Cargo handling control device of forklift truck

[0001] This application claims priority to Japanese Patent Application No. 2024-001289 filed on Jan. 9, 2024, the entire disclosure of which is incorporated herein by reference.

[0002] The present disclosure relates to a cargo handling control device of a forklift truck.

[0003] A known cargo handling control device of a forklift truck is mentioned in Japanese Patent Application Publication No. 2023-110711. This known cargo handling control device includes a detection sensor that is disposed at a proximal portion of an insertion part of a fork and configured to detect contact between the fork and the first upper surface of a pallet that faces the fork, and a controller that is configured to control a cargo handling device so that the cargo handling device lowers the fork to place the pallet on a loading surface and then pulls the fork out of the pallet. When the detection sensor detects that the fork has moved away from the first upper surface while the cargo handling device is lowering the fork, the controller stops the fork and controls so that the fork tilts forward until a tilt angle of the fork reaches a limit value. If the tilt angle of the fork no longer change before reaching the limit value, the controller controls so that the fork tilts backward.

[0004] According to the known cargo handling control device, the fork tilts forward and then tilt backward to load the pallet on the loading surface, even when the pallet is not tilted. It may take time to place the pallet.

[0005] The present disclosure is directed to providing a cargo handling control device of a forklift truck capable of reducing time to place a pallet.

[0006] In accordance with an aspect of the present disclosure, there is provided a cargo handling control device of a forklift truck. The forklift truck includes a lift cylinder configured to raise and lower a fork holding a pallet and a tilt cylinder configured to tilt the fork. The cargo handling control device includes: a first detector; a second detector; a third detector; a fork lowering controller; a pallet posture determination unit; a pallet placement controller; and a fork withdrawal controller. The first detector is configured to detect whether a distal portion of the fork inserted into a pallet hole of the pallet is adjacent to an upper wall of the pallet. The first detector is configured to output a detection signal. The second detector is configured to detect whether the distal portion of the fork inserted into the pallet hole of the pallet is adjacent to a lower wall of the pallet. The second detector is configured to output a detection signal. The third detector is configured to detect whether a proximal portion of the fork inserted into the pallet hole of the pallet is in contact with the upper wall of the pallet. The third detector is configured to output a detection signal. The fork lowering controller is configured to control the lift cylinder so that the lift cylinder lowers the fork holding the pallet toward a loading surface. The pallet posture determination unit is configured to determine a posture of the pallet relative to the fork based on the detection signals from the first detector, the second detector, and the third detector, after the fork lowering controller starts controlling the lift cylinder. The pallet placement controller is configured to control the lift cylinder and the tilt cylinder so that the fork is placed on the loading surface depending on the posture of the pallet relative to the fork determined by the pallet posture determination unit. The fork withdrawal controller is configured to control the forklift truck so that the forklift truck pulls the fork out of the pallet after the pallet placement controller completes controlling the lift cylinder and the tilt cylinder. The pallet placement controller judges based on the posture of the pallet whether the fork is allowed to be pulled out of the pallet. When the pallet placement controller judges that the fork is allowed to be pulled out of the pallet, the pallet placement controller controls the lift cylinder so that the lift cylinder stops lowering the fork. When the pallet placement controller judges that the fork is not allowed to be pulled out of the pallet, the pallet placement controller controls the tilt cylinder so that tilt cylinder tilts the fork along the loading surface.

[0007] Other aspects and advantages of the disclosure will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the disclosure.

[0022] The following will describe an embodiment of the present disclosure in detail with reference to the accompanying drawings.

[0023] FIG. 1 is a block diagram of a cargo handling control device of a forklift truck according to an embodiment of the present disclosure. In FIG. 1, a cargo handling control device 1 according to a present embodiment is mounted on a forklift truck 2, such as a counterbalance forklift truck. As illustrated in FIG. 2, the forklift truck 2 includes a mast 3, and a right-left pair of forks 6 that is mounted to the mast 3 via a spacer 4 and movable up and down. The forks 6 hold a pallet 5.

[0024] The forklift truck 2 includes a lift cylinder 7 configured to raise or lower the forks 6, and a tilt cylinder 8 configured to tilt the forks 6 by tilting the mast 3 (see FIG. 1).

[0025] The pallet 5 is a bed on which a cargo M is placed. The pallet 5 is a flat pallet, for example. The pallet 5 has an approximately square shape in a plane view. The pallet 5 is placed on a loading surface 9a of a loading platform 9 of a truck or the like. As illustrated in FIG. 3, the pallet 5 has two pallet holes 10 into which the forks 6 are inserted, respectively. The pallet 5 has an upper wall 11 and a lower wall 12 that form the pallet holes 10. The upper wall 11 and the lower wall 12 respectively have an inner wall surface 11a and an inner wall surface 12a that face each other.

[0026] The cargo handling control device 1 is a device that controls the forklift truck 2 so that the forklift truck 2 automatedly places the pallet 5. The cargo handling device 1 controls the forklift truck 2 so that the forklift truck 2 places the pallet 5 held by the forks 6 on the loading surface 9a of the loading platform 9 and pulls the forks 6 out of the pallet 5.

[0027] The cargo handling control device 1 includes an upper sensor 21, a lower sensor 22, a pallet sensor 23, a tilt angle sensor 24, a vehicle speed sensor 25, a travelling drive unit 26, a cargo handling drive unit 27, an alarm 28, and a controller 30.

[0028] The upper sensor 21 and the lower sensor 22 are arranged vertically inside a distal portion 6a of each fork 6, as illustrated in FIG. 3. The upper sensor 21 and the lower sensor 22 may be reflective photoelectric sensors that irradiate one dimension light, for example. Alternatively, the upper sensor 21 and the lower sensor 22 may be proximity switches and the like.

[0029] The upper sensor 21 serves as the first detector of the present disclosure that detects whether the distal portion 6a of the fork 6 inserted into the pallet hole of the pallet 5 is adjacent to the upper wall 11 of the pallet 5. Specifically, the upper sensor 21 detects whether the distal portion 6a of the fork 6 inserted into the pallet hole 10 of the pallet 5 is adjacent to the upper wall 11 of the pallet 5 by detecting whether a distance from the distal portion 6a of the fork 6 to the upper wall 11 of the pallet 5 is equal to or less than a predetermined value. The predetermined value is, for example, on the order of several millimeters. The upper sensor 21 outputs an ON signal as a detection signal when the distal portion 6a of the fork 6 is adjacent to the upper wall 11 of the pallet 5. In this context, “the distal portion 6a of the fork 6 is adjacent to the upper wall 11 of the pallet 5” may include “the distal portion 6a of the fork 6 is in contact with the upper wall 11 of the pallet 5” depending on the predetermined value.

[0030] The lower sensor 22 serves as the second detector of the present disclosure that detects whether the distal portion 6a of the fork 6 inserted into the pallet hole 10 of the pallet 5 is adjacent to the lower wall 12 of the pallet 5. Specifically, the lower sensor 22 detects whether the distal portion 6a of the fork 6 inserted into the pallet hole 10 of the pallet 5 is adjacent to the lower wall 12 of the pallet 5 by detecting whether a distance from the distal portion 6a of the fork 6 to the lower wall 12 of the pallet 5 is equal to or less than a predetermined value.

[0031] The predetermined value is the same as that for the upper sensor 21. The lower sensor 22 outputs an ON signal as a detection signal when the distal portion 6a of the fork 6 is adjacent to the lower wall 12 of the pallet 5. In this context, “the distal portion 6a of the fork 6 is adjacent to the lower wall 12 of the pallet 5” may include “the distal portion 6a of the fork 6 is in contact with the lower wall 12 of the pallet 5” depending on the predetermined value.

[0032] The pallet sensor 23 is disposed at a proximal portion 6b of the fork 6, as illustrated in FIG. 3. The pallet sensor 23 may be a limit switch, for example. The pallet sensor 23 is mounted on an upper surface 6c of the fork 6. The pallet sensor 23 serves as the third detector of the present disclosure that detects whether the proximal portion 6b of the fork 6 inserted into the pallet hole 10 of the pallet 5 is in contact with the upper wall 11 of the pallet 5. The pallet sensor 23 outputs an ON signal as a detection signal when the proximal portion 6b of the fork 6 comes into contact with the upper wall 11 of the pallet 5.

[0033] The tilt angle sensor 24 is a sensor that detects a tilt angle of the fork 6. The tilt angle sensor 24 is mounted on the spacer 4, for example. The vehicle speed sensor 25 detects a travel speed of the forklift truck 2.

[0034] The travelling drive unit 26 is configured to drive the forklift truck 2 to travel. The travelling drive unit 26 includes a traveling motor (not illustrated) for rotating a front wheel 15 (see FIG. 2) as a drive wheel and a steering motor (not illustrated) for steering a rear wheel (not illustrated) as a steered wheel, for example.

[0035] The cargo handling drive unit 27 is configured to operate the lift cylinder 7 and the tilt cylinder 8. The cargo handling drive unit 27 is, for example, an oil control valve (not illustrated), and disposed between a hydraulic pump and the lift cylinder 7 and the tilt cylinder 8.

[0036] The alarm 28 alerts in the vicinity of the forklift truck 2 through audible signals or visual indicators.

[0037] The controller 30 includes a central processing unit (CPU), a random access memory (RAM), a read-only memory (ROM), and input/output interfaces. The controller 30 includes a fork lowering controller 31, a pallet posture determination unit 32, a pallet placement controller 33, a withdrawal and travel controller 34, and a fork height controller 35.

[0038] The fork lowering controller 31 controls the lift cylinder 7 via the cargo handling drive unit 27 so that the fork 6 holding the pallet 5 is lowered toward the loading surface 9a of the loading platform 9.

[0039] The pallet posture determination unit 32 determines a posture of the pallet 5 relative to the fork 6 based on detection signals from the upper sensor 21, the lower sensor 22, and the pallet sensor 23 after the fork lowering controller 31 starts controlling the lift cylinder 7.

[0040] When the upper sensor 21 detects that the distal portion 6a of the fork 6 is not adjacent to the upper wall 11 of the pallet 5 and the pallet sensor 23 detects that the proximal portion 6b of the fork 6 is not in contact with the upper wall 11 of the pallet 5, the pallet posture determination unit 32 determines that the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5.

[0041] When the upper sensor 21 detects that the distal portion 6a of the fork 6 is adjacent to the upper wall 11 of the pallet 5, the lower sensor 22 detects that the distal portion 6a of the fork 6 is not adjacent to the lower wall 12 of the pallet 5, and the pallet sensor 23 detects that the proximal portion 6b of the fork 6 is not in contact with the upper wall 11 of the pallet 5, the pallet posture determination unit 32 determines that the posture of the pallet 5 is a posture that requires the fork 6 to tilt forward (downward).

[0042] When the upper sensor 21 detects that the distal portion 6a of the fork 6 is not adjacent to the upper wall 11 of the pallet 5, the lower sensor 22 detects that the distal portion 6a of the fork 6 is adjacent to the lower wall 12 of the pallet 5, and the pallet sensor 23 detects that the proximal portion 6b of the fork 6 is in contact with the upper wall 11 of the pallet 5, the pallet posture determination unit 32 determines that the posture of the pallet 5 is a posture that requires the fork 6 to tilt backward (upward).

[0043] When the upper sensor 21 detects that the distal portion 6a of the fork 6 is adjacent to the upper wall 11 of the pallet 5 and the lower sensor 22 detects that the distal portion 6a of the fork 6 is adjacent to the lower wall 12 of the pallet 5, the pallet posture determination unit 32 determines that the posture of the pallet 5 is a posture that requires the fork 6 to immediately stop.

[0044] The pallet placement controller 33 controls the lift cylinder 7 and the tilt cylinder 8 via the cargo handling drive unit 27 so that the fork 6 is placed on the loading surface 9a of the loading platform 9 depending on the posture of the pallet 5 relative to the fork 6 determined by the pallet posture determination unit 32.

[0045] The pallet placement controller 33 judges based on the posture of the pallet 5 relative to the fork 6 whether the fork 6 is allowed to be pulled out of the pallet 5. When the pallet placement controller 33 judges that the fork 6 is allowed to be pulled out of the pallet 5, the pallet placement controller 33 controls the lift cylinder 7 via the cargo handling drive unit 27 so that the lift cylinder 7 stops lowering the fork 6. When the pallet placement controller 33 judges that the fork 6 is not allowed to be pulled out of the pallet 5, the pallet placement controller 33 controls the tilt cylinder 8 via the cargo handling drive unit 27 so that tilt cylinder 8 tilts the fork 6 along the loading surface 9a of the loading platform 9.

[0046] Specifically, when the pallet posture determination unit 32 determines that the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5, the pallet placement controller 33 controls the lift cylinder 7 via the cargo handling drive unit 27 so that the lift cylinder 7 stops lowering the fork 6.

[0047] When the pallet posture determination unit 32 determines that the posture of the pallet 5 requires the fork 6 to tilt forward, the pallet placement controller 33 controls the tilt cylinder 8 via the cargo handling drive unit 27 so that the tilt cylinder 8 tilts the fork 6 forward along the loading surface 9a of the loading platform 9. When the pallet posture determination unit 32 determines that the posture of the pallet 5 requires the fork 6 to tilt backward, the pallet placement controller 33 controls the tilt cylinder 8 via the cargo handling drive unit 27 so that the tilt cylinder 8 tilts the fork 6 backward along the loading surface 9a of the loading platform 9.

[0048] More specifically, when the pallet posture determination unit 32 determines that the posture of the pallet 5 requires the fork 6 to tilt forward, the pallet placement controller 33 controls the tilt cylinder 8 via the cargo handling drive unit 27 so that the tilt cylinder 8 tilts the fork 6 forward incrementally by a certain amount until the pallet posture determination unit 32 determines that the posture of the pallet 5 does not require the fork 6 to tilt forward. When the pallet posture determination unit 32 determines that the posture of the pallet 5 requires the fork 6 to tilt backward, the pallet placement controller 33 controls the tilt cylinder 8 via the cargo handling drive unit 27 so that the tilt cylinder 8 tilts the fork 6 backward incrementally by a certain amount until the pallet posture determination unit 32 determines that the posture of the pallet 5 does not require the fork 6 to tilt backward.

[0049] When the pallet posture determination unit 32 determines that the posture of the pallet 5 requires the fork 6 to immediately stop, the pallet placement controller 33 controls the travelling drive unit 26 and the cargo handling drive unit 27 of the forklift truck 2 so that the operation of the forklift truck 2 stops.

[0050] The withdrawal and travel controller 34 controls the travelling drive unit 26 so that the forklift truck 2 travels backward to pull the fork 6 out of the pallet 5 after the pallet placement controller 33 completes controlling the lift cylinder 7 and the tilt cylinder 8.

[0051] The fork height controller 35 controls the lift cylinder 7 via the cargo handling drive unit 27 so that the fork 6 remains at a constant height while the forklift truck 2 travels backward.

[0052] Specifically, when the fork 6 is tilted forward, the fork height controller 35 controls the lift cylinder 7 via the cargo handling drive unit 27 so that the fork 6 is raised while the forklift truck 2 travels backward. When the fork 6 is tilted backward, the fork height controller 35 controls the lift cylinder 7 via the cargo handling drive unit 27 so that the fork 6 is lowered while the forklift truck 2 travels backward.

[0053] The withdrawal and travel controller 34 and the fork height controller 35 cooperate to serve as the fork withdrawal controller of the present disclosure that controls the forklift truck 2 so that the forklift truck 2 pulls the fork 6 out of the pallet after the pallet placement controller 33 completes controlling the lift cylinder 7 and the tilt cylinder 8.

[0054] FIG. 4 is a flowchart of a pallet placement control process executed by the controller 30. This process is executed by the fork lowering controller 31, the pallet posture determination unit 32, and the pallet placement controller 33 when a placement operation is instructed while the pallet 5 is held by the fork 6.

[0055] At the start of this process, the pallet 5 held by fork 6 is positioned directly above the loading surface 9a of the loading platform 9 (see accompanied figures, such as FIG. 11A). When the pallet 5 is normally held by the fork 6, the distal portion 6a and the proximal portion 6b of the fork 6 are in contact with the inner wall surface 11a of the upper wall 11 of the pallet 5, but the distal portion 6a of the fork 6 is not in contact with the inner wall surface 12a of the lower wall 12 of the pallet 5.

[0056] In FIG. 4, the controller 30 first controls the cargo handling drive unit 27 so that the fork 6 is lowered toward the loading surface 9a of the loading platform 9 (step S101). The controller 30 then receives the detection signals from the upper sensor 21, the lower sensor 22, and the pallet sensor 23 (step S102).

[0057] The controller 30 determines a posture of the pallet 5 relative to the fork 6 based on the detection signals from the upper sensor 21, the lower sensor 22, and the pallet sensor 23 (step S103).

[0058] FIG. 5 is a table illustrating the relationship between the detection signals from the upper sensor 21, the lower sensor 22, and the pallet sensor 23, and the posture of the pallet 5 relative to the fork 6.

[0059] When the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output ON signal, OFF signal, and ON signal (see FIG. 5), the distal portion 6a of the fork 6 is adjacent to the inner wall surface 11a of the upper wall 11 of the pallet 5, the distal portion 6a of the fork 6 is not adjacent to the inner wall surface 12a of the lower wall 12 of the pallet 5, and the proximal portion 6b of the fork 6 is in contact with the inner wall surface 11a of the upper wall 11 of the pallet 5, as illustrated in FIG. 6A. In this state, the pallet 5 is being lowered by the fork 6.

[0060] When the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output ON signal, OFF signal, and OFF signal, the distal portion 6a of the fork 6 is adjacent to the inner wall surface 11a of the upper wall 11 of the pallet 5, the distal portion 6a of the fork 6 is not adjacent to the inner wall surface 12a of the lower wall 12 of the pallet 5, and the proximal portion 6b of the fork 6 is not in contact with the inner wall surface 11a of the upper wall 11 of the pallet 5, as illustrated in FIG. 6B. In this state, the pallet 5 is tilted downward from the proximal portion 6b to the distal portion 6a such that a part of the pallet 5 near the proximal portion 6b is positioned higher than a part of the pallet 5 near the distal portion 6a. Accordingly, the posture of the pallet 5 is a posture that requires the fork 6 to tilt forward.

[0061] When the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output OFF signal, ON signal, and ON signal, the distal portion 6a of the fork 6 is not adjacent to the inner wall surface 11a of the upper wall 11 of the pallet 5, the distal portion 6a of the fork 6 is adjacent to the inner wall surface 12a of the lower wall 12 of the pallet 5, and the proximal portion 6b of the fork 6 is in contact with the inner wall surface 11a of the upper wall 11 of the pallet 5, as illustrated in FIG. 7A. In this state, the pallet 5 is tilted upward from the proximal portion 6b to the distal portion 6a such that a part of the pallet 5 near the proximal portion 6b is positioned lower than a part of the pallet 5 near the distal portion 6a. Accordingly, the posture of the pallet 5 is a posture that requires the fork 6 to tilt backward.

[0062] When the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output OFF signal, ON signal, and OFF signal, the distal portion 6a of the fork 6 is not adjacent to the inner wall surface 11a of the upper wall 11 of the pallet 5, the distal portion 6a of the fork 6 is adjacent to the inner wall surface 12a of the lower wall 12 of the pallet 5, and the proximal portion 6b of the fork 6 is not in contact with the inner wall surface 11a of the upper wall 11 of the pallet 5, as illustrated in FIG. 7B. In this state, the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5, even when the pallet 5 is tilted upward from the proximal portion 6b to the distal portion 6a such that a part of the pallet 5 near the proximal portion 6b is positioned lower than a part of the pallet 5 near the distal portion 6a.

[0063] When the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output OFF signal, OFF signal, and ON signal, the distal portion 6a of the fork 6 is not adjacent to the inner wall surface 11a of the upper wall 11 of the pallet 5, the distal portion 6a of the fork 6 is not adjacent to the inner wall surface 12a of the lower wall 12 of the pallet 5, and the proximal portion 6b of the fork 6 is in contact with the inner wall surface 11a of the upper wall 11 of the pallet 5, as illustrated in FIG. 8A. In this state, the pallet 5 is being lowered by the fork 6, even when the pallet 5 is tilted upward from the proximal portion 6b to the distal portion 6a such that a part of the pallet 5 near the proximal portion 6b is positioned lower than a part of the pallet 5 near the distal portion 6a.

[0064] When the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output OFF signal, OFF signal, and OFF signal, the distal portion 6a of the fork 6 is not adjacent to the inner wall surface 11a of the upper wall 11 of the pallet 5, the distal portion 6a of the fork 6 is not adjacent to the inner wall surface 12a of the lower wall 12 of the pallet 5, and the proximal portion 6b of the fork 6 is not in contact with the inner wall surface 11a of the upper wall 11 of the pallet 5, as illustrated in FIG. 8B. In this state, there is a clearance between the fork 6 and the pallet 5 (the upper wall 11 and the lower wall 12) along the whole length of the fork 6. Accordingly, the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5.

[0065] When the upper sensor 21 and the lower sensor 22 respectively output ON signal and ON signal, the distal portion 6a of the fork 6 is adjacent to the inner wall surface 11a of the upper wall 11 of the pallet 5, and the distal portion 6a of the fork 6 is adjacent to the inner wall surface 12a of the lower wall 12 of the pallet 5. In this state, an abnormality is suspected to have occurred regardless of the detection signal from the pallet sensor 23. Accordingly, the posture of the pallet 5 is a posture that requires the fork 6 to immediately stop.

[0066] As illustrated in FIG. 4, the controller 30 judges whether the posture of the pallet 5 requires the fork 6 to immediately stop, based on the determination in step S103 (step S104). When the controller 30 judges that the posture of the pallet 5 does not require the fork 6 to immediately stop, the controller 30 judges whether the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5 (step S105). That is, the controller 30 judges whether the fork 6 is allowed to be pulled out of the pallet 5.

[0067] When the controller 30 judges that the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5, the controller 30 controls the cargo handling drive unit 27 so that the lift cylinder 7 stops lowering the fork 6 (step S106).

[0068] When the controller 30 judges that the posture of the pallet 5 does not allow the fork 6 to be pulled out of the pallet 5, the controller 30 controls the cargo handling drive unit 27 so that the lift cylinder 7 temporarily stops lowering the fork 6 (step S107). The controller 30 then judges whether the posture of the pallet 5 requires the fork 6 to tilt forward (step S108).

[0069] When the controller 30 judges that the posture of the pallet 5 requires the fork 6 to tilt forward, the controller 30 controls the cargo handling drive unit 27 so that the fork 6 tilts forward (step S109). Specifically, the controller 30 controls the cargo handling drive unit 27 so that the tilt cylinder 8 tilts the fork 6 forward by a predetermined angle.

[0070] The controller 30 controls the cargo handling drive unit 27 so that the lift cylinder 7 raises the fork 6 (step S110). Specifically, the controller 30 controls the cargo handling drive unit 27 so that the lift cylinder 7 slightly raises the fork 6 by a predetermined amount. Then, the controller 30 executes step S102 again.

[0071] When the controller 30 judges in step S108 that the posture of the pallet 5 does not require the fork 6 to tilt forward, the controller 30 judges whether the posture of the pallet 5 requires the fork 6 to tilt backward (step S111).

[0072] When the controller 30 judges that the posture of the pallet 5 requires the fork 6 to tilt backward, the controller 30 controls the cargo handling drive unit 27 so that the tilt cylinder 8 tilts the fork 6 backward (step S112). Specifically, the controller 30 controls the cargo handling drive unit 27 so that the tilt cylinder 8 tilts the fork 6 backward by a predetermined angle.

[0073] The controller 30 controls the cargo handling drive unit 27 so that the lift cylinder 7 lowers the fork 6 (step S113). Specifically, the controller 30 controls the cargo handling drive unit 27 so that the lift cylinder 7 slightly lowers the fork 6 by a predetermined amount. Then, the controller 30 executes step S102 again.

[0074] When the controller 30 judges in step S111 that the posture of the pallet 5 does not require the fork 6 to tilt backward, the controller 30 judges that the pallet is being lowered by the fork 6. Then, the controller 30 executes step S102 again.

[0075] When the controller 30 judges in step S104 that the posture of the pallet 5 requires the fork 6 to immediately stop, the controller 30 controls the alarm 28 so that the alarm 28 alerts (step S114). The controller 30 then controls the cargo handling drive unit 27 so that the lift cylinder 7 stops lowering the fork 6 (step S106).

[0076] Step S101 is executed by the fork lowering controller 31. Step S102 and step S103 are executed by the pallet posture determination unit 32. Step S104 to step S114 are executed by the pallet placement controller 33.

[0077] FIG. 9 is a flowchart of a fork withdrawal control process executed by the controller 30. This process is executed by the withdrawal and travel controller 34 and the fork height controller 35 after the pallet 5 is placed on the loading surface 9a of the loading platform 9 in accordance with the process illustrated in FIG. 4.

[0078] As illustrated in FIG. 9, the controller 30 first judges whether the posture of the pallet 5 requires the fork 6 to immediately stop, based on the determination in step S103 (step S121). When the controller 30 judges that the posture of the pallet 5 requires the fork 6 to immediately stop, the controller 30 does not execute the fork withdrawal control process.

[0079] When the controller 30 judges that the posture of the pallet 5 does not require the fork 6 to immediately stop, the controller 30 controls the travelling drive unit 26 so that the forklift truck 2 travels backward (step S122). This initiates the withdrawal of the fork 6 from the pallet 5.

[0080] The controller 30 then obtains detection values detected by the tilt angle sensor 24 and the vehicle speed sensor 25 (step S123). The controller 30 judges whether the tilt angle of the fork 6 is equal to or less than a predetermined threshold based on the detection value of the tilt angle sensor 24 (step S124). The threshold is set to an angle that allows the fork 6 to be pulled out of the pallet 5.

[0081] When the controller 30 judges that the tilt angle of the fork 6 is equal to or less than the threshold, the controller 30 calculates the travel distance of the forklift truck 2 from the start of the withdrawal of the fork 6 from the pallet 5 based on the detection value of the vehicle speed sensor 25 (step S125). Specifically, the controller 30 calculates the travel distance of the forklift truck 2 by multiplying the travel speed of the forklift truck 2 by a travel time.

[0082] The controller 30 then judges whether the travel distance of the forklift truck 2 from the start of the withdrawal of the fork 6 is greater than a fork length L of the fork 6 (see FIG. 3) (step S126). The fork length L is the dimension of an insertion part of the fork 6 inserted into the pallet hole 10 of the pallet 5. Specifically, the fork length L is the dimension of the fork 6 from the proximal end to the distal end of fork 6.

[0083] When the controller 30 judges that the travel distance of the forklift truck 2 from the start of the withdrawal of the fork 6 is equal to or less than the fork length L of the fork 6, the controller 30 executes step S123 again.

[0084] When the controller 30 judges that the travel distance of the forklift truck 2 from the start of the withdrawal of the fork 6 is equal to or greater than the fork length L of the fork 6, the controller 30 controls the travelling drive unit 26 so that the forklift truck 2 stops travelling backward (step S127). This completes the placement of the pallet 5.

[0085] When the controller 30 judges based on the detection value of the tilt angle sensor 24 in step S124 that the tilt angle of the fork 6 is greater than the predetermined threshold, the controller 30 judges whether the fork 6 is tilted forward (step S128).

[0086] When the controller 30 judges that the fork 6 is tilted forward, the controller 30 calculates the amount of change in the clearance between the pallet and the fork 6 based on the detection values of the tilt angle sensor 24 and the vehicle speed sensor 25 (step S129).

[0087] Specifically, the controller 30 calculates the amount of change in the clearance (i.e., clearance change amount AH) between the pallet 5 and the fork 6 using the following formula (A). As illustrated in FIG. 10A, the tilt angle of the fork 6 is denoted by θ, and the reverse distance of the forklift truck 2 is denoted by AD. When the fork 6 is tilted forward, the clearance change amount AH in the clearance between pallet 5 and the fork 6 is the amount of change in the clearance between the lower wall 12 of the pallet 5 and the fork 6. The reverse distance of the forklift truck 2 is calculated in the manner in step S125.

[0088] $\begin{array}{cc}\Delta H=\Delta D*\sin \theta & \left(A\right)\end{array}$

[0089] The controller 30 then controls the cargo handling drive unit 27 so that the fork 6 is raised (step S130). Specifically, the controller 30 calculates the raise amount of the fork 6 based on the clearance change amount between the pallet 5 and the fork 6 such that the clearance between the pallet 5 and the fork 6 becomes constant, and controls the cargo handling drive unit 27 so that the fork 6 is raised according to the calculated raise amount of the fork 6. Then, the controller 30 executes step S125.

[0090] When the controller 30 judges in step S128 that the fork 6 is tilted backward, the controller 30 calculates the clearance change amount between the pallet 5 and the fork 6 based on the detection values of the tilt angle sensor 24 and the vehicle speed sensor 25 (step S131). Specifically, the controller 30 calculates the clearance change amount ΔH between the pallet 5 and the fork 6 using the formula (A). As illustrated in FIG. 10B, the tilt angle of the fork 6 is denoted by θ, and the reverse distance of the forklift truck 2 is denoted by AD. When the fork 6 is backward, the clearance change amount ΔH in the clearance between pallet 5 and the fork 6 is the amount of change in the clearance between the upper wall 11 of the pallet 5 and the fork 6.

[0091] The controller 30 then controls the cargo handling drive unit 27 so that the fork 6 is lowered (step S132). Specifically, the controller 30 calculates the lowering amount of the fork 6 based on the clearance change amount between the pallet 5 and the fork 6 such that the clearance between the pallet 5 and the fork 6 becomes constant, and controls the cargo handling drive unit 27 so that the fork 6 is lowered according to the calculated lowering amount of the fork 6. Then, the controller 30 executes step S125.

[0092] Step S121, step S122, and steps S125 to S127 are executed by the withdrawal and travel controller 34. Step S123, step S124, and steps S128 to S132 are executed by the fork height controller 35.

[0093] The forklift truck 2 may load the pallet 5 onto a horizontal surface that serves as the loading surface 9a, as illustrated in FIG. 11A. In this case, first, the lift cylinder 7 lowers the fork 6 holding the pallet 5 toward the loading platform 9. In this state, the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output ON signal, OFF signal, and ON signal. Accordingly, the pallet 5 is being lowered by the fork 6 (see FIG. 6A).

[0094] As illustrated in FIG. 11B, the pallet 5 comes into contact with the loading surface 9a of the loading platform 9. A bottom surface 5a of the pallet 5 is parallel to the loading surface 9a of the loading platform 9. Accordingly, the pallet 5 does not tilt relative to the fork 6 when the pallet 5 comes into contact with the loading surface 9a.

[0095] The fork 6 is still lowered after the pallet 5 comes into contact with loading surface 9a. In this state, the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output OFF signal, OFF signal, and OFF signal. Accordingly, the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5 (see FIG. 8B).

[0096] The lift cylinder 7 stops lowering the fork 6. The forklift truck 2 travels backward to pull the fork 6 out of the pallet 5 as illustrated in FIG. 11C. This completes the placement of the pallet 5.

[0097] The forklift truck 2 may load the pallet 5 onto a surface that serves as the loading surface 9a and slopes downward away from the forklift truck 2 side, as illustrated in FIG. 12A. In this case, first, the lift cylinder 7 lowers the fork 6 holding the pallet 5 toward the loading platform 9. The pallet 5 comes into contact with the loading surface 9a of the loading platform 9.

[0098] Specifically, as illustrated in FIG. 12B, the pallet 5 comes into contact, at the end of the bottom surface 5a near the forklift truck 2, with the loading surface 9a that slopes downward from the forklift truck 2 side. Accordingly, the pallet 5 tilts downward along the sloping loading surface 9a. In this state, the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output ON signal, OFF signal, and OFF signal. Accordingly, the posture of the pallet 5 requires the fork 6 to tilt forward (see FIG. 6B).

[0099] The lift cylinder 7 stops lowering the fork 6. As illustrated in FIG. 12C, the forklift truck 2 places the pallet 5 on the loading surface 9a. Specifically, the tilt cylinder 8 tilts the fork 6 forward so that the fork 6 approaches and becomes parallel to the inner wall surface 12a of the lower wall 12 of the pallet 5, and the lift cylinder 7 slightly raises the fork 6 so that the fork 6 does not come into contact with the inner wall surface 12a of the lower wall 12 of the pallet 5.

[0100] In this state, the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output OFF signal, OFF signal, and OFF signal. Accordingly, the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5 (see FIG. 8B).

[0101] The forklift truck 2 travels backward to pull the fork 6 out of the pallet 5. While the fork 6 is pulled out, the fork 6 is raised so that the fork 6 does not come into contact with the inner wall surface 12a of the lower wall 12 of the pallet 5. This prevents the fork 6 from getting caught on the lower wall 12 of the pallet 5, when the fork 6 is pulled out of the pallet 5.

[0102] The forklift truck 2 may load the pallet 5 onto a surface that serves as the loading surface 9a and slopes upward away from the forklift truck 2 side, as illustrated in FIG. 13A. In this case, first, the lift cylinder 7 lowers the fork 6 holding the pallet 5 toward the loading platform 9. The pallet 5 comes into contact with the loading surface 9a of the loading platform 9.

[0103] Specifically, as illustrated in FIG. 13B, the pallet 5 comes into contact, at the end of the bottom surface 5a farthest from the forklift truck 2, with the loading surface 9a that slopes upward from the forklift truck 2 side. Accordingly, the pallet tilts upward along the sloping loading surface 9a. In this state, the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output OFF signal, ON signal, and ON signal. Accordingly, the posture of the pallet 5 requires the fork 6 to tilt backward (see FIG. 7A).

[0104] The lift cylinder 7 stops lowering the fork 6. As illustrated in FIG. 13C, the forklift truck 2 places the pallet 5 on the loading surface 9a. Specifically, the tilt cylinder 8 tilts the fork 6 backward so that the fork 6 approaches and becomes parallel to the inner wall surface 11a of the upper wall 11 of the pallet 5, and the lift cylinder 7 slightly lowers the fork 6 so that the fork 6 does not come into contact with the inner wall surface 11a of the upper wall 11 of the pallet 5.

[0105] In this state, the upper sensor 21, the lower sensor 22, and the pallet sensor 23 respectively output OFF signal, OFF signal, and OFF signal. Accordingly, the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5 (see FIG. 8B).

[0106] The forklift truck 2 travels backward to pull the fork 6 out of the pallet 5. While the fork 6 is pulled out, the fork 6 is lowered so that the fork 6 does not come into contact with the inner wall surface 11a of the upper wall 11 of the pallet 5. This prevents the fork 6 from getting caught on the upper wall 11 of the pallet 5, when the fork 6 is pulled out of the pallet 5.

[0107] According to the present embodiment, firstly, the controller 30 starts controlling the lift cylinder 7 so that the lift cylinder 7 lowers the fork 6 holding the pallet 5 toward the loading surface 9a of the loading platform 9. After the controller 30 starts controlling the lift cylinder 70, the upper sensor 21 detects whether the distal portion 6a of the fork 6 is adjacent to the upper wall 11 of the pallet 5, the lower sensor 22 detects whether the distal portion 6a of the fork 6 is adjacent to the lower wall 12 of the pallet 5, and the pallet sensor 23 detects whether the proximal portion 6b of the fork 6 is in contact with the upper wall 11 of the pallet 5. The controller 30 determines a posture of the pallet 5 relative to the fork 6 based on the detection signals from the upper sensor 21, the lower sensor 22, and the pallet sensor 23. The controller 30 controls the lift cylinder 7 and tilt cylinder 8 so that the fork 6 is placed on the loading surface 9a of the loading platform 9 depending on the posture of the pallet 5 relative to the fork 6. The posture of pallet 5 relative to fork 6 changes depending on whether the pallet 5 is tilted. The controller 30 judges whether the posture of the pallet 5 relative to the fork 6 allows the fork 6 to be pulled out of the pallet 5. When the controller 30 judges that the fork 6 is allowed to be pulled out of the pallet 5, the controller 30 controls the lift cylinder 7 so that the lift cylinder 7 stops lowering the fork 6. When the controller 30 judges that the fork 6 is not allowed to be pulled out of the pallet 5, the controller 30 controls the tilt cylinder 8 so that the tilt cylinder 8 tilts the fork 6 along the loading surface 9a of the loading platform 9. This eliminates the need for tilting the fork 6 when the controller 30 judges that the posture of the pallet 5 relative to the fork 6 allows the fork 6 to be pulled out of the pallet 5. This also eliminates the need for tilting the fork 6 backward after tilting the fork 6 forward and simply needs to tilt the fork 6 along the loading surface 9a when the controller judges that the fork 6 is not allowed to be pulled out of the pallet 5. This therefore reduces the time to place the pallet 5.

[0108] According to the present embodiment, when the upper sensor 21 detects that the distal portion 6a of the fork 6 is not adjacent to the upper wall 11 of the pallet 5 and the pallet sensor 23 detects that the proximal portion 6b of the fork 6 is not in contact with the upper wall 11 of the pallet 5, the controller 30 judges that the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5. This facilitates the judgement of whether the posture of the pallet 5 allows the fork 6 to be pulled out of the pallet 5 based on the detection signals from the upper sensor 21 and the pallet sensor 23. This therefore simplifies the placement of pallet 5.

[0109] According to the present embodiment, when the upper sensor 21 detects that the distal portion 6a of the fork 6 is adjacent to the upper wall 11 of the pallet 5, the lower sensor 22 detects that the distal portion 6a of the fork 6 is not adjacent to the lower wall 12 of the pallet 5, and the pallet sensor 23 detects that the proximal portion 6b of the fork 6 is not in contact with the upper wall 11 of the pallet 5, the controller 30 determines that the posture of the pallet 5 is a posture that requires the fork 6 to tilt forward. When the upper sensor 21 detects that the distal portion 6a of the fork 6 is not adjacent to the upper wall 11 of the pallet 5, the lower sensor 22 detects that the distal portion 6a of the fork 6 is adjacent to the lower wall 12 of the pallet 5, and the pallet sensor 23 detects that the proximal portion 6b of the fork 6 is in contact with the upper wall 11 of the pallet 5, the controller 30 determines that the posture of the pallet 5 is a posture that requires the fork 6 to tilt backward. This facilitates the judgement of whether the posture of the pallet 5 requires the fork 6 to tilt forward or whether the posture of the pallet 5 requires the fork 6 to tilt backward based on the detection signals from the upper sensor 21, the lower sensor 22, and the pallet sensor 23. This further simplifies the placement of pallet 5.

[0110] According to the present embodiment, when the controller 30 determines that the posture of the pallet 5 requires the fork 6 to tilt forward, the controller 30 controls the tilt cylinder 8 so that the tilt cylinder 8 tilts the fork 6 forward incrementally by a certain amount until the controller 30 determines that the posture of the pallet 5 does not require the fork 6 to tilt forward. When the controller 30 determines that the posture of the pallet 5 requires the fork 6 to tilt backward, the controller 30 controls the tilt cylinder 8 so that the tilt cylinder 8 tilts the fork 6 backward incrementally by a certain amount until the controller 30 determines that the posture of the pallet 5 does not require the fork 6 to tilt backward. Accordingly, the fork 6 gradually tilts forward when the posture of the pallet 5 requires the fork 6 to tilt forward, and gradually tilts backward when the posture of the pallet 5 requires the fork 6 to tilt backward. This allows the fork 6 to tilt smoothly, regardless of the tilt angle of the pallet 5.

[0111] According to the present embodiment, when the upper sensor 21 detects that the distal portion 6a of the fork 6 is adjacent to the upper wall 11 of the pallet and the lower sensor 22 detects that the distal portion 6a of the fork 6 is adjacent to the lower wall 12 of the pallet 5, the controller 30 determines that the posture of the pallet 5 is a posture that requires the fork 6 to immediately stop. This facilitates the judgement of whether the posture of the pallet 5 requires the fork 6 to immediately stop based on the detection signals from the upper sensor 21 and the lower sensor 22. This therefore allows immediate stop of the placement of the pallet 5 if an abnormality occurs in the forklift truck 2, for example.

[0112] The present disclosure is not limited to the above-described embodiment. For example, according to the above-described embodiment, the fork 6 is slightly raised to tilt forward to place the pallet 5 on the loading surface 9a of the loading platform 9, and slightly lowered to tilt backward to place the pallet 5 on the loading surface 9a of the loading platform 9. However, the present disclosure is not limited thereto. When tilting the fork 6 forward, it is not necessary to raise the fork 6 if the fork 6 does not come into contact with the lower wall 12 of the pallet 5. When tilting the fork 6 backward, it is not necessary to lower the fork 6 if the fork 6 does not come into contact with the upper wall 11 of the pallet 5.

[0113] According to the above-described embodiment, the controller 30 calculates the travel distance of the forklift truck 2 by multiplying the travel speed of the forklift truck 2 by a travel time. However, a sensor, such as an odometry sensor, may detect the travel distance of the forklift truck 2.

[0114] According to the above-described embodiment, a counterbalance forklift truck serves as the forklift truck 2. However, the present disclosure is applicable to a reach forklift truck. In this case, the reach forklift truck may retract the mast 3 using a reach cylinder to pull the fork 6 out of the pallet 5.