Method and apparatus for lifting steel plates

Abstract

To provide a method and a device for lifting a steel plate, which can efficiently cut off only the excessively adsorbed steel plate, and a method for manufacturing the steel plate, which can improve work efficiency of steel plate manufacturing.SOLUTION: The method for lifting a steel plate comprises lifting, as an object 20 to be lifted, at least one steel plate among a plurality of steel plates stacked using a lifting magnet 1 having a magnetic pole 3 and an electromagnetic coil 2. This method comprises: a lifting step; a determination step of determining the number of lifted steel plates; and a lifted number adjustment step of cutting off excessive steel plates exceeding a target number when it is determined that the number of lifted steel plates is larger than the target number. The lifted number adjustment step includes reducing a voltage applied to the electromagnetic coil 2 until the reduced amount of a magnetic flux density while a voltage applied to the electromagnetic coil 2 is reduced with respect to a magnetic flux density in the lifted state of the steel plates becomes equal to or larger than a cutting-off detection threshold value.SELECTED DRAWING: Figure 4B

Description

[Technical Field]

[0001] This disclosure relates to a method and apparatus for lifting steel plates, and to a method for manufacturing steel plates, including a process for transporting the lifted steel plates. [Background technology]

[0002] A steel mill's heavy plate factory is broadly divided into three sections: a rolling mill (rolling process) that rolls block steel plates to the desired thickness; a finishing mill (finishing process) that cuts the plates to the shipping size, deburrs the edges, cleans up surface defects, or inspects for internal defects; and a product warehouse for storing steel plates awaiting shipment. Due to space constraints, steel plates in the finishing process or those awaiting shipment in the product warehouse are stored stacked in piles of several to a dozen or so plates. When rearranging or shipping, these stacked steel plates are moved by using an electromagnet-type lifting magnet attached to a crane to lift one to several plates at a time.

[0003] A method has been proposed (Patent Document 1) for controlling the current applied to the coil of a lifting magnet based on the relationship between the current applied to the coil and the total thickness of the steel plates being lifted, in order to lift and move a predetermined number of steel plates. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2-295889 [Overview of the project] [Problems that the invention aims to solve]

[0005] In the method described in Patent Document 1, the output magnetic flux is controlled by controlling the coil current to change the penetration depth of the magnetic flux. Here, the coils of lifting magnets commonly used in the thick plate factories of steel mills are designed to apply a large amount of magnetic flux to steel plates in order to lift thick steel plates of 100 mm or more. When controlling the number of thin steel plates with a thickness of 10 mm or less using such a coil, the magnetic flux density may become excessive due to a slight error in the voltage applied to the coil. When the magnetic flux density becomes excessive, the number of steel plates attracted to the lifting magnet will exceed the target number of plates to be lifted. In other words, the lifting magnet will attract an excessive number of steel plates. In order to control the number of plates to be lifted, it becomes necessary to detach the excessively attracted steel plates and adjust the number to be lifted.

[0006] In the method described in Patent Document 1, the power supply is cut off for a predetermined time to control the number of steel plates that are suspended, thereby detaching any excess steel plates that are attracted. However, the rate at which the magnetic flux density decreases when the power supply is cut off varies depending on the state of the attraction surface between the coil and the steel plate, or the composition of the steel plate. Therefore, it is difficult to set the optimal power supply cutoff time. As a result, it may not be possible to detach the excess steel plates in a single detachment operation, or conversely, steel plates other than the excess steel plates may be detached together.

[0007] In view of the above facts, this disclosure aims to provide a method and apparatus for lifting steel plates that can efficiently separate only the excessively adsorbed steel plates, and a method for manufacturing steel plates that can improve the work efficiency of steel plate manufacturing. [Means for solving the problem]

[0008] (1) A method for lifting a steel plate according to one embodiment of the present disclosure is: A method for lifting steel plates, comprising using a lifting magnet having magnetic poles and an electromagnet coil configured to control the voltage for exciting the magnetic poles, to lift at least one steel plate from a stack of multiple steel plates as the target for lifting, A lifting step in which a voltage is applied to the electromagnet coil and the lifting magnet is raised so that the first magnetic flux density, which is the magnetic flux density of the magnetic pole in the state before the lifting magnet begins to rise, becomes the target magnetic flux density required for the magnetic pole to lift the object to be lifted, and the object to be lifted is lifted by raising the lifting magnet, A determination step for determining the number of steel plates lifted in the lifting step, In the determination step, if it is determined that the number of lifted steel plates is greater than the target number, a lifting number adjustment step is performed in which the voltage applied to the electromagnet coil is reduced until the decrease in the third magnetic flux density (the magnetic flux density of the magnetic pole when the steel plates are lifted) becomes equal to or greater than the separation detection threshold, in order to separate the excess steel plates that are greater than the target number. Includes.

[0009] (2) In one embodiment of the present disclosure, in the lifting step of the steel plate lifting method of (1) above, the voltage applied to the electromagnet coil may be controlled so that the difference between the first magnetic flux density and the target magnetic flux density is less than or equal to the lifting control threshold.

[0010] (3) In one embodiment of the present disclosure, in the determination step of the steel plate lifting method of (1) or (2) above, the number of lifted steel plates may be determined based on the second magnetic flux density.

[0011] (4) In one embodiment of the present disclosure, in the lifting number adjustment step of any one of the steel plate lifting methods described in (1) to (3) above, the voltage applied to the electromagnet coil may be increased after the decrease in the third magnetic flux density relative to the second magnetic flux density becomes equal to or greater than the separation detection threshold.

[0012] (5) In one embodiment of the present disclosure, in the lifting number adjustment step of the steel plate lifting method of (4) above, the voltage applied to the electromagnet coil may be increased after a waiting time has elapsed since it was determined that the amount of decrease in the third magnetic flux density relative to the second magnetic flux density has become equal to or greater than the separation detection threshold. The waiting time may be within the range of 0.1 seconds or more and 2.0 seconds or less.

[0013] (6) In one embodiment of the present disclosure, in the lifting step of any one of the steel plate lifting methods described in (1) to (5) above, the target magnetic flux density may be calculated based on the plate thickness and saturation magnetic flux density of at least one steel plate to be lifted, and the dimensions of the magnetic poles.

[0014] (7) A steel plate lifting device according to one embodiment of the present disclosure is A steel plate lifting device for lifting at least one steel plate from among a stack of multiple steel plates, A lifting magnet having magnetic poles and an electromagnet coil configured to control the voltage for exciting the magnetic poles, A voltage application device for applying a voltage to the aforementioned electromagnet coil, A magnetic flux sensor for measuring the magnetic flux density of the aforementioned magnetic pole, A control device for controlling the voltage applied to the electromagnetic coil Equipped with, The magnetic flux sensor measures the magnetic flux density of the magnetic pole as the first magnetic flux density when the object to be lifted is attracted to the magnetic pole and before the lifting magnet begins to rise. The control device applies a voltage to the electromagnet coil so that the first magnetic flux density becomes the target magnetic flux density required for the magnetic poles to lift the object to be lifted, thereby raising the lifting magnet and lifting the object to be lifted. The magnetic flux sensor measures the magnetic flux density of the magnetic pole when the steel plate is suspended, and defines this as the second magnetic flux density. When the control device determines that the number of the lifted steel plates is greater than a target number, it reduces the voltage applied to the electromagnet coil so as to separate excess steel plates that are more than the target number, the magnetic flux sensor measures the magnetic flux density of the magnetic pole when the voltage applied to the electromagnet coil is being reduced as a third magnetic flux density, the control device reduces the voltage applied to the electromagnet coil until a decrease amount of the third magnetic flux density relative to the second magnetic flux density is equal to or greater than a separation detection threshold.

[0015] (8) As one embodiment of the present disclosure, in the steel plate lifting apparatus according to (7) above, the control device may control the voltage applied to the electromagnet coil such that a difference between the first magnetic flux density and the target magnetic flux density is equal to or less than a lifting control threshold.

[0016] (9) As one embodiment of the present disclosure, in the steel plate lifting apparatus according to (7) or (8) above, the control device may determine the number of lifted steel plates based on the second magnetic flux density.

[0017] (10) As one embodiment of the present disclosure, in the steel plate lifting apparatus according to any one of (7) to (9) above, after a decrease amount of the third magnetic flux density relative to the second magnetic flux density becomes equal to or greater than a separation detection threshold, the control device may increase the voltage applied to the electromagnet coil.

[0018] (11) As one embodiment of the present disclosure, in the steel plate lifting apparatus according to (10) above, the control device may increase the voltage applied to the electromagnet coil after a standby time elapses from when it is determined that the decrease amount of the third magnetic flux density relative to the second magnetic flux density becomes equal to or greater than a separation detection threshold. The standby time may be a time within a range of 0.1 seconds or more and 2.0 seconds or less.

[0019] (12) In one embodiment of the present disclosure, in any one of the steel plate lifting devices described in (7) to (11) above, the control device may calculate the target magnetic flux density based on the thickness and saturation magnetic flux density of at least one steel plate to be lifted and the dimensions of the magnetic poles.

[0020] (13) A method for manufacturing a steel plate according to one embodiment of the present disclosure includes the step of lifting and transporting at least one steel plate as a lifting target using any one of the steel plate lifting devices described in (7) to (12) above. [Effects of the Invention]

[0021] This disclosure provides a method and apparatus for lifting steel plates that can efficiently separate only the excessively adsorbed steel plates, as well as a method for manufacturing steel plates that can improve the production efficiency of steel plates. [Brief explanation of the drawing]

[0022] [Figure 1] This is a block diagram showing an example of the configuration of a steel plate lifting device according to this disclosure. [Figure 2] This is a cross-sectional view showing an example of the configuration of a lifting magnet. [Figure 3] This is a cross-sectional view illustrating the magnitude of the magnetic flux in each steel plate when multiple steel plates are attracted to each other by a lifting magnet. [Figure 4A] This flowchart shows an example of the procedure for lifting steel plates according to the steel plate lifting method described herein, when the number of plates to be lifted is appropriate or insufficient. [Figure 4B] This flowchart shows an example of the procedure when lifting a large number of steel plates in the steel plate lifting method according to this disclosure. [Figure 5] This graph shows the change in magnetic flux density when the coil voltage is reduced to detach the excessively attracted steel plate in the lifting method according to this disclosure. [Figure 6]This graph shows the change in magnetic flux density when the number of lifted steel plates is controlled by reducing the coil voltage to detach the excessively attracted steel plates in the lifting method according to this disclosure, and then increasing the coil voltage again. [Figure 7] This graph shows the change in magnetic flux density when a steel plate that has been excessively attracted is detached using the lifting method described in the comparative example. [Modes for carrying out the invention]

[0023] Embodiments of the steel plate lifting device, steel plate lifting method, and steel plate manufacturing method relating to this disclosure will be described below with reference to the drawings. Each drawing is schematic and may differ from the actual one. Furthermore, the following embodiments are illustrative examples of devices or methods for realizing the technical idea of ​​this disclosure and do not limit the configuration to those described below. In other words, the technical idea of ​​this disclosure can be modified in various ways within the technical scope described in the claims.

[0024] (Embodiment) The steel plate lifting device 10 (see Figure 1) according to one embodiment of the present disclosure, described below, uses a lifting magnet 1 (see Figure 1, etc.) to attract a steel plate, which is the object to be lifted 20 (see Figure 3). The steel plate lifting device 10 lifts the steel plate attracted to the lifting magnet 1 by raising the lifting magnet 1 with a crane 8 (see Figure 1). The configuration and operation examples of the steel plate lifting device 10 according to this embodiment will be described below.

[0025] <Example of configuration of steel plate lifting device 10> As shown in Figure 1, a steel plate lifting device 10 according to one embodiment of the present disclosure comprises a lifting magnet 1, a control device 5, a voltage application device 6, and a crane 8. The crane 8 is configured to hold the lifting magnet 1 and to raise and lower or traverse the lifting magnet 1. The crane 8 may be configured to raise the lifting magnet 1 by winding up a wire.

[0026] <<Lifting Magnet 1>> The lifting magnet 1 comprises an electromagnet coil 2, a magnetic pole 3, and a yoke 7, as shown in Figures 1 and 2. The lifting magnet 1 may comprise multiple units, each consisting of an electromagnet coil 2, a magnetic pole 3, and a yoke 7, as illustrated in Figure 2.

[0027] The electromagnet coil 2 may be configured as a ring-shaped excitation coil made by winding enameled copper wire many times and insulating it. The diameter of the portion of the electromagnet coil 2 in which the copper wire is wound may be 100 mm to several hundred mm. When a voltage is applied to the electromagnet coil 2 from the voltage application device 6 described later, a current corresponding to the applied voltage flows through the copper wire. A magnetic flux is generated by the current flowing through the copper wire. The larger the voltage applied to the electromagnet coil 2, the larger the current flowing through the copper wire and the larger the magnetic flux generated by the electromagnet coil 2. When the voltage application device 6 is off and no voltage is applied, the electromagnet coil 2 does not generate a magnetic flux. In other words, the electromagnet coil 2 is configured to be controllable to either an on state or an off state. Furthermore, the electromagnet coil 2 is configured to control the magnitude of the magnetic flux it generates depending on the magnitude of the applied voltage.

[0028] A magnetic pole 3 is positioned inside the electromagnet coil 2. The magnetic pole 3 functions as the magnetic core of the electromagnet coil 2. The diameter of the magnetic pole 3 is denoted by D. The electromagnet coil 2 energizes the magnetic pole 3. In other words, the electromagnet coil 2 is configured to control the voltage used to energize the magnetic pole 3. The yoke 7 is positioned on the upper end of the magnetic pole 3 and outside the electromagnet coil 2. The electromagnet coil 2 is wound around the magnetic pole 3 as its axis inside the yoke 7. That is, the electromagnet coil 2 is wound between the magnetic pole 3 and the yoke 7. When viewed in the direction of the axis around which the electromagnet coil 2 is wound, the electromagnet coil 2 and the yoke 7 may be arranged concentrically around the magnetic pole 3. The shapes of the magnetic pole 3, electromagnet coil 2, and yoke 7 when viewed in the direction of the axis around which the electromagnet coil 2 is wound may be circular, rectangular, or various other shapes. The yoke 7 is configured to have a U-shaped cross-section. The yoke 7 is configured such that its cross-section, when combined with the magnetic pole 3, forms an E shape. The magnetic pole 3 and the yoke 7 protrude beyond the electromagnet coil 2 in cross-section, and the protruding portion attracts the steel plate.

[0029] The magnetic pole 3 and the yoke 7 may be made of a soft magnetic material such as mild steel. The magnetic pole 3 and the yoke 7 may be made of a part or all of a single integrated component. The magnetic pole 3 and the yoke 7 may be made of separate components and combined. A non-magnetic material such as resin may be filled between the magnetic pole 3 and the yoke 7 to fix the electromagnet coil 2.

[0030] The lifting magnet 1 further includes a magnetic flux sensor 4. The magnetic flux sensor 4 is installed on the magnetic pole 3 and measures the magnetic flux density of the magnetic flux passing through the magnetic pole 3. The magnetic flux density of the magnetic flux passing through the magnetic pole 3 is also called the magnetic pole magnetic flux density. The magnetic pole magnetic flux density is determined based on the amount of magnetic flux generated by the electromagnet coil 2 and the cross-sectional area of ​​the magnetic pole 3 in a cross section normal to the direction of the axis around which the electromagnet coil 2 is wound.

[0031] The magnetic flux sensor 4 may be configured to include, for example, a search coil or a Hall element. In this embodiment, the magnetic flux sensor 4 is assumed to be composed of a search coil. The installation position of the magnetic flux sensor 4 is not particularly limited as long as it is a position in which the magnetic pole flux density can be measured. In this embodiment, the magnetic flux sensor 4 is assumed to be installed at the lower outer end of the magnetic pole 3. Multiple magnetic flux sensors 4 may be installed at different positions on the magnetic pole 3.

[0032] <<Voltage application device 6>> The voltage application device 6 applies a voltage to the electromagnet coil 2 so that current flows through it. The voltage application device 6 may be configured as a voltage source with controllable voltage. The voltage application device 6 may be replaced with a current source with controllable current.

[0033] <<Control device 5>> The control device 5 is connected to the voltage application device 6 as shown in Figure 1, and controls the magnitude of the voltage applied by the voltage application device 6 to the electromagnet coil 2. The control device 5 acquires the measured magnetic pole flux density from the magnetic flux sensor 4. The control device 5 may also be connected to the crane 8 and configured to control the raising and lowering of the lifting magnet 1 by the crane 8.

[0034] The control device 5 may be configured to include at least one processor, such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit). The control device 5 may consist of one processor or multiple processors. The processors constituting the control device 5 may realize the functions of the steel plate lifting device 10 by reading and executing a program stored in the memory unit described later.

[0035] The control device 5 may include a memory unit. The memory unit stores various types of information or data. For example, the memory unit may store programs executed in the control device 5, or data or processing results used in processes executed in the control device 5. The memory unit may also function as the work memory of the control device 5. The memory unit may include, but is not limited to, semiconductor memory. For example, the memory unit may be configured as the internal memory of the processor used as the control device 5, or as a hard disk drive (HDD) accessible from the control device 5. The memory unit may be configured as a non-temporary readable medium. The memory unit may be configured integrally with the control device 5, or as a separate unit from the control device 5.

[0036] The control device 5 may include a communication unit. The communication unit may include a communication interface for communicating with other devices by wire or wireless means. The communication interface may be configured to communicate with other devices via a network. The communication unit may include input / output ports for inputting and outputting data to and from other devices. The communication unit sends and receives necessary data and signals to and from a process computer or higher-level system. The communication unit may communicate based on wired communication standards or wireless communication standards. For example, wireless communication standards may include cellular phone communication standards such as 3G, 4G, and 5G. Also, for example, wireless communication standards may include IEEE 802.11 and Bluetooth®. The communication unit may support one or more of these communication standards. The communication unit may communicate with other devices and input / output data based on various standards, but is not limited to these examples. The communication unit may be configured integrally with the control device 5 or as a separate unit from the control device 5.

[0037] The control device 5 may include an input device that receives information or data from an operator using the steel plate lifting device 10. The input device may include, for example, a touch panel or touch sensor, or a pointing device such as a mouse. The input device may also include physical keys. The input device may also include an audio input device such as a microphone. The control device 5 may be configured to be connectable to an external input device. The control device 5 may be configured to acquire information or data input to an external input device from that external input device.

[0038] The control device 5 may be configured to include an output device that outputs information or data to the operator. The output device may include, for example, a display device that outputs visual information such as images, characters, or figures. The display device may include, for example, an LCD (Liquid Crystal Display), an organic EL (Electro-Luminescence) display, an inorganic EL display, or a PDP (Plasma Display Panel). The display device is not limited to these displays and may include various other types of displays. The display device may include a light-emitting device such as an LED (Light Emitting Diode) or an LD (Laser Diode). The display device may include various other devices. The output device may include, for example, an audio output device such as a speaker that outputs auditory information such as sound. The output device is not limited to these examples and may include various other devices. The control device 5 may be configured to be connectable to an external output device. The control device 5 may be configured to output information or data to an external output device.

[0039] <Example of operation of steel plate lifting device 10> In this embodiment, the steel plate lifting device 10 lifts the steel plate by using a lifting magnet 1 to attract at least one steel plate from among a stack of multiple steel plates and raising the lifting magnet 1 with a crane 8.

[0040] <<Controlling the amount of steel plate to be adsorbed>> The control device 5 can control the number of steel plates attracted by the lifting magnet 1 by controlling the voltage applied to the electromagnet coil 2. The process of attracting and lifting the steel plates is also called the lifting process.

[0041] When the lifting magnet 1 attracts a steel plate, a magnetic circuit is formed through which magnetic flux passes in a link with the electromagnet coil 2, passing through the magnetic pole 3, the yoke 7, and the attracted steel plate. As the amount of attracted steel plate increases, the magnetic resistance of the magnetic circuit decreases, and the amount of magnetic flux passing through the magnetic circuit increases. The control device 5 can calculate the amount of attracted steel plate based on the amount of magnetic flux passing through the magnetic circuit, that is, the magnetic flux density of the magnetic flux passing through the magnetic pole 3. The control device 5 may obtain a measured value of the magnetic pole magnetic flux density from the magnetic flux sensor 4 and calculate the amount of attracted steel plate based on the measured value of the magnetic pole magnetic flux density. The control device 5 may calculate the amount of attracted steel plate as the sum of the thicknesses of multiple steel plates. If the thickness of each steel plate is the same or approximately the same, the control device 5 may calculate the amount of attracted steel plate as the number of steel plates.

[0042] The control device 5 may acquire information specified by the operator, such as the number of steel plates to be attracted or the total thickness of the steel plates to be attracted. The control device 5 calculates a target value of magnetic flux density required to attract the specified amount of steel plates with the lifting magnet 1. The target value of magnetic flux density is also called the target magnetic flux density. The target magnetic flux density is denoted as Br.

[0043] The total thickness of the steel plates that the lifting magnet 1 attracts is determined by the depth to which the magnetic flux penetrates the steel plates (magnetic flux penetration depth). The lifting magnet 1 can control the number of steel plates it attracts by controlling the magnetic flux penetration depth. The control device 5 may determine a target magnetic flux density based on the relationship between the magnetic pole magnetic flux density and the magnetic flux penetration depth.

[0044] The control device 5 attracts a specified amount of steel plates with the lifting magnet 1 by controlling the voltage application device 6 so that the magnetic pole flux density reaches the target magnetic flux density. The control device 5 may calculate the target magnetic flux density based on the thickness of the steel plates to be lifted 20 and the saturation magnetic flux density of the steel plates. The control device 5 can automatically control the number of steel plates to be lifted by feedback control of the voltage applied to the electromagnet coil 2 so that the magnetic pole flux density reaches the target magnetic flux density, based on the diameter of the magnetic pole 3 and the magnetic pole flux density measured by the magnetic flux sensor 4.

[0045] Specifically, the control device 5 determines the diameter (D) of the magnetic pole 3 of the lifting magnet 1, i.e., the dimensions of the magnetic pole 3, and the thickness (t1~t) of each of the n steel plates to be lifted 20. n ) and the saturation magnetic flux density (Bs1~Bs) of each steel plate n The target magnetic flux density may be calculated using the following equation (1) based on the above.

number

[0046] Here, the target magnetic flux density of magnetic pole 3 is derived by the following theory. As shown by the white arrows in Figure 3, the magnetic flux generated by the lifting magnet 1 flows into the region 141 directly below magnetic pole 3 from the upper surface of the steel plate to be lifted 20 and flows out from the side of region 141. The amount of magnetic flux outflow (Φ) in the k-th steel plate from the side closest to the electromagnet coil 2 k ) is the lateral surface area πD·t k and saturation magnetic flux density Bs k Based on that, Φ k =πD·Bs k ·t k It is calculated as follows.

[0047] In that case, if n steel plates are to be lifted 20, starting from the side closest to the electromagnet coil 2, the target magnetic flux (Φr) required to pass through the nth steel plate can be calculated using the following equation (2).

number

[0048] Further, when the cross-sectional area (A) of the magnetic pole 3 satisfies A=πD 2 / 4, the target magnetic flux density (Br) of the magnetic pole 3 is calculated by the following formula (3).

Numerical formula

[0049] The control device 5 acquires the magnetic pole magnetic flux density of the magnetic pole 3 measured by the magnetic flux sensor 4, and calculates the difference between the magnetic pole magnetic flux density and the target magnetic flux density. Here, the magnetic pole magnetic flux density being adjusted for adsorbing steel plates is also referred to as a first magnetic flux density, and is represented by Ba1. In other words, the first magnetic flux density is the magnetic pole magnetic flux density in a state before the lifting magnet 1 starts to rise. The control device 5 can calculate the difference as |Ba1-Br|. The control device 5 determines whether the difference becomes equal to or less than a lifting control threshold. The lifting control threshold is represented by B1. When the difference is larger than the lifting control threshold, that is, when |Ba1-Br|>B1, the control device 5 feedback-controls the voltage application device 6 to adjust the voltage applied to the electromagnet coil 2 so that the difference becomes equal to or less than the lifting control threshold. When the difference becomes equal to or less than the lifting control threshold, that is, when |Ba1-Br|≦B1, the control device 5 may cause the crane 8 to start lifting the lifting magnet 1.

[0050] <<Determination of the number of lifted steel plates>> The control device 5 lifts the target steel plate by lifting the lifting magnet 1 with the crane 8. After the lifting of the steel plates is completed, the control device 5 determines the number of lifted steel plates based on the measured value of the magnetic pole magnetic flux density of the magnetic pole 3. The control device 5 may determine the number of lifted steel plates after the steel plates are lifted off the ground. The step of determining the number of lifted steel plates is also referred to as a determination step.

[0051] Specifically, the control device 5 obtains the measured magnetic flux density of the steel plates when they are suspended from the magnetic flux sensor 4. The magnetic flux density when the steel plates are suspended is also called the second magnetic flux density and is represented by Ba2. The control device 5 calculates the absolute value of the difference between the magnetic flux density when the steel plates are suspended and the magnetic flux density when the target number of plates are suspended. The magnetic flux density when the target number of plates are suspended is also called the target suspension magnetic flux density. The target suspension magnetic flux density may be set to the same value as the initial value of the target magnetic flux density. If the absolute value of the difference is less than or equal to the suspension number determination threshold, the control device 5 determines that the number of suspended steel plates is the specified number and transports the steel plates as is. If the absolute value of the difference is greater than the suspension number determination threshold, the control device 5 determines that the number of suspended steel plates is more or less than the specified number.

[0052] As described above, the number of lifted plates may be determined by the control device 5, but it may also be determined by other means. For example, the number of lifted plates may be determined by visually counting the number of steel plates being lifted by an operator or other person operating the lifting device 10. The number of lifted plates may also be determined by photographing the lifted steel plates with a camera and processing the images. Alternatively, the weight of the lifted steel plates may be measured with a weight sensor such as a load cell, and the number of lifted plates may be determined based on the difference between the measured weight and the target weight.

[0053] <<If the number of items to be lifted is small>> The control device 5 determines that the number of steel plates lifted is insufficient if the absolute value of the difference is greater than the lifting count determination threshold and the second magnetic flux density is less than the target lifting magnetic flux density. If the control device 5 determines that the number of steel plates lifted is insufficient, it lowers the lifting magnet 1 to lower the steel plates, increases the target magnetic flux density, and restarts the lifting of the steel plates. The control device 5 may determine the amount of increase in the target magnetic flux density based on the value of Ba2-Br. The control device 5 may change the target magnetic flux density by adding the value of Br-Ba2 to the original value. The control device 5 may change the target magnetic flux density by adding the value obtained by multiplying the value of Br-Ba2 by a predetermined coefficient to the original value. The predetermined coefficient may be less than 1 or greater than 1.

[0054] <<If a large number of items are lifted>> The control device 5 determines that too many steel plates have been lifted if the absolute value of the difference is greater than the lifting count determination threshold and the second magnetic flux density is greater than the target lifting magnetic flux density. If the control device 5 determines that too many steel plates have been lifted, it reduces the voltage applied to the electromagnet coil 2 to detach the steel plates that are excessively attracted to the lifting magnet 1. The process of adjusting the lifting count by detaching the excess steel plates is also called the lifting count adjustment process.

[0055] The control device 5 acquires measured values ​​of the magnetic pole flux density from the magnetic flux sensor 4 while reducing the voltage applied to the electromagnet coil 2, that is, during the process of separating the excess steel plate. The magnetic pole flux density when the voltage applied to the electromagnet coil 2 is being reduced is also called the third magnetic flux density and is represented by Ba3.

[0056] The control device 5 reduces the voltage applied to the electromagnet coil 2 until the decrease in the third magnetic flux density relative to the second magnetic flux density (which is the magnetic pole flux density when the steel plate is suspended) reaches the disconnection detection threshold B2. Specifically, the control device 5 measures the third magnetic flux density while reducing the voltage applied to the electromagnet coil 2, and stops reducing the voltage applied to the electromagnet coil 2 when the decrease in the third magnetic flux density relative to the second magnetic flux density becomes equal to or greater than the disconnection detection threshold B2. In other words, the reduction in the voltage applied to the electromagnet coil 2 stops when Ba2 - Ba3 ≥ B2 is satisfied.

[0057] Here, the nth steel plate from the side closest to the electromagnet coil 2 has a magnetic flux Φ represented by the following equation (4). n It is passing through.

[0058]

number

[0059] If the nth steel plate falls by decreasing the voltage applied to the electromagnet coil 2, the third magnetic flux density will be Φ relative to the second magnetic flux density. n It decreases by only that much. Then the detachment detection threshold B2 is Φ n By setting it to a value smaller than Φn, the separation of the nth steel plate is detected. As an example, it is preferable to set B2 to a value between 10% and 50% of Φn. If the separation detection threshold B2 is too small, separation is likely to be falsely detected. Conversely, if the separation detection threshold B2 is too large, the separation of the steel plate is difficult to detect. As an example, it is preferable to set the separation detection threshold B2 to a value within the range of 0.01T or more and 0.20T or less, and more preferably to a value within the range of 0.05T or more and 0.10T or less.

[0060] If the reduction in the voltage applied to the electromagnet coil 2 is stopped only when the decrease in the third magnetic flux density relative to the second magnetic flux density exceeds the separation detection threshold B2, the magnetic pole flux density may decrease too much, causing more steel plates than necessary to be separated along with the steel plate to be separated.

[0061] Therefore, the control device 5 may increase the voltage applied to the electromagnet coil 2 after the decrease in the third magnetic flux density relative to the second magnetic flux density becomes equal to or greater than the separation detection threshold B2. By doing so, the control device 5 can complete the separation of the steel plate to be separated when the decrease in the third magnetic flux density relative to the second magnetic flux density becomes equal to or greater than the separation detection threshold B2, while maintaining the attraction of steel plates that are not to be separated by increasing the voltage applied to the electromagnet coil 2.

[0062] Furthermore, if the voltage applied to the electromagnet coil 2 is increased immediately after the decrease in the third magnetic flux density relative to the second magnetic flux density exceeds the detachment detection threshold B2, the steel plate that was detached from the lifting magnet 1 may be attracted to the lifting magnet 1 again. Therefore, the control device 5 may increase the voltage applied to the electromagnet coil 2 after a waiting time T has elapsed following the decrease in the third magnetic flux density relative to the second magnetic flux density exceeding the detachment detection threshold B2. The waiting time T is preferably set to a time within the range of 0.1 seconds or more and 2.0 seconds or less, and more preferably to a time within the range of 0.2 seconds or more and 1.0 second or less.

[0063] After detecting the separation of the steel plates, or after increasing the voltage applied to the electromagnet coil 2, the control device 5 obtains a measurement of the magnetic pole flux density from the magnetic flux sensor 4. Based on the measurement of the magnetic pole flux density, the control device 5 determines the number of plates to be lifted. If the number of plates to be lifted is too large, the control device 5 repeats the process of separating the steel plates by reducing the voltage applied to the electromagnet coil 2 until the number of plates to be lifted becomes appropriate.

[0064] As described above, the control device 5 disconnects the excessively lifted steel plates by reducing the voltage applied to the electromagnet coil 2. The control device 5 may control the rate at which the voltage applied to the electromagnet coil 2 decreases. The rate at which the voltage decreases is calculated as the value at which the voltage decreases per unit time. The rate at which the voltage decreases may be slow or fast. When the voltage is reduced, a reverse current is generated in the coil, so the decrease in magnetic pole flux density does not immediately follow the decrease in voltage. Therefore, even if the rate at which the voltage decreases is fast, it will not occur that more steel plates than necessary are disconnected. The control device 5 may instantaneously reduce the voltage applied to the electromagnet coil 2 to zero.

[0065] <<Example of a flowchart>> The control device 5 may execute a steel plate lifting method that includes the steps of the flowchart illustrated in Figures 4A and 4B. The steel plate lifting method may be implemented as a steel plate lifting program to be executed by a processor included in the control device 5. The steel plate lifting program may be stored in a non-temporary computer-readable medium.

[0066] The control device 5 sets a target magnetic flux density (Br) corresponding to the number of steel plates to be lifted (step S1 in Figure 4A). Based on the target magnetic flux density, the control device 5 determines the voltage to be applied to the electromagnet coil 2. The voltage applied to the electromagnet coil 2 is also called the coil voltage. The control device 5 controls the voltage application device 6 to apply the coil voltage to the electromagnet coil 2 (step S2). The control device 5 measures the magnetic pole flux density (Ba1) using the magnetic flux sensor 4 (step S3).

[0067] The control device 5 determines whether the difference between the magnetic pole flux density and the target magnetic flux density is less than or equal to the lifting control threshold (step S4). In other words, the control device 5 determines whether |Ba1-Br|≦B1. If the difference is not less than or equal to the lifting control threshold (step S4:NO), that is, if the difference is greater than the lifting control threshold, the control device 5 returns to the procedure in step S2 and re-determines the coil voltage.

[0068] When the difference is equal to or less than the lifting control threshold (step S4: YES), the control device 5 determines that the specified number of steel plates to be lifted are attracted, and causes the crane 8 to lift the lifting magnet 1 (step S5). The control device 5 uses the magnetic flux sensor 4 to measure a second magnetic flux density (Ba2), which is the magnetic pole magnetic flux density after the lifting magnet 1 starts to rise (step S6).

[0069] The control device 5 determines whether the number of lifted steel plates is appropriate based on the second magnetic flux density (step S7). When the control device 5 determines that the number of lifted steel plates is appropriate (step S7: appropriate), it ends the execution of the procedure in the flowcharts of FIG. 4A and FIG. 4B, and conveys the lifted steel plates.

[0070] When the control device 5 determines that the number of lifted steel plates is too small (step S7: too small), it lowers the lifting magnet 1 (step S8). After lowering the lifting magnet 1 and lowering the steel plates, the control device 5 increases the target magnetic flux density (step S9). After increasing the target magnetic flux density, the control device 5 returns to the procedure of step S2 to re-determine the coil voltage.

[0071] When the control device 5 determines that the number of lifted steel plates is too large (step S7: too large), it decreases the coil voltage (step S10 in FIG. 4B). The control device 5 measures a third magnetic flux density (Ba3), which is the magnetic pole magnetic flux density when the coil voltage is being decreased (step S11).

[0072] The control device 5 determines whether the amount of decrease in the third magnetic flux density relative to the second magnetic flux density is equal to or greater than the separation detection threshold B2 (step S12). That is, the control device 5 determines whether Ba2-Ba3≧B2 is satisfied. When Ba2-Ba3<B2, that is, when Ba2-Ba3≧B2 is not satisfied (step S12: NO), the control device 5 returns to the procedure of step S10 to further decrease the coil voltage.

[0073] If Ba2-Ba3≧B2 is satisfied (step S12:YES), the control device 5 waits for a waiting time T (step S13). After waiting, the control device 5 increases the coil voltage (step S14). The control device 5 measures the magnetic pole flux density and determines whether the number of plates to be lifted is appropriate (step S15). If the number of plates to be lifted is too large (step S15: too large), the control device 5 returns to the procedure in step S10 and further decreases the coil voltage. If the number of plates to be lifted is appropriate (step S15: appropriate), the control device 5 finishes executing the procedures in the flowcharts of Figures 4A and 4B and transports the lifted steel plates.

[0074] After the steel plate has been lifted in the procedure described above, the control device 5 may further increase the coil voltage to prevent the steel plate from falling when the steel plate is being transported by the traverse of the crane 8. The control device 5 may increase the coil voltage to the maximum possible value when the steel plate is being transported by the traverse of the crane 8.

[0075] The control device 5 does not need to perform the waiting procedure in step S13 in the flowchart of Figure 4B. The control device 5 may simply stop decreasing the coil voltage without performing the coil voltage increase procedure in step S14.

[0076] The procedure for determining the number of items to be lifted, step S7 in the flowchart of Figure 4A, or step S15 in the flowchart of Figure 4B, may be performed by the control device 5, by a human, or by other means such as an image processing device including a camera. The procedure for applying the coil voltage, step S2 in the flowchart of Figure 4A, may be performed by human operation. The procedure for raising the lifting magnet 1, step S5 in the flowchart of Figure 4A, or the procedure for lowering the lifting magnet 1, step S8, may be performed by human operation. The procedure for decreasing the coil voltage, step S10 in the flowchart of Figure 4B, may be performed by human operation.

[0077] <Method of manufacturing steel plates> The steel plate lifting device 10 may be used in the steel plate transport process during the manufacturing of steel plates. In one embodiment, a steel plate manufacturing method may be implemented that includes a step of lifting and transporting at least one steel plate as the lifting target 20 using the steel plate lifting device 10 according to the above embodiment.

[0078] <Summary> As described above, the steel plate lifting device 10 according to this disclosure can separate only the excess steel plates while the steel plates remain lifted, even when a large number of steel plates have been lifted. Compared to the method of lowering the steel plates and then lifting the target number of plates again, the efficiency of the steel plate lifting and transporting operations is improved. Furthermore, the production efficiency of steel plates is increased.

[0079] (Examples) Hereinafter, an embodiment of the steel plate lifting device 10 according to one embodiment of the present disclosure will be described.

[0080] <Testing the control of the number of sheets to be lifted> To evaluate the accuracy of controlling the number of steel plates separated by a steel plate lifting device 10 and lifting method according to one embodiment of the present disclosure, the following tests were performed. In the lifting magnet 1 used in the tests, the diameter of the magnetic pole 3 was 150 mm. The outer diameter of the yoke 7 was 350 mm. The thickness of the yoke 7 was 20 mm. The height (length along the winding axis) of the electromagnet coil 2 was 150 mm.

[0081] The test was conducted using a steel plate designated as SS400 (Structural Steel) with a saturation magnetic flux density of 1.5T as the lifting target 20. The thickness of the steel plate used as the lifting target 20 was set to 4.5 mm. Ten steel plates were prepared as the lifting target 20. The lifting control threshold (B1) was set to 0.1T. The detachment detection threshold (B2) was set to 0.1T. The waiting time T was set to 0.2 seconds. The target number of steel plates to be lifted was set to 2. The target magnetic flux density (Br) for lifting 2 steel plates is 0.36T, but the target magnetic flux density (Br) was set to 0.54T in order to lift more steel plates than the target number of 2.

[0082] The parameters set under the above conditions are shown in Table 1 below.

[0083] [Table 1]

[0084] The test was conducted 10 times following the procedures in the flowcharts in Figures 4A and 4B. In each test, the stacking order of the 10 steel plates was changed.

[0085] Under the conditions described above, Figure 5 shows the test results when the coil voltage was reduced by the minimum necessary value to detach the excess steel plate. In the graph in Figure 5, the horizontal axis represents the elapsed time since the start of lifting the steel plate, and the vertical axis represents the magnetic flux density. In the state with three plates suspended, the magnetic flux density was 0.54T. By reducing the coil voltage by 50%, the magnetic flux density decreased by more than 0.1T, which is the detachment detection threshold B2. At this point, one steel plate was detached, and the state with two plates suspended remained. When the decrease in coil voltage was stopped when the magnetic flux density had fallen by more than 0.2T, the second steel plate was not detached, and the state with two plates suspended was maintained. In the state with two plates suspended, the magnetic flux density approached 0.36T.

[0086] Under the conditions described above, Figure 6 shows the test results when the coil voltage was reduced to 50% of the original voltage to detach the excess steel plate, and then the coil voltage was increased back to the original voltage. In the graph in Figure 6, the horizontal axis represents the elapsed time since the start of lifting the steel plate, and the vertical axis represents the magnetic flux density of the magnetic poles.

[0087] In the state with three plates suspended, the magnetic flux density was 0.54T. By reducing the coil voltage by 50%, the magnetic flux density dropped to 0.1T or more, which is the detachment detection threshold B2. At this point, one steel plate was detached, resulting in a state with two plates suspended. Then, after a waiting time T had elapsed since the magnetic flux density dropped to 0.54T or more, the coil voltage was increased back to its original voltage, preventing the second steel plate from detaching and maintaining the two-plate suspension state. In the two-plate suspension state, the magnetic flux density approached 0.36T.

[0088] Furthermore, Table 2 shows the results of 10 tests in which the coil voltage was reduced to 50% of the original voltage to disconnect the excess steel plates, and then the coil voltage was increased back to the original voltage. The applied voltage is the voltage applied to electromagnet coil 2 at the end of the test. The actual value is the magnetic flux density of the pole at the end of the test. The number of lifted plates is the number of steel plates that were lifted at the end of the test. In each test, the number of lifted steel plates was controlled to 2.

[0089] [Table 2]

[0090] Figure 7 shows the test results for a comparative example of a lifting method in which the coil voltage was temporarily reduced to zero to detach the excess steel plate, a 2-second wait was made, and then the coil voltage was increased back to the original voltage. In the graph in Figure 7, the horizontal axis represents the elapsed time since the start of lifting the steel plate, and the vertical axis represents the magnetic flux density. In the state of three plates being lifted, the magnetic flux density was 0.54T. By reducing the coil voltage to zero, the magnetic flux density decreased to 0.1T or more, which is the detachment detection threshold B2. At this time, one steel plate was detached, and the state became two plates being lifted. During the 2-second wait before increasing the coil voltage back to the original voltage, the magnetic flux density sometimes decreased further. If the magnetic flux density decreased further, the second steel plate was detached, and the state became one plate being lifted.

[0091] Table 3 shows the results of each of the 10 tests conducted on the comparative example. The applied voltage is the voltage applied to electromagnet coil 2 at the end of the test. The actual value is the magnetic flux density at the end of the test. The number of lifted plates is the number of steel plates that were lifted at the end of the test. In each test, the number of lifted steel plates was sometimes controlled to two, but sometimes it was only one.

[0092] [Table 3]

[0093] Based on the test results described above, the lifting method according to this disclosure can control the number of steel plates detached with higher precision than the lifting method according to the comparative example, which does not control the magnetic flux density, by controlling the decrease in magnetic flux density. As a result, with the steel plate lifting device 10 and lifting method according to this disclosure, only the excessively attracted steel plates are efficiently detached. Furthermore, the productivity of steel plates is improved by the improved transport efficiency of the steel plates.

[0094] <Tests related to waiting time> In the above-mentioned test to control the number of steel plates lifted, the waiting time was set to 0.2 seconds. To confirm the effect of the waiting time on the control of the number of steel plates lifted, the test was conducted under the condition that the waiting time was changed to a different time while other parameters remained unchanged. Specifically, the waiting time was set to 0.05 seconds, 0.2 seconds, 1.5 seconds, and 2.5 seconds, and 10 tests were conducted in which the number of steel plates lifted was controlled to 2 for each set waiting time. The results of each of the 10 tests in which the number of steel plates lifted was controlled to 2 with the waiting time set to 0.05 seconds and 0.2 seconds are shown in Table 4. The results of each of the 10 tests in which the number of steel plates lifted was controlled to 2 with the waiting time set to 1.5 seconds and 2.5 seconds are shown in Table 5.

[0095] [Table 4]

[0096] [Table 5]

[0097] When the waiting time was set to 0.2 seconds (see Table 4) and when the waiting time was set to 1.5 seconds (see Table 5), the number of steel plates lifted was successfully controlled to 2 in 9 out of 10 attempts. In other words, when the waiting time was set to 0.1 seconds or more and 2.0 seconds or less, the number of steel plates lifted was successfully controlled to 2 in 9 out of 10 attempts.

[0098] On the other hand, when the waiting time was set to 0.05 seconds (see Table 4), the separated steel plates sometimes reattached, resulting in a total of 3 plates being lifted. Controlling the number of lifted steel plates to 2 was only successful in 4 out of 10 attempts. Furthermore, when the waiting time was set to 2.5 seconds (see Table 5), the second steel plate, which was not intended for separation, sometimes fell, resulting in a total of 1 plate being lifted. Controlling the number of lifted steel plates to 2 was only successful in 3 out of 10 attempts. In other words, when the waiting time was less than 0.1 seconds, or greater than 2.0 seconds, the number of times the number of lifted steel plates was successfully controlled to 2 decreased.

[0099] In light of the test results described above, it is preferable to set the waiting time to a time within the range of 0.1 seconds and 2.0 seconds or less.

[0100] While embodiments of this disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art can make various modifications or alterations based on this disclosure. Therefore, it should be noted that these modifications or alterations are included within the scope of this disclosure. For example, the functions included in each component or step can be rearranged in a logically consistent manner, and multiple components or steps can be combined into one or divided. Embodiments relating to this disclosure can also be realized as programs executed by a processor in the device or as storage media recording such programs. These should also be understood to be included within the scope of this disclosure. [Explanation of Symbols]

[0101] 10. Steel plate lifting device (5: control device, 6: voltage application device, 8: crane) 1. Lifting magnet (2: electromagnet coil, 3: magnetic pole, 4: magnetic flux sensor, 7: yoke) 20 Lifting targets (141: area)

Claims

1. A method for lifting steel plates, comprising using a lifting magnet having magnetic poles and an electromagnet coil configured to control the voltage for exciting the magnetic poles, to lift at least one steel plate from a stack of multiple steel plates as the target for lifting, A lifting step in which a voltage is applied to the electromagnet coil and the lifting magnet is raised so that the first magnetic flux density, which is the magnetic flux density of the magnetic pole in the state before the lifting magnet begins to rise, becomes the target magnetic flux density required for the magnetic pole to lift the object to be lifted, and the object to be lifted is lifted by raising the lifting magnet, A determination step for determining the number of steel plates lifted in the lifting step, In the determination step, if it is determined that the number of lifted steel plates is greater than the target number, a lifting number adjustment step is performed in which the voltage applied to the electromagnet coil is reduced until the amount of decrease in the third magnetic flux density, which is the magnetic flux density of the magnetic pole when the voltage applied to the electromagnet coil is reduced relative to the second magnetic flux density, which is the magnetic flux density of the magnetic pole when the steel plates are lifted, is less than the amount of decrease from the saturation magnetic flux density when the steel plates including the excess steel plates are lifted to the saturation magnetic flux density when the excess steel plates are separated, is greater than or equal to a separation detection threshold, in order to separate the excess steel plates that are greater than the target number. A method for lifting steel plates, including [the specified method].

2. The method for lifting a steel plate according to Claim 1, wherein the separation detection threshold is set to a value of 10% to 50% of the decrease in the amount from the saturation magnetic flux density when the steel plate including the excess steel plate is lifted to the saturation magnetic flux density when the excess steel plate is separated.

3. The method for lifting a steel plate according to claim 1, wherein in the lifting step, the voltage applied to the electromagnet coil is controlled so that the difference between the first magnetic flux density and the target magnetic flux density is less than or equal to a lifting control threshold.

4. The method for lifting steel plates according to claim 1, wherein in the determination step, the number of lifted steel plates is determined based on the second magnetic flux density.

5. A method for lifting steel plates according to any one of claims 1 to 4, wherein in the lifting number adjustment step, the voltage applied to the electromagnet coil is increased after the amount of decrease in the third magnetic flux density relative to the second magnetic flux density becomes equal to or greater than the separation detection threshold.

6. In the lifting number adjustment step, after a waiting period has elapsed since it was determined that the decrease in the third magnetic flux density relative to the second magnetic flux density has exceeded the separation detection threshold, the voltage applied to the electromagnet coil is increased. The method for lifting a steel plate according to claim 5, wherein the waiting time is within the range of 0.1 seconds or more and 2.0 seconds or less.

7. A method for lifting steel plates according to any one of claims 1 to 4, wherein in the lifting step, the target magnetic flux density is calculated based on the thickness and saturation magnetic flux density of at least one steel plate to be lifted, and the dimensions of the magnetic poles.

8. A steel plate lifting device for lifting at least one steel plate from among a stack of multiple steel plates, A lifting magnet having magnetic poles and an electromagnet coil configured to control the voltage for exciting the magnetic poles, A voltage application device for applying a voltage to the aforementioned electromagnet coil, A magnetic flux sensor for measuring the magnetic flux density of the aforementioned magnetic pole, A control device for controlling the voltage applied to the electromagnetic coil Equipped with, The magnetic flux sensor measures the magnetic flux density of the magnetic pole as the first magnetic flux density when the object to be lifted is attracted to the magnetic pole and before the lifting magnet begins to rise. The control device applies a voltage to the electromagnet coil and raises the lifting magnet so that the first magnetic flux density becomes the target magnetic flux density required for the magnetic pole to lift the object to be lifted, thereby lifting the object to be lifted. The magnetic flux sensor measures the magnetic flux density of the magnetic pole when the steel plate is suspended, and defines this as the second magnetic flux density. If the control device determines that the number of steel plates lifted is greater than the target number, it reduces the voltage applied to the electromagnet coil in order to detach the excess steel plates that are greater than the target number. The magnetic flux sensor measures the magnetic flux density of the magnetic pole as the third magnetic flux density when the voltage applied to the electromagnet coil is reduced. The control device reduces the voltage applied to the electromagnet coil until the amount of decrease in the third magnetic flux density relative to the second magnetic flux density becomes less than or equal to a separation detection threshold, which is a value less than the amount of decrease from the saturation magnetic flux density when the steel plate including the excess steel plate is suspended to the saturation magnetic flux density when the excess steel plate is separated. A lifting device for steel plates.

9. The steel plate lifting device according to claim 8, wherein the separation detection threshold is set to a value of 10% to 50% of the decrease in the amount from the saturation magnetic flux density when the steel plate including the excess steel plate is lifted to the saturation magnetic flux density when the excess steel plate is separated.

10. The steel plate lifting device according to claim 8, wherein the control device controls the voltage applied to the electromagnet coil so that the difference between the first magnetic flux density and the target magnetic flux density is less than or equal to a lifting control threshold.

11. The steel plate lifting device according to any one of claims 8 to 10, wherein the control device increases the voltage applied to the electromagnet coil after the amount of decrease in the third magnetic flux density relative to the second magnetic flux density becomes equal to or greater than a separation detection threshold.

12. The control device increases the voltage applied to the electromagnet coil after a waiting period has elapsed since it determined that the decrease in the third magnetic flux density relative to the second magnetic flux density has exceeded the disconnection detection threshold. The steel plate lifting device according to claim 11, wherein the waiting time is within the range of 0.1 seconds or more and 2.0 seconds or less.

13. The steel plate lifting device according to any one of claims 8 to 10, wherein the control device calculates the target magnetic flux density based on the thickness and saturation magnetic flux density of each of the at least one steel plate to be lifted and the dimensions of the magnetic poles.

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