriately. [Solution] The calibration tool 10 for a two-dimensional LiDAR sensor 104 for detecting the orientation of a pallet located in front of a forklift 100 comprises a target plate 12 to which laser light from the two-dimensional LiDAR sensor 104 is irradiated, and a support portion 14 for supporting the target plate 12 perpendicularly to the upper surface 102a of the forks 102 of the forklift 100. The target plate 12 is configured to reflect laser light toward the two-dimensional LiDAR sensor 104 over a predetermined range in the width direction of the target plate only when the laser light from the two-dimensional LiDAR sensor 104 is scanned parallel to the upper surface 102a of the forks 102 at a predetermined height from the upper surface 102a of the forks 102.

Description

Technical Field

[0001] The present invention relates to a calibration tool and a calibration method for a two-dimensional LiDAR sensor.

Background Art

[0002] Conventionally, an automated guided forklift (AGF) configured to be able to detect the posture of a pallet by mounting a two-dimensional (2D)-LiDAR (Light Detction And Ranging) sensor has been known (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the above technology, it is necessary to calibrate the two-dimensional LiDAR sensor so that the measurement plane of the two-dimensional LiDAR sensor is parallel to the upper surface of the fork. However, since the laser light used by the two-dimensional LiDAR sensor is usually non-visible light such as near-infrared light, it is difficult for an operator to perform calibration while visually observing the laser beam emitted by the two-dimensional LiDAR sensor with the naked eye.

[0005] The present invention has been made in view of such a situation, and an object thereof is to provide a technique for suitably calibrating a two-dimensional LiDAR sensor in a forklift equipped with the two-dimensional LiDAR sensor for detecting the posture of a pallet.

Means for Solving the Problems

[0006] To solve the above problems, a calibration tool according to one aspect of the present invention is a calibration tool for a two-dimensional LiDAR sensor for detecting the orientation of a pallet located in front of a forklift, comprising: a target plate onto which laser light from the two-dimensional LiDAR sensor is irradiated; and a support portion for supporting the target plate perpendicularly to the upper surface of the forks of the forklift. The target plate is configured to reflect laser light toward the two-dimensional LiDAR sensor over a predetermined range in the width direction of the target plate only when the laser light from the two-dimensional LiDAR sensor is scanned parallel to the upper surface of the forks at a predetermined height from the upper surface of the forks.

[0007] Another aspect of the present invention is a method for calibrating a two-dimensional LiDAR sensor for detecting the orientation of a pallet located in front of a forklift. This method comprises the steps of: supporting the target plate of the calibration tool described above perpendicularly to the upper surface of the forks of the forklift; detecting the target plate using a two-dimensional LiDAR sensor; and adjusting the optical axis of the two-dimensional LiDAR sensor based on the detection result of the target plate. [Effects of the Invention]

[0008] According to the present invention, in a forklift equipped with a two-dimensional LiDAR sensor for detecting the orientation of a pallet, the calibration of the two-dimensional LiDAR sensor can be performed effectively. [Brief explanation of the drawing]

[0009] [Figure 1] This is a perspective view taken from the front at an angle, showing the calibration tool for the 2D LiDAR sensor according to the embodiment mounted on the forks of a forklift. [Figure 2] This is a perspective view from the rear at an angle, showing the calibration tool for the 2D LiDAR sensor according to the embodiment mounted on the forks of a forklift. [Figure 3] Figures 3(a) and 3(b) are diagrams illustrating the target plate in detail. [Figure 4] This is a flowchart illustrating the calibration method for a 2D LiDAR sensor using a calibration tool. [Figure 5] This figure shows the spread of the light beam emitted by a 2D LiDAR sensor. [Figure 6] This figure shows a target plate related to a modified example. [Modes for carrying out the invention]

[0010] In the following, identical or equivalent components and members shown in each drawing will be denoted by the same reference numeral, and redundant explanations will be omitted as appropriate. Furthermore, the dimensions of the members in each drawing will be enlarged or reduced as appropriate for ease of understanding. Additionally, some members that are not important for explaining the embodiment will be omitted from the drawings.

[0011] Figure 1 is a perspective view from the front at an angle, showing the calibration tool 10 for the 2D LiDAR sensor according to the embodiment mounted on the fork 102 of the forklift 100, and Figure 2 is a perspective view of the same state from the rear at an angle.

[0012] For the sake of clarity, the following explanation assumes an XYZ Cartesian coordinate system. The X-axis direction is horizontal. The Z-axis direction is vertical. The Y-axis direction is perpendicular to the X and Y axes. In Figures 1 and 2, the positive X-axis direction may be referred to as "forward," the negative X-axis direction as "backward," the positive Z-axis direction as "up," the negative Z-axis direction as "down," the positive Y-axis direction as "right," and the negative Y-axis direction as "left."

[0013] The forklift 100 may be an automated guided vehicle (AGF) used in factories and warehouses. The forklift 100 has two forks 102 extending forward (in the positive X-axis direction) from its main body. The forks 102 are supported by a mast, which can be used to move the forks 102 up and down. When the forklift 100 transports goods placed on a pallet, the forks 102 are inserted into the openings of the pallet.

[0014] The forklift 100 is equipped with a two-dimensional LiDAR sensor 104 to detect the orientation of a pallet located in front of it. In this embodiment, the two-dimensional LiDAR sensor 104 is located between the bases of two forks 102. The two-dimensional LiDAR sensor measures the distance to the object to be measured by scanning a laser beam horizontally and receiving the reflected laser beam. Since the two-dimensional LiDAR sensor scans the laser beam horizontally from one point over a predetermined angle, the measurement plane 106 of the two-dimensional LiDAR sensor 104 is a sector-shaped plane, as shown in Figures 1 and 2.

[0015] The measurement plane 106 of the 2D LiDAR sensor 104 needs to be set parallel to the upper surface 102a of the fork 102. Calibration (optical axis adjustment) of the 2D LiDAR sensor 104 is necessary to set the measurement plane 106 in this way. However, since the laser light used by the 2D LiDAR sensor 104 is usually invisible light such as near-infrared light, it is difficult for an operator to perform calibration while viewing the laser beam emitted from the 2D LiDAR sensor 104 with the naked eye. The calibration tool 10 according to this embodiment can suitably calibrate the 2D LiDAR sensor 104 even when the laser light of the 2D LiDAR sensor 104 is invisible light.

[0016] The calibration tool 10 includes a target plate 12 onto which laser light from a 2D LiDAR sensor 104 is irradiated, a support part 14 for vertically supporting the target plate 12 on the upper surface 102a of the fork 102, and a parallel adjustment member 16 for making the target plate 12 and the front surface 102b of the fork 102 parallel.

[0017] The target plate 12 is a substantially rectangular plate-like body extending in the width direction (Y-axis direction) of the fork 102. The target plate 12 is disposed across two forks 102. As shown in FIGS. 1 and 2, the target plate 12 is provided with a plurality (six) of holes 18 (first hole 18a to sixth hole 18f). The holes 18 are elongated holes in the Y-axis direction. The first hole 18a, the third hole 18c, the fourth hole 18d, and the fifth hole are through holes, and the second hole 18b and the sixth hole 18f are notch holes.

[0018] When the target plate 12 is viewed from the two-dimensional LiDAR sensor 104, the first hole 18a is provided at the lower left end portion of the target plate 12, the second hole 18b is provided at the upper left end portion of the target plate 12, the third hole 18c is provided at the upper portion intermediate between the width center and the left end of the target plate 12, the fourth hole 18d is provided at the upper portion intermediate between the width center and the right end of the target plate 12, the fifth hole 18e is provided at the lower right end portion of the target plate 12, and the sixth hole 18f is provided at the upper right end portion of the target plate 12. The first hole 18a to the third hole 18c and the fourth hole 18d to the sixth hole 18f are arranged symmetrically left and right.

[0019] The portions of the target plate 12 other than the six holes 18 are configured to reflect the laser light from the two-dimensional LiDAR sensor 104. On the other hand, the six holes 18 do not reflect the laser light from the two-dimensional LiDAR sensor 104 and allow it to pass through. Therefore, the two-dimensional LiDAR sensor 104 can detect the portions of the target plate 12 other than the holes 18, but cannot detect the holes 18. By checking which portions of the target plate 12 can be detected and which portions cannot be detected by the two-dimensional LiDAR sensor 104, it is possible to determine whether the measurement plane 106 of the two-dimensional LiDAR sensor 104 is parallel to the upper surface 102a of the fork 102 or inclined.

[0020] FIG. 3(a) and (b) are diagrams for explaining the target plate 12 in detail. FIG. 3(a) is a schematic view of the target plate 12 as seen from the two-dimensional LiDAR sensor 104. FIG. 3(b) is a diagram showing the detection result by the two-dimensional LiDAR sensor 104. In FIG. 3(b), "1" indicates that the target plate 12 is detected. Also, "0" indicates that an object farther than the target plate 12 is detected or nothing is detected. Also, "2" indicates that an object closer than the target plate 12 is detected.

[0021] In FIG. 3(a), the width (length in the Y-axis direction) of the target plate 12 is 800 mm, and the height (length in the X-axis direction) of the target plate 12 is 50 mm. As described above, six hole portions 18 (first hole portion 18a to sixth hole portion 18f) are provided in the target plate 12.

[0022] As shown in Figure 3(a), below the first hole 18a and the fifth hole 18e, there is a portion of the target plate 12 without holes, running parallel to the upper surface 102a of the fork 102, across its entire width. The scan line SL2 shown in Figure 3(a) is irradiated onto this portion without holes. The laser light from the 2D LiDAR sensor 104 is reflected back towards the 2D LiDAR sensor 104 across its entire width only when the laser light from the 2D LiDAR sensor 104 is scanned parallel to the upper surface 102a of the fork 102 at a predetermined height, as shown by the scan line SL2. At this time, as shown by SL2 in Figure 3(b), the detection result is "1" across the entire width of the target plate 12, meaning the target plate 12 can be detected across its entire width. In such cases, it can be determined that the scan line SL2 is in the correct position, that is, that the measurement plane 106 of the 2D LiDAR sensor 104 is parallel to the upper surface 102a of the fork 102. In this embodiment, the target plate 12 is configured so that laser light is reflected toward the 2D LiDAR sensor 104 across its entire width. However, it is sufficient if the target plate 12 is configured so that laser light is reflected toward the 2D LiDAR sensor 104 across a predetermined range in the width direction of the target plate 12.

[0023] The scan line SL1 shown in Figure 3(a) is parallel to the upper surface 102a of the fork 102, but is deviated downwards from the scan line SL2. In this case, as shown in SL1 of Figure 3(b), the detection result for the part of the fork 102 that is illuminated is "2", and the detection result for the rest of the part that is outside the target plate 12 is "0". When such detection results are obtained, it can be determined that the measurement plane 106 of the 2D LiDAR sensor 104 is parallel to the upper surface 102a of the fork 102, but is deviated downwards from the correct height, so the optical axis should be adjusted so that the measurement plane 106 is raised.

[0024] The scan line SL4 shown in Figure 3(a) is parallel to the upper surface 102a of the fork 102, but is deviated upward from the scan line SL2. In this case, the scan line SL4 overlaps with one of the holes 18, and the laser light passes through the hole 18. As shown in SL4 of Figure 3(b), the detection result is "0" for the part where the laser light passes through the hole 18, and "1" for the other parts where the laser light is reflected. When such a detection result is obtained, it can be determined that the measurement plane 106 of the 2D LiDAR sensor 104 is parallel to the upper surface 102a of the fork 102, but is deviated upward from the correct height. Therefore, the optical axis should be adjusted so that the measurement plane 106 is lowered.

[0025] The scan line SL3 shown in Figure 3(a) is slightly inclined upward to the right with respect to the upper surface 102a of the fork 102. In this case, the scan line SL3 overlaps with the fifth hole 18e, and the laser light passes through the fifth hole 18e. As shown in SL3 of Figure 3(b), the detection result is "0" for the part where the laser light has passed through the fifth hole 18e, and "1" for the rest of the part where the laser light has been reflected. When such a detection result is obtained, it can be determined that the measurement plane 106 of the 2D LiDAR sensor 104 is slightly inclined upward to the right with respect to the upper surface 102a of the fork 102. Therefore, the optical axis should be adjusted so that the right side of the measurement plane 106 is slightly lower.

[0026] The scan line SL5 shown in Figure 3(a) is significantly inclined upward to the right with respect to the upper surface 102a of the fork 102. In this case, the scan line SL5 overlaps with the first hole 18a, the fourth hole 18d, and the sixth hole 18f, and the laser light passes through them. As shown in SL5 in Figure 3(b), the detection result is "0" in the areas where the laser light has passed through, and "1" in the other areas where the laser light has been reflected. When such a detection result is obtained, it can be determined that the measurement plane 106 of the 2D LiDAR sensor 104 is significantly inclined upward to the right with respect to the upper surface 102a of the fork 102. Therefore, the optical axis should be adjusted so that the right side of the measurement plane 106 is significantly lower.

[0027] In this way, the area where the laser beam from the 2D LiDAR sensor 104 is not reflected differs depending on the degree of inclination of the fork 102 with respect to the upper surface 102a. Therefore, by checking the detection results from the 2D LiDAR sensor 104, it is possible to determine what kind of optical axis adjustment is necessary.

[0028] Figure 4 is a flowchart illustrating the calibration method for the 2D LiDAR sensor 104 using the calibration tool 10. First, the calibration tool 10 described above is prepared (S10).

[0029] Next, the target plate 12 of the calibration tool 10 is supported vertically on the upper surface 102a of the fork 102 (S12).

[0030] Next, the target plate 12 is detected by scanning the laser beam with the 2D LiDAR sensor 104 and receiving the reflected laser light (S14).

[0031] Next, the optical axis of the 2D LiDAR sensor 104 is adjusted based on the detection results of the target plate 12 (S16). Specific examples of optical axis adjustment are shown in Figures 3(a) and (b) as described above.

[0032] The calibration tool 10 and calibration method for the 2D LiDAR sensor 104 according to this embodiment have been described above. According to the calibration tool 10 according to this embodiment, the 2D LiDAR sensor 104 can be suitably calibrated using a simple procedure.

[0033] In the above-described embodiment, six holes 18 were formed in the target plate 12, but the number and arrangement of the holes 18 are not limited to the illustrated embodiment.

[0034] Furthermore, in the above-described embodiment, a portion that does not reflect laser light toward the 2D LiDAR sensor 104 is configured by providing holes 18 in the target plate 12. However, instead of providing holes 18 in the target plate 12, the reflected laser light may be attenuated by reducing the reflectivity of a part of the target plate 12.

[0035] The shape and size of the portion that reflects laser light toward the 2D LiDAR sensor 104 over a predetermined range and the holes 18 formed in the target plate 12 should be appropriately set considering the spread of the laser beam from the 2D LiDAR sensor 104.

[0036] Figure 5 shows the spread of the light beam emitted by the 2D LiDAR sensor 104. Note that the spread of the light beam in Figure 5 is exaggerated for illustrative purposes. In the case of an ideal laser beam without light beam spread, the scan line on the target plate 12 would be a straight line. However, in reality, as shown in Figure 5, light beam spread occurs, and the width of the scan line on the target plate 12 may be narrowest in the center and wider towards the edges. When such light beam spread exists, even when the scan line is within the allowable range of the pitch angle, as in scan line SL2 in Figure 3, light escapes from the first hole 18a at the lower left end of the target plate 12 and the fifth hole 18e at the lower right end of the target plate 12. As a result, the detection result by the 2D LiDAR sensor 104 becomes "0", and there is a risk that the inclination of the measurement plane of the 2D LiDAR sensor 104 cannot be accurately determined. To address this, simply widening the width of the target plate 12 would result in the detection result of the 2D LiDAR sensor 104 being "1" even if the scan line is outside the acceptable range of the pitch angle in the center.

[0037] Figure 6 shows a modified target plate 112. In order to accommodate the spread of light rays, this target plate 112 has a strip-shaped portion 114 located at the bottom of the target plate 112 that is detected linearly by the 2D LiDAR sensor 104 only when the measurement plane of the 2D LiDAR sensor 104 is parallel to the top surface of the fork. This strip-shaped portion 114 is shaped like a "bow tie" when viewed from the front (narrower in the center and wider towards the ends). This bow tie-shaped strip portion 114 is formed by angling the bottom edge of the target plate 112 toward the center, and by angling the bottom edges of the first hole 18a and the fifth hole 18e. By shaping the strip portion 114 in a way that corresponds to the spread of light rays, the inclination of the measurement plane of the 2D LiDAR sensor 104 can be determined more accurately.

[0038] The target plate 112 shown in Figure 6 is further equipped with two protrusions 116 at the bottom center. If the pitch angle of the 2D LiDAR sensor 104 is shifted downwards, it may not be possible to clearly determine the gap between the fork 102 and the target plate. This is because, regarding the space that should be measured at a distance greater than the target plate, a value close to the distance to the target plate is detected, resulting in the determination that the target plate is present. To avoid this situation, the target plate 112 has a characteristic shape (in this case, two protrusions 116) in the central part of the target plate 112 where the light beam spread is small. When the pitch angle is shifted downwards, this characteristic shape (two protrusions 116) is detected. Once the characteristic shape is found, it can be determined that the pitch angle is shifted downwards, regardless of the detection results of other parts. Considering the resolution of the 2D LiDAR sensor 104, it is desirable that the width of the protrusions 116 be greater than the minimum required width, and that the distance between the two protrusions 116 be greater than the minimum required distance. Furthermore, it is desirable that the height of the projection 116 be at least the minimum required height, taking into account the spread of the light rays.

[0039] Any combination of the embodiments and modifications described above is also useful as an embodiment of the present invention. The new embodiments resulting from these combinations possess the effects of both the combined embodiments and modifications. [Explanation of Symbols]

[0040] 10 Calibration tool, 12, 112 Target plate, 14 Support part, 16 Parallel adjustment member, 18 Hole, 100 Forklift, 102 Fork, 104 2D LiDAR sensor, 106 Measurement plane, 116 Protrusion.

Claims

1. A calibration tool for a two-dimensional LiDAR sensor for detecting the orientation of a pallet located in front of a forklift, A target plate onto which laser light from the two-dimensional LiDAR sensor is irradiated, A support portion for vertically supporting the target plate on the upper surface of the forks of the forklift, Equipped with, A calibration tool characterized in that the target plate is configured to reflect laser light toward the two-dimensional LiDAR sensor over a predetermined range in the width direction of the target plate only when the laser light from the two-dimensional LiDAR sensor is scanned parallel to the upper surface of the fork at a predetermined height from the upper surface of the fork.

2. The calibration tool according to claim 1, characterized in that the target plate is configured such that, when the laser beam from the two-dimensional LiDAR sensor is scanned at an angle relative to the upper surface of the fork, a portion of the target plate does not reflect or attenuates the reflection of the laser beam toward the two-dimensional LiDAR sensor.

3. The calibration tool according to claim 2, characterized in that the target plate is configured such that the region that does not reflect laser light toward the two-dimensional LiDAR sensor differs depending on the degree of inclination of the laser light from the two-dimensional LiDAR sensor with respect to the upper surface of the fork.

4. The calibration tool according to claim 2 or 3, characterized in that the holes provided in the target plate constitute a portion that does not reflect or attenuates the reflection of laser light toward the two-dimensional LiDAR sensor.

5. The calibration tool according to claim 2, characterized in that the shape and size of the portion that reflects laser light toward the two-dimensional LiDAR sensor over the predetermined range and the portion that does not reflect laser light toward the two-dimensional LiDAR sensor or attenuates the reflection are set taking into consideration the spread of the laser beam from the two-dimensional LiDAR sensor.

6. The calibration tool according to claim 1, further comprising a parallel adjustment member for making the target plate and the front surface of the fork parallel.

7. A calibration method for a two-dimensional LiDAR sensor for detecting the orientation of a pallet located in front of a forklift, The steps of supporting the target plate of the calibration tool according to claim 1 vertically on the upper surface of the forks of the forklift, The steps include detecting the target plate using the two-dimensional LiDAR sensor, The steps include adjusting the optical axis of the two-dimensional LiDAR sensor based on the detection result of the target plate, A calibration method characterized by comprising the following:

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