EP3100289A1

EP3100289A1 - Magnet and method for handling metal sheets

Info

Publication number: EP3100289A1
Application number: EP15708853.5A
Authority: EP
Inventors: Reijo NÄTTI
Original assignee: IXTUR Oy
Current assignee: IXTUR Oy
Priority date: 2014-01-30
Filing date: 2015-01-30
Publication date: 2016-12-07
Anticipated expiration: 2035-01-30
Also published as: FI20145101L; WO2015114220A1; EP3100289B1

Abstract

The present invention relates to a magnet (100), which comprises a body (101) and a slide (110) that is arranged to be movable relative to the body (101). The slide (110) comprises a permanent magnet (111), and a first and a second pole piece (112, 113) that are attached to opposite magnetic pole surfaces of the permanent magnet (111). The body (101) comprises a first and a second section (103, 104) made of magnetic material for directing magnetic flux to an object (200) to be attached. The first and the second section (103, 104) are separated and attached to a third section (105) of the body (101), which third section (105) is made of non-magnetic material. The first section (103) comprises a hole (106) that opens at a first end into a first cavity (107) and at a second end into a second cavity (108) of the body (101), the first and the second cavity (107, 108) containing a medium. The magnet (100) further comprises means (116, 117, 118, 119, 120) for transferring the medium into and out of the first and the second cavity (107, 108) in order to move the slide (110) that is movably arranged in the hole (106). The invention also relates to a method for handling metal sheets (200).

Description

Magnet and method for handling metal sheets

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a magnet and to a method for handling metal sheets according to the preambles of the appended independent claims.

BACKGROUND OF THE INVENTION

Magnets are used in many fields of technology to perform various operations, such as to control motion, to switch electrical circuits and to move objects. A magnet is typically provided with a certain functionality to change its magnetic state. A mag- net may comprise, for example, a coil which produces a magnetic field that is dependent on the amount and the direction of the electric current supplied to the coil. The coil may be used alone, or it can be combined with a permanent magnet, whereby the coil is used to increase or decrease the magnetic field produced by the permanent magnet. Alternatively, a magnet may comprise a movable part, the position of which determines the magnetic state of the magnet. The movable part, which comprises a permanent magnet, is moved relative to a body of the magnet, for example, with a magnetic force produced by a coil that is arranged to encircle the movable part.

An example of a magnet that comprises a movable part for changing the magnetic state of the magnet is disclosed in the document WO 2012/160262. The magnet of the document WO 2012/160262 is a so-called bi-stable magnet, wherein the movable part, which comprises a permanent magnet, is arranged to be movable relative to a body of the magnet between two positions. In the first position, the movable part is in contact with the body, whereby the magnetic flux generated by the permanent magnet may be directed through the body to an object to be attached. In the second position, the movable part is separated from the body so that the flow of the magnetic flux in the body is significantly reduced and thus the holding force of the magnet is almost negligible. The body of the magnet comprises a coil that is arranged around the movable part. The movable part is moved between the two positions by supplying a sufficient amount of electric current through the coil in a suitable direction.

A problem associated with the magnet of the document WO 2012/160262 relates to the bi-stability of the magnet. The magnet can only be in either of its two states and the state of the magnet is changed by moving the movable part from one stable position to the other. Consequently, the holding force of the magnet that is dependent on the position of the movable part has only two fixed values. In other words, the holding force of the magnet of the document WO 2012/160262 cannot be adjusted, which reduces the applicability of the magnet.

Another problem associated with the magnet of the document WO 2012/160262 relates to the second position of the movable part, i.e. to the position wherein the movable part is not in contact with the body. In order to make sure that the movable part stays in its second position, either electric current must constantly be sup- plied to the coil, or the magnet must be provided with springs or other suitable means to push the movable part towards the second position. In the first case, a disadvantage is the power consumption of the coil, whereas in the second case, a disadvantage is the complicated structure, which may easily be damaged, thus resulting in a malfunction of the magnet. OBJECTIVES OF THE INVENTION

It is the main objective of the present invention to reduce or even eliminate prior art problems presented above.

It is an objective of the present invention to provide a magnet whose holding force can be changed easily and which consumes very little energy. In more detail, it is an objective of the invention to provide a magnet whose holding force is steplessly adjustable and which enables to maintain the holding force constant with very little energy.

It is a further objective of the invention to provide a magnet having a structure that enables to achieve a large holding force with a small size. It is still a further objec- tive of the invention to provide a magnet that has a simple structure, low manufacturing costs, a long life expectancy and a great reliability.

It is also an objective of the present invention to provide a method for handling metal sheets. In more detail, it is an objective of the invention to provide a method enabling to lift a desired number of metal sheets from a stack of metal sheets. In order to realise the above-mentioned objectives, the magnet and the method according to the invention are characterised by what is presented in the characterising parts of the appended independent claims. Advantageous embodiments of the invention are described in the dependent claims. DESCRIPTION OF THE INVENTION

A typical magnet according to the invention comprises a body that comprises a first and a second section made of magnetic material for directing magnetic flux to an object to be attached, the first and the second section being separated and at- tached to a third section of the body, which third section is made of non-magnetic material, and a slide that comprises a permanent magnet, and a first and a second pole piece attached to opposite magnetic pole surfaces of the permanent magnet, the first and the second pole piece being made of magnetic material and the slide being arranged to be movable relative to the body in order to alter the path of the magnetic flux generated by the permanent magnet. In a typical magnet according to the invention the first section comprises a hole in which the slide is movably arranged, which hole opens at a first end into a first cavity of the body and at a second end into a second cavity of the body, the bottom of the first cavity being defined at least partly by the second section, the first and the second cavity contain a medium, and the magnet comprises means for transferring the medium into and out of the first and the second cavity in order to move the slide.

In the magnet according to the invention the first and the second section of the body are separated by the third section, so that inside the magnet the flow of magnetic flux between the first and the second section is insignificant. An object that is arranged in contact with both the first and the second section closes the magnetic circuit, so that magnetic flux can flow between the first and the second section through the object. The position of the slide determines the amount of magnetic flux that can be conducted through the object.

The slide is movably arranged in the hole of the first section. This means that part of the slide is always inside the hole. The slide is arranged to be movable in two opposite directions, which are parallel to the longitudinal direction of the hole. The slide is moved in a first direction when it is moved towards the first cavity and in a second direction when it is moved towards the second cavity. The slide is arranged in the hole in such a manner that the second pole piece is directed towards the bottom of the first cavity.

The slide is moved relative to the body by transferring medium into and out of the first and the second cavity. When medium is transferred into the second cavity and out of the first cavity, the slide is moved in the first direction. And when medium is transferred into the first cavity and out of the second cavity, the slide is moved in the second direction. The first and the second cavity can be connected together through the means for transferring the medium, whereby medium can be transferred between the first and the second cavity, i.e. from one cavity to the other. Alternatively, the means for transferring the medium can be arranged to process the first and the second cavity separately from one another. In this case, the means for transferring the medium can comprise a container for each cavity for receiving the medium that has been transferred out of the cavity.

The holding force of the magnet depends on the position of the slide. The holding force is at its minimum when the slide is at a first position, in which the permanent magnet, and at least part of the first and the second pole piece are located inside the hole. At the first position of the slide, magnetic flux generated by the permanent magnet is short-circuited by the first section. This means that the magnetic flux flows from one pole piece to the other mainly through a portion of the first section that surrounds the hole. The magnetic circuit is thus closed by the first section. At the first position, the magnetic flux is mainly directed from one pole piece to the first section, and from the first section to the other pole piece in a direction, which is essentially perpendicular to a wall of the hole. If the slide that is at the first position is moved in the first or the second direction, the permanent magnet produces a counterforce that works against the moving force and tries to pull the slide back to the first position. The holding force of the magnet is at its maximum when the slide is at a second position, in which the second pole piece is in contact with the bottom of the first cavity. At the second position of the slide, magnetic flux generated by the permanent magnet flows efficiently through an object that is in contact with the first and the second section. Magnetic flux flows between the permanent magnet and the object through the first pole piece and the first section, and through the second pole piece and the second section. At the second position, the second pole piece is located outside the hole, and at least part of the first pole piece is located inside the hole. Preferably, also the permanent magnet is located outside the hole. The magnetic flux is mainly directed between the first pole piece and the first section in a direction essentially perpendicular to the wall of the hole, and between the second pole piece and the bottom of the first cavity in a direction essentially perpendicular to the bottom wall. The direction of the magnetic flux to/from the second pole piece is thus turned by about 90 degrees between the first and the second position of the slide. At the second position of the slide, the distance between the second pole piece and the second section is preferably less than 5 mm, less than 1 mm, or less than 0.1 mm. Preferably, at the second position of the slide, the second pole piece is in contact with the portion of the second section that defines the bottom of the first cavity.

The holding force of the magnet can be changed by moving the slide between the first and the second position. The position of the slide is steplessly adjustable by transferring medium into and out of the first and the second cavity. Because the slide can be moved continuously between the first and the second position, the holding force can be set at a desired value by positioning the slide at a suitable position. The holding force can be increased by moving the slide in the first direction, i.e. towards the second position. The holding force can be decreased by mov- ing the slide in the second direction, i.e. towards the first position.

Depending on the application, the sections of the body may have various forms and sizes. The sections of the body can be formed of one or more parts. The sections of the body can be arranged within each other in such a manner that the first section surrounds the third section, which surrounds the second section. The third section can be a sleeve that is arranged around the second section that can be cylindrical. The first and the second section of the body are made of magnetic material that is suitable for conducting magnetic flux. Magnetic material of the first and the second section is ferromagnetic material, such as iron, nickel, cobalt or their alloys. The third section is made of non-magnetic material, which can be par- amagnetic material, such as resin, brass or aluminium, or diamagnetic material, such as acid-proof steel or stainless steel.

The slide has a sandwich structure, wherein the permanent magnet is arranged between the first and the second pole piece. The first and the second pole piece are attached to different poles of the permanent magnet and are made of magnetic material so that the magnetic flux generated by the permanent magnet may be conducted through them. Magnetic material of the first and the second pole piece is ferromagnetic material, preferably iron. The permanent magnet can be, for example, a neodymium magnet, an alnico magnet or a samarium-cobalt magnet.

The first and the second pole piece can be, for example, disc-shaped and have the same diameter. The permanent magnet can be, for example, disc-shaped and have the same diameter as or smaller diameter than the first and the second pole piece. Typically, the thickness of the permanent magnet is smaller than the thickness of the first pole piece and the thickness of the second pole piece. The use of thicker pole pieces prevents the magnetic flux from saturating. The permanent magnet may consist of one or more magnet pieces arranged in one or more layers. The permanent magnet can, for example, be formed of sector pieces arranged in one layer in such a manner that the same poles of the sector pieces are disposed on the same side of the permanent magnet. The number of sector pieces can be, for example, 2, 3, 4-6, or 7-10. The permanent magnet can alternatively be formed of magnet pieces arranged one on the other. The magnet pieces can be arranged one on the other in such a manner that ferromagnetic discs are arranged between the magnet pieces and the different poles of the magnet pieces are arranged to face each other. According to an embodiment of the invention the slide and the hole are cylindrical. The slide and the hole can, however, have other forms, such as rectangular. Preferably, the diameter of the slide is only slightly smaller than the diameter of the hole, whereby the wall of the hole can efficiently support the slide while it is moved between the first and the second position. The diameter of the hole can be, for example, less than 10 mm, 10-50 mm, 50-200 mm or 200-500 mm. The diameter of the slide can be, for example, less than 2 mm, less than 1 mm, less than 0.5 mm, less than 0.1 mm or 0.005-0.5 mm smaller than the diameter of the hole. The thickness of the slide is preferably larger than the depth of the hole. The thickness of the slide can be, for example, less than 3 mm, 3-5 mm, 5-10 mm, 10-100 mm or 100-500 mm.

Depending on the application, the first and the second cavity may have various forms and volumes. Preferably, the first cavity is cylindrical and its diameter is at least equal to the diameter of the hole. The bottom of the first cavity is defined partly or totally by the second section, whereas the wall of the first cavity may be defined partly or totally by the third section, or by some other section of the body. Preferably, the depth of the first cavity is smaller than the thickness of the slide, whereby the slide may extend, at its second position, from the bottom of the first cavity into the hole.

The magnet according to the invention is suitable for moving objects and can thus be used as a lifting magnet. The magnet can be used, for example, as follows to move an object from one location to another. First, an attachment surface of the magnet is arranged in contact with an object in such a manner that the first and the second section are in contact with the object. Next, the holding force of the magnet is increased by moving the slide in the first direction until the magnet is attached to the object. And then, the object is moved with the magnet to a desired location, in which the magnet is detached from the object by moving the slide in the second direction until the object is detached from the magnet.

An advantage of the magnet according to the invention is that its holding force is steplessly adjustable and the holding force can be adjusted easily. Another ad- vantage of the magnet is that it consumes very little energy for maintaining the holding force constant. Still another advantage of the magnet is that the slide can stay in place at the first and the second position even without the use of the medium, with the help of the magnetic field produced by the permanent magnet. When the slide is at the first or the second position, the magnet does not need any ener- gy. Still another advantage of the magnet is its simple structure that makes the magnet very robust and reliable. Still another advantage of the magnet is that the demagnetization of the permanent magnet can be avoided by arranging the slide at its first position, when the magnet is not in use. Still another advantage of the magnet is that its holding force is minimal when the magnet is OFF, i.e. when the slide is at the first position.

According to an embodiment of the invention the magnet comprises a sealing ring attached around the slide or attached to the wall of the hole. The sealing ring divides the airtight space inside the body into two portions and prevents the medium from flowing between said portions. The sealing ring makes it easier to create a sufficient pressure difference and enables moving the slide efficiently. The sealing ring can be made of, for example, silicone, ethylenepropylene, polyurethane, ni- trile-butadiene rubber or acetal plastics, or their compounds.

According to an embodiment of the invention the sealing ring is attached to a groove in the first pole piece. The sealing ring is attached to the first pole piece in such a position that the sealing ring stays inside the hole at all positions of the slide.

According to an embodiment of the invention the slide comprises a cap attached on top of the first pole piece for holding the sealing ring in place. The cap may comprise a groove in which the sealing ring is installed. The groove is preferably located close to the first pole piece. The groove may also be arranged to the cap and/or the first pole piece so that the sealing ring is hold in place between the cap and the first pole piece. The cap can be made of magnetic or non-magnetic material. According to an embodiment of the invention the means for transferring the medium into and out of the first and the second cavity comprises a first and a second conduit integrated into the body, a first end of the first conduit being in communication with the first cavity and a first end of the second conduit being in communica- tion with the second cavity. Medium can be transferred into and out of the first and the second cavity through the first and the second conduit, respectively. Preferably, the first end of the first conduit is arranged to the bottom of the first cavity, and the first end of the second conduit is arranged to the bottom of the second cavity. Second ends of the conduits may be in communication with each other so that the medium that has been transferred out of one cavity can be transferred into the other cavity. The magnet may comprise a plurality of first and second conduits. The number of the first and the second conduits can be, for example, 2-4, 5-10 or 10-30.

According to an embodiment of the invention a second end of the first conduit and a second end of the second conduit open outside the magnet. The first and the second conduit thus extend through the body.

According to an embodiment of the invention the means for transferring the medium into and out of the first and the second cavity comprises a pneumatic or a hydraulic system coupled to the second end of the first conduit and the second end of the second conduit.

According to an embodiment of the invention the means for transferring the medium into and out of the first and the second cavity comprises a pump coupled to the second end of the first and the second conduit. The pump can be configured to transfer medium in two directions, whereby the medium can be transferred from the first cavity into the second cavity, and vice versa. The means for transferring the medium into and out of the first and the second cavity may comprise two pumps, the first pump being coupled to the second end of the first conduit and the second pump being coupled to the second end of the second conduit. Depending on the type of the medium, the pump can be a hydraulic pump or a pneumatic pump, such as a piston pump, a screw pump or a gear pump. It is also possible to use an existing hydraulic or pneumatic system coupled to the second ends of the conduits for transferring the medium into and out of the cavities.

According to an embodiment of the invention the means for transferring the medium into and out of the first and the second cavity comprises a first pipe coupled between the pump and the second end of the first conduit, and a second pipe coupled between the pump and the second end of the second conduit.

According to an embodiment of the invention the medium is gas or liquid. A suitable gas for the magnet is, for example, air. Suitable liquids for the magnet are, for example, oil and water. Preferably, the liquid that is used also functions as a lubricant, reducing the friction between the slide and the wall of the hole.

According to an embodiment of the invention the body comprises a fourth section that defines the wall and the bottom of the second cavity, the fourth section being made of non-magnetic material. The fourth section is attached to the first section. Because the fourth section is made of non-magnetic material, the slide cannot attach to it. Non-magnetic material of the fourth section can be paramagnetic material, such as aluminium or plastic, or diamagnetic material, such as acid-proof steel.

The magnet can be manufactured, for example, as follows. First, a circular groove is machined into a second end of a circular block of magnetic material, the circular groove extending from the second end towards the first end. Next, the circular groove is filled with non-magnetic liquid material, which is hardened into solid form, and a bore, which is concentric with the circular groove, is drilled into the block from its first end towards the second end. The diameter of the bore is at least the inner diameter, but less than the outer diameter of the circular groove. The sum of the depth of the bore and the depth of the circular groove is larger than the thickness of the circular block. Then, a conduit is drilled for the first cavity, and a slide is inserted to the bore. And finally, a cover having a conduit for the second cavity and being made of non-magnetic material is attached to the first end of the block to cover the bore. The inner part of the machined block corresponds to the second section, the outer part corresponds to the first section, the intermediate part corresponds to the third section and the cover corresponds to the fourth section.

According to an embodiment of the invention the first section defines the second cavity of the body.

According to an embodiment of the invention the magnet comprises a magnetic flux sensor configured to measure magnetic flux density in the first section, and means for determining the position of the slide based on the measured magnetic flux density. By a magnetic flux sensor it is meant a transducer that varies its out- put voltage and/or current in response to magnetic flux density. Since the path of the magnetic flux in the first section is dependent on the position of the slide, the position of the slide can be determined from the output voltage and/or current of the magnetic flux sensor. The means for determining the position of the slide may comprise, for example, a comparator circuit for providing, as a response to the output voltage and/or current of the magnetic flux sensor, an output signal identifying whether the slide is at the first or the second position. The magnetic flux sensor may also be configured to indicate the position of the slide directly as a binary output. The magnetic flux sensor may also be used to detect whether an object is attached to the magnet or not.

The magnetic flux sensor can be arranged inside the first section, or attached on its surface. Preferably, the magnetic flux sensor is arranged inside the portion of the first section that surrounds the hole.

The magnet may comprise a plurality of magnetic flux sensors, which are config- ured to measure magnetic flux densities in different spatial locations and/or directions. The magnetic flux sensors can be configured to measure magnetic flux densities in orthogonal and/or opposite directions. The number of magnetic flux sensors can be, for example, 2, 3, or more than 3.

According to an embodiment of the invention the magnetic flux sensor is one of the following: a Hall sensor, an AMR magnetometer, a MEMS sensor or reed relay.

According to an embodiment of the invention the slide comprises a guiding rod extending to a bore in the bottom of the first cavity. The guiding rod, which extends in the longitudinal direction of the hole, may be arranged to extend partly or com- pletely through the slide. The guiding rod is preferably dimensioned in such a manner that part of the guiding rod stays in the bore all the time. The purpose of the guiding rod is to reduce the movement of the slide in other directions than the longitudinal direction of the hole. The guiding rod is made of non-magnetic material, which can be paramagnetic material, such as resin, brass or aluminium, or di- amagnetic material, such as acid-proof steel or stainless steel.

According to an embodiment of the invention the magnet comprises a coil attached to the body, the coil being configured to generate a magnetic force that increases or decreases the holding force of the magnet depending on the direction of electric current that is supplied to the coil. The coil is preferably arranged inside the third section in such a manner that the coil encircles the slide at least partly, when the slide is at the second position. Preferably, the coil is used to change the holding force of the magnet, when the slide is at its second position.

According to an embodiment of the invention the magnet comprises means for supplying electric current to the coil. The means for supplying electric current may comprise, for example, a battery that is connected to the coil via a control unit. The control unit is configured to control the amount and the direction of electric current supplied to the coil. The control unit may comprise one or more operating switches for using the magnet, and/or a wireless receiver for receiving control commands from a remote controller. The control unit may also comprise one or more indicator lights for indicating the status of the magnet, and/or a wireless transmitter for transmitting the status information to the remote controller.

According to an embodiment of the invention the coil is configured to generate a magnetic force for moving the slide towards the first or the second position de- pending on the direction of electric current that is supplied to the coil. The position of the slide can be changed with an electric current pulse that has a certain duration, magnitude and polarity. The duration and the magnitude of the electric current pulse that is needed to move the slide from the first position to the second position, and vice versa are highly dependent of the structure and the size of the magnet. The polarity of the electric current pulse depends on the direction to which the slide needs to be moved. Typically, the duration of an electric current pulse is 30-300 ms.

The present invention also relates to a method for handling metal sheets with a magnet according to the invention. A typical method according to the invention comprises placing the magnet according to the invention on an uppermost metal sheet of a stack of metal sheets, increasing the holding force of the magnet by moving the slide towards the uppermost metal sheet, lifting the magnet, and decreasing the holding force of the magnet by moving the slide away from the uppermost metal sheet until a predetermined number of metal sheets remains at- tached to the magnet.

An advantage of the method according to the invention is that it enables to lift a desired number of metal sheets from a stack of metal sheets. It also enables to detach metal sheets from the magnet one by one. According to an embodiment of the invention the method comprises, after the step of decreasing the holding force of the magnet, increasing again the holding force of the magnet by moving the slide towards the uppermost metal sheet. This ensures that the metal sheets remain attached to the magnet while the magnet with the predetermined number of metal sheets is moved from one location to another.

The exemplary embodiments of the invention presented in this text are not interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this text as an open limitation that does not exclude the existence of also unrecited features. The features recited in the dependent claims are mutually freely combinable unless otherwise explicitly stated.

The exemplary embodiments presented in this text and their advantages relate by applicable parts to the magnet as well as the method according to the invention, even though this is not always separately mentioned.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the description of specific embodiments when read in connection with the accompanying drawings. Fig. 1 illustrates a cross-sectional view of a magnet according to an embodiment of the invention, and figs. 2-4 illustrate the magnetic field of the magnet of fig. 1 at three different positions of the slide.

DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a cross-sectional view of a magnet according to an embodiment of the invention. The magnet 100 comprises a cylindrical body 101 that comprises at its one end an attachment surface 102, which is meant to be arranged in contact with an object to be attached.

The body 101 comprises a first and a second section 103, 104 made of magnetic material . The first and the second section 103, 104 are attached together with a third section 105 of the body 101 , which third section 105 is made of non-magnetic material. The third section 105 has the form of a sleeve and is attached around the second section 104 that is cylindrical. The first section 103 is attached around the third section 105.

The first section 103 comprises a hole 106 that opens at a first end into a first cavity 107 of the body 101 and at a second end into a second cavity 108 of the body 101 . The first and the second cavity 107, 108 are cylindrical and contain liquid. The bottom of the first cavity 107 is defined by the second section 104, and the wall of the first cavity 107 is defined by the third section 105. The bottom and the wall of the second cavity 108 are defined by a fourth section 109 of the body 101 . The fourth section 109 is made of non-magnetic material. The magnet 100 comprises a slide 1 10 that is movably arranged in the hole 106 of the first section 103. The slide 1 10 is cylindrical and comprises a permanent magnet 1 1 1 and a first and a second pole piece 1 12, 1 13 that are attached to opposite magnetic pole surfaces of the permanent magnet 1 1 1 . The second pole piece 1 13 is directed towards the bottom of the first cavity 107. The slide 1 10 comprises a sealing ring 125 that is attached around the first pole piece 1 12. The slide 1 10 comprises a guiding rod 1 14 that extends from the second pole piece 1 13 to a bore 1 15 in the bottom of the first cavity 107. The guiding rod 1 14 and the bore 1 15 are dimensioned in such a manner that part of the guiding rod 1 14 stays in the bore 1 15 all the time. The position of the slide 1 10 is changed by transferring liquid from one cavity to the other. For this purpose the magnet 100 comprises a first and a second conduit 1 16, 1 17 that are integrated into the body 101 . The first conduit 1 16 is used for transferring liquid into and out of the first cavity 107, and the second conduit 1 17 is used for transferring liquid into and out of the second cavity 108. The first and the second conduit 1 16, 1 17 are connected to a pump 1 18 with a first and a second pipe 1 19, 120, respectively. By using the pump 1 18, liquid can be transferred from one cavity to the other so that the slide 1 10 is moved from one position to the other. The position of the slide 1 10 is determined with a magnetic flux sensor 121 that is arranged inside the first section 103, close to the hole 106. The magnet 100 comprises a coil 122 that is arranged to generate a magnetic force for increasing or decreasing the holding force of the magnet 100 depending on the direction of electric current that is supplied to the coil 122. The coil 122 is arranged inside the third section 105 and partly around the first cavity 107. The magnet 100 comprises a battery 123 that is connected via a control unit 124 to the coil 122. The control unit 124 controls the amount and the direction of electric current supplied from the battery 123 to the coil 122.

Fig. 2 illustrates the magnetic field generated by the magnet of fig. 1 , when the slide 1 10 is at a position where the permanent magnet 1 1 1 , and part of the first and the second pole piece 1 12, 1 13 are located inside the hole 106. At this position, essentially all of the magnetic field lines pass from the first pole piece 1 12 to the second pole piece 1 13 through a portion of the first section 103 that surrounds the hole 106. Magnetic flux generated by the permanent magnet 1 1 1 is thus short- circuited by the first section 103. At this position of the slide 1 10, the holding force of the magnet 100 is minimal, and therefore the magnet 100 cannot attach to a metal sheet 200.

Fig. 3 illustrates the magnetic field generated by the magnet of fig. 1 , when the slide 1 10 is at a position where the second pole piece 1 13 is located outside the hole 106, and the permanent magnet 1 1 1 and the first pole piece 1 12 are located inside the hole 106. At this position, part of the magnetic field lines pass from the first pole piece 1 12 to the second pole piece 1 13 through a portion of the first section 103 that surrounds the hole 106, and part of the magnetic field lines pass from the first pole piece 1 12 to the second pole piece 1 13 through the first and the second section 103, 104, and through the metal sheet 200. At this position of the slide 1 10, the magnet 100 is weakly attached to the metal sheet 200.

Fig. 4 illustrates the magnetic field generated by the magnet of fig. 1 , when the slide 1 10 is at a position where the second pole piece 1 13 is in contact with the bottom of the first cavity 107. At this position, the second pole piece 1 13 and the permanent magnet 1 1 1 are located outside the hole 106, and part of the first pole piece 1 12 is located inside the hole 106. Essentially all of the magnetic field lines pass from the first pole piece 1 12 to the second pole piece 1 13 through the first and the second section 103, 104, and through the metal sheet 200. At this position of the slide 1 10, the holding force is maximal, and therefore the magnet 100 is tightly attached to the metal sheet 200. Only advantageous exemplary embodiments of the invention are described in the figures. It is clear to a person skilled in the art that the invention is not restricted only to the examples presented above, but the invention may vary within the limits of the claims presented hereafter. Some possible embodiments of the invention are described in the dependent claims, and they are not to be considered to re- strict the scope of protection of the invention as such.

Claims

1 . A magnet (100), comprising:

- a body (101 ) that comprises a first and a second section (103, 104) made of magnetic material for directing magnetic flux to an object to be attached (200), the first and the second section (103, 104) being separated and attached to a third section (105) of the body (101 ), which third section (105) is made of non-magnetic material, and

- a slide (1 10) that comprises a permanent magnet (1 1 1 ), and a first and a second pole piece (1 12, 1 13) attached to opposite magnetic pole surfaces of the permanent magnet (1 1 1 ), the first and the second pole piece (1 12,

1 13) being made of magnetic material, and the slide (1 10) being arranged to be movable relative to the body (101 ) in order to alter the path of the magnetic flux generated by the permanent magnet (1 1 1 ); characterised in that: - the first section (103) comprises a hole (106) in which the slide (1 10) is movably arranged, which hole (106) opens at a first end into a first cavity (107) of the body (101 ) and at a second end into a second cavity (108) of the body (101 ), the bottom of the first cavity (107) being defined at least partly by the second section (104), - the first and the second cavity (107, 108) contain a medium, and

- the magnet (100) comprises means (1 16, 1 17, 1 18, 1 19, 120) for transferring the medium into and out of the first and the second cavity (107, 108) in order to move the slide (1 10).

2. The magnet according to claim 1 , characterised in that the slide (1 10) and the hole (106) are cylindrical.

3. The magnet according to claim 1 or 2, characterised in that the magnet (100) comprises a sealing ring (125) attached around the slide (1 10) or attached to the wall of the hole (106).

4. The magnet according to any of the preceding claims, characterised in that the means (1 16, 1 17, 1 18, 1 19, 120) for transferring the medium into and out of the first and the second cavity (107, 108) comprises a first and a second conduit (1 16, 1 17) integrated into the body (101 ), a first end of the first conduit (1 16) being in communication with the first cavity (107) and a first end of the second conduit (1 17) being in communication with the second cavity (108).

5. The magnet according to claim 4, characterised in that a second end of the first conduit (1 16) and a second end of the second conduit (1 17) open outside the magnet (100).

6. The magnet according to claim 4 or 5, characterised in that the means (1 16, 1 17, 1 18, 1 19, 120) for transferring the medium into and out of the first and the second cavity (107, 108) comprises a pump (1 18) coupled to the second end of the first and the second conduit (1 16, 1 17).

7. The magnet according to any of the preceding claims, characterised in that the medium is gas or liquid.

8. The magnet according to any of the preceding claims, characterised in that the body (101 ) comprises a fourth section (109) that defines the wall and the bottom of the second cavity (108), the fourth section (109) being made of non-magnetic material.

9. The magnet according to any of the preceding claims, characterised in that the magnet (100) comprises:

- a magnetic flux sensor (121 ) configured to measure magnetic flux density in the first section (103), and

- means for determining the position of the slide (1 10) based on the measured magnetic flux density.

10. The magnet according to claim 9, characterised in that the magnetic flux sensor (121 ) is one of the following: a Hall sensor, an AMR magnetometer, a MEMS sensor or a reed relay.

1 1 . The magnet according to any of the preceding claims, characterised in that the slide (1 10) comprises a guiding rod (1 14) extending to a bore (1 15) in the bottom of the first cavity (107).

12. The magnet according to any of the preceding claims, characterised in that the magnet (100) comprises a coil (122) attached to the body (101 ), the coil (122) being configured to generate a magnetic force that increases or decreases the holding force of the magnet (100) depending on the direction of electric current that is supplied to the coil (122).

13. The magnet according to claim 12, characterised in that the magnet (100) comprises means (123, 124) for supplying electric current to the coil (122).

14. A method for handling metal sheets, characterised in that the method comprises:

- placing the magnet (100) according to claim 1 on an uppermost metal sheet of a stack of metal sheets,

- increasing the holding force of the magnet (100) by moving the slide (1 10) towards the uppermost metal sheet,

- lifting the magnet (100), and

- decreasing the holding force of the magnet (100) by moving the slide (1 10) away from the uppermost metal sheet until a predetermined number of metal sheets remains attached to the magnet (100).

15. The method according to claim 14, characterised in that the method comprises, after the step of decreasing the holding force of the magnet (100), increasing again the holding force of the magnet (100) by moving the slide (1 10) towards the uppermost metal sheet.