10 is not less than the predetermined threshold, the second slippage detecting section 35 determines that slippage of not less than the predetermined amount has occurred at the second wheel 21b.

The second slippage detecting section 35 makes the above-described threshold determination every time the second traveling motor 28b makes predetermined rotation, for example, every time the second traveling motor 28b makes two rotations. Assume here that L2 (mm) denotes a traveling distance of the traveling vehicle 10, the traveling distance being based on two rotations of the second traveling motor 28b. For example, in a case where a difference between the traveling distance L2 (mm) of the traveling vehicle 10 and an amount of change of a position, in the traveling direction, of the traveling vehicle 10 detected by the traveling sensor 29 is not less than 0.4×L2 (mm), the second slippage detecting section 35 determines that slippage of not less than the predetermined amount has occurred at the second wheel 21b.

With this, it is possible to detect, at an early timing, occurrence of slippage of not less than the predetermined amount at the second wheel 21b. Further, by appropriately setting the threshold for the threshold determination, it is possible to more quickly detect occurrence of slippage of not less than the predetermined amount at the second wheel 21b.

In a case where the second slippage detecting section 35 does not detect slippage of not less than the predetermined amount at the second wheel 21b (S2: NO), the process returns to step S1. Meanwhile, in a case where detection of slippage of not less than the predetermined amount at the second wheel 21b is detected (S2: YES), the traveling control section 31 controls the first servo amplifier 37 to stop rotation of the first traveling motor 28a (S3).

After step S3, the traveling control section 31 controls the second servo amplifier 38 to stop rotation of the second traveling motor 28b (S4). Consequently, the traveling vehicle 10 stops. As discussed above, the slippage detecting process carried out by the traveling control section 31 shown in FIG. 4 is ended.

In this manner, in a case where all the slippage detecting sections, i.e., the first slippage detecting section 34 and the second slippage detecting section 35 detect, at least in a state where the traveling vehicle is accelerated, slippage of not less than the predetermined amount, the traveling control section 31 causes the traveling vehicle 10 to stop.

[Flow of Sensor Abnormality Detection Process by Traveling Control Section]

Next, the following will describe, with reference to FIG. 5, a flow of a sensor abnormality detection process carried out by the traveling control section 31. FIG. 5 is a flowchart illustrating an example of a flow of the sensor abnormality detection process carried out by the traveling control section 31. In the present embodiment, the sensor abnormality detection process shown in FIG. 5 is executed at predetermined time periods, for example.

In the flowchart shown in FIG. 5, first, the traveling control section 31 determines whether or not the traveling vehicle 10 is traveling (S11). In step S11, the traveling control section 31 determines whether or not the traveling vehicle 10 is traveling, on the basis of detection of rotation of the first traveling motor 28a and rotation of the second traveling motor 28b, the detection being carried out by the first encoder 27a and the second encoder 27b.

In a case where the first encoder 27a does not detect rotation of the first traveling motor 28a and the second encoder 27b does no detect rotation of the second traveling motor 28b, the traveling control section 31 determines that the traveling vehicle 10 is not traveling (S11: NO), and the process returns to step S11.

Meanwhile, in a case where the first encoder 27a detects rotation of the first traveling motor 28a and the second encoder 27b detects the second traveling motor 28b, the traveling control section 31 determines that the traveling vehicle 10 is traveling (S11: YES), the traveling control section 31 determines whether or not the position of the traveling vehicle 10 detected by the traveling sensor 29 changes (S12).

In a case where the position of the traveling vehicle 10 detected by the traveling sensor 29 changes (S12: YES), the traveling control section 31 determines that the traveling sensor 29 is normally operating, and the process returns to step S11.

Meanwhile, in a case where the position of the traveling vehicle 10 detected by the traveling sensor 29 does not change (S12: NO), the traveling control section 31 determines that an abnormality has occurred in the traveling sensor 29 (S13), and the traveling control section 31 stops rotation of the first traveling motor 28a and rotation of the second traveling motor 28b so as to cause the traveling vehicle 10 to stop (S14). With this process, the sensor abnormality detection process shown in FIG. 5 is ended.

The above-described stacker crane 1 in accordance with the present embodiment is configured as follows. Only in a case where all the slippage detecting sections, that is, the first slippage detecting section 34 and the second slippage detecting section 35 detect slippage of not less than the predetermined amount (S2: YES), the traveling control section 31 causes the traveling vehicle 10 to stop (S4).

Note here that, in the stacker crane 1, the masts 11 are provided to the traveling vehicle 10 in a standing manner. Therefore, when accelerated, the traveling vehicle 10 is likely to be tilted in the traveling direction. Thus, slippage is likely to occur at the wheel on the front side in the traveling direction of the traveling vehicle 10, whereas slippage is difficult to occur at the wheel on the rear side in the traveling direction since a large load is applied thereto.

The stacker crane 1 configured as above enables the following operation. That is, even when slippage occurs at one of the first wheel 21a and the second wheel 21b while the conveyance object M is being conveyed in the stacker crane 1, the traveling vehicle 10 can travel with the other wheel. This can avoid unnecessary stop of the traveling vehicle 10, thereby suppressing reduction in efficiency with which the stacker crane 1 conveys the conveyance object M.

Further, the first wheel 21a, which is a wheel of the first driving unit 41, is disposed at the front side of the traveling vehicle 10. Meanwhile, the second wheel 21b, which is a wheel of the second driving unit 42, is disposed at the rear side of the traveling vehicle 10. By arranging the first wheel 21a and the second wheel 21b at different positions, in the traveling direction, of the traveling vehicle 10 in this manner, it is possible to suppress a situation in which the stacker crane 1, which includes the masts 11 provided to the traveling vehicle 10 in a standing manner, is tilted in the traveling direction at the time of acceleration.

Further, in the sensor abnormality detection process shown in FIG. 5, in a case where it is determined that an abnormality such as a failure has occurred in the traveling sensor 29 (S13), the traveling control section 31 causes the traveling vehicle 10 to stop (S14).

Note here that, in a case where an abnormality occurs in the traveling sensor 29, a traveling speed calculated on the basis of changes in traveling position per unit time detected by the traveling sensor 29 would not change. In this case, the first servo amplifier 37 supplies an excessive electric current to the first traveling motor 28a so as to make the traveling speed closer to the target traveling speed. Consequently, the number of rotations of the first traveling motor 28a becomes excessively large, so that the traveling vehicle 10 is excessively accelerated.

In order to address this, according to the present embodiment, in a case where it is determined that an abnormality has occurred in the traveling sensor 29, the traveling control section 31 causes the traveling vehicle 10 to stop, thereby making it possible to prevent excessive acceleration of the traveling vehicle 10. With this, it is possible to secure safety when the stacker crane 1 conveys the conveyance object M.

OTHER EMBODIMENTS

In the description of the above-described embodiment, the traveling vehicle 10 includes two driving units, i.e., the first driving unit 41 and the second driving unit 42. However, the present invention is not limited to this. Alternatively, for example, the traveling vehicle 10 may include four driving units. This configuration may be configured as follows. That is, in a case where all slippage detection sections in the four driving units detect slippage of not less than a predetermined amount at the wheels, the traveling vehicle 10 may be caused to stop. Further, in a case where slippage detection sections in two or more of the four driving units detect slippage of not less than a predetermined amount at the wheels, the traveling vehicle 10 may be caused to stop.

In the description of the above-described embodiment, used as the position detecting section which detects the position of the traveling vehicle 10 along the track is the traveling sensor 29, which is a laser-type distance sensor. However, the present embodiment is not limited to this. For example, used as the position detecting section may be a bar code type distance meter including (a) a bar code provided to extend along the track and (b) a reading section which is provided to the traveling vehicle 10 and which reads the bar code.

Further, used as the position detecting section may be a magnetic-force detecting type distance meter which includes a magnet section having N poles and S poles arranged alternately at predetermined intervals along the track and which detects a position of the traveling vehicle 10 on the track on the basis of detection of changes in magnetic force of the magnet section. Furthermore, used as the position detecting section may be a distance meter including (a) a scale disposed on the traveling rail R1 and (b) an encoder which is disposed in the traveling vehicle 10 and which detects the scale.

In the description of the above-described embodiment, in a case where all the slippage detection sections detect, at least in a state where the traveling vehicle 10 is accelerated, slippage of not less than a predetermined amount, the traveling control section 31 causes the traveling vehicle 10 to stop. However, the present invention is not limited to this. Alternatively, for example, in a case where all the slippage detection sections detect slippage of not less than a predetermined amount in a state where the traveling vehicle 10 is traveling at a constant speed or in a state where the traveling vehicle 10 is decelerated, the traveling control section 31 may cause the traveling vehicle 10 to stop.

In the description of the above-described embodiment, the first slippage detecting section 34 detects slippage at the first wheel 21a on the basis of (a) the number of rotations of the first traveling motor 28a detected by the first encoder 27a and (b) an amount of change of a position, in the traveling direction, of the traveling vehicle 10 detected by the traveling sensor 29. However, the present invention is not limited to this. Alternatively, for example, the first slippage detecting section 34 may detect slippage at the first wheel 21a by determining whether or not a difference between (a) a traveling distance of the traveling vehicle 10 calculated on the basis of a torque instruction value from the first servo amplifier 37 and (b) an amount of change of a position, in the traveling direction, of the traveling vehicle 10 detected by the traveling sensor 29 is not less than a predetermined threshold.

Further, in the description of the above-described embodiment, in a case where a position, in the traveling direction, of the traveling vehicle 10 detected by the traveling sensor 29 does not change while the traveling vehicle 10 is traveling, the traveling vehicle 10 is caused to stop. However, the present invention is not limited to this. Alternatively, for example, a torque sensor which detects a torque loaded to the second wheel 21b may be provided. In a case where a torque detected by the torque sensor is not less than a predetermined threshold, the traveling vehicle 10 may be caused to stop. With this, it is possible to prevent excessive rotation and overheating of the second traveling motor 28b.

Furthermore, in the description of the above-described embodiment, the first servo amplifier 37 carries out torque control for obtaining a torque instruction value so as to make a difference between the traveling speed and the target traveling speed 0 (zero). However, the present invention is not limited to this. The first servo amplifier 37 may carry out proportional integral control for carrying out proportional control and integral control on the basis of the difference between the traveling speed and the target traveling speed.

Aspects of the present invention can also be expressed as follows:

A conveyance device in accordance with a first aspect of the present disclosure includes: a traveling vehicle which travels along a track; a mast provided to the traveling vehicle in a standing manner; a raising and lowering section which holds a conveyance object and which is raised and lowered along the mast; and a control section. The traveling vehicle includes a plurality of driving units each including a wheel which rolls on the track, a motor which drives the wheel, and a slippage detection section which detects slippage at the wheel. The control section causes the traveling vehicle to stop, in a case where two or more slippage detection sections out of the slippage detection sections detect, at least in a state where the traveling vehicle is accelerated, slippage of not less than a predetermined amount at the wheels.