Patent Publication Number: US-2017372825-A1

Title: Magnet

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a magnet according to the preamble of the appended independent claim. 
     BACKGROUND OF THE INVENTION 
     Magnets that comprise a movable part for changing the magnetic state of the magnet are known in the prior art. An example of such a 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 stable positions. In the first position, the movable part is separated from the body so that in the body the magnetic flux generated by the permanent magnet is low and thus the holding force of the magnet is negligible. In the second 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. 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 is that the length of the movement of the movable part is large, as a result of which the magnet is tall. Because of the structure of the magnet, the distance between the first and second positions of the movable part needs to be large so that the movable part can be easily held in place in its first position. The larger the distance between the first and second positions, the smaller the force that is needed to keep the movable part in the first position. In the magnet of the document WO 2012/160262, the movable part is held in place in its first position using a coil that is supplied with an electric current or using springs that push the movable part towards the first position. A disadvantage of the coil is that it consumes energy, whereas a disadvantage of the springs is the complicated structure, which may be damaged easily. 
     Objectives of the Invention 
     It is the main objective of the present invention to reduce or even eliminate the prior art problems presented above. 
     It is an objective of the present invention to provide a magnet having a structure that enables to achieve a large holding force with a small size. It is also an objective of the invention to provide a magnet comprising a movable part for changing the magnetic state of the magnet, wherein the length of the movement of the movable part is small. 
     It is a further objective of the present invention to provide a magnet that has stable magnetic states at which the magnet does not consume energy. It is also an objective of the invention to provide a magnet, whose magnetic state can be changed easily using very little energy. 
     It is still a further objective of the invention to provide a magnet that has a simple structure, low manufacturing costs, a long life expectancy and a great reliability. 
     In order to realise the above-mentioned objectives, the magnet according to the invention is characterised by what is presented in the characterising part of the appended independent claim. 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 attached 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 slide being arranged to be movable relative to the body between a first and a second position 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 that opens at its first end into a cavity of the body, the bottom of the cavity being defined at least partly by the second section, in which hole the slide is movably arranged so that at least part of the first pole piece stays inside the hole and the second pole piece is directed towards the bottom of the cavity,
 
wherein the movement of the slide is limited by the first and the second section in such a manner that at the first position of the slide, the second pole piece is in contact with the first section, and at the second position of the slide, the second pole piece is in contact with the second section.
       

     The magnet according to the invention is an attachment magnet, the magnetic state of which can be changed by moving the slide relative to the body between the first and the second position. At the first position of the slide (OFF state), the magnetic flux generated by the permanent magnet is essentially short-circuited by the first section, and therefore the magnet cannot attach to an object or it can attach to an object only very weakly. At the second position of the slide (ON state), the magnetic flux is conducted through the first and the second section to an object that is arranged in contact with the first and the second section. The object closes the magnetic circuit and therefore the magnet is attached to the object. The first and the second position are stable positions at which the slide tends to stay still with the help of the magnetic field produced by the permanent magnet. 
     At the first position of the slide, the second pole piece is in contact with the first section and at least part of the first pole piece is located inside the hole of the first section. Preferably, the first pole piece is in contact or nearly in contact with the wall of the hole. Preferably, at the first position of the slide, the first pole piece is located completely and the permanent magnet at least partly inside the hole. Preferably, at the first position of the slide, the second pole piece is located outside the hole. 
     When the slide is at the first position, the magnetic flux generated by the permanent magnet is essentially short-circuited by the first section, which means that the magnetic flux flows from one pole piece to the other mainly through the part of the first section that surrounds the hole and the part of the first section that is in contact with the second pole piece. The first position of the slide is a position to which the magnetic force of the permanent magnet actively pulls the slide when the slide is close to the first position. When the slide is moved away from the first position towards the second position, the permanent magnet produces a counterforce that works against the moving force and tries to pull the slide back to the first position. 
     At the second position of the slide, the second pole piece is in contact with the second section and at least part of the first pole piece is located inside the hole. Preferably, the first pole piece is in contact or nearly in contact with the wall of the hole. At the second position of the slide, the second pole piece and preferably also the permanent magnet are located outside the hole. 
     When the slide is at the second position, the magnetic flux generated by the permanent magnet may flow through the first and the second section to an object to be attached. The object that is arranged in contact with both the first and the second section closes the magnetic circuit, whereby magnetic flux is conducted from one pole piece to the other through the first and the second section and the object. 
     The first and the second section of the body are separated from each other by the third section. Because the third section is made of non-magnetic material, the first and the second section are magnetically separated, so that the flow of the magnetic flux directly between the first and the second section is essentially prevented. 
     The slide is movably arranged in the hole of the first section in such a manner that part of the first pole piece always stays inside the hole. The slide is arranged to be linearly movable between the first and the second position, in the longitudinal direction of the hole. The slide can be designed to be moved between the first and the second position manually, for example with a shaft attached to the slide, or with the help of suitable driving means. 
     The movement of the slide is mechanically limited by the first and the second section of the body in such a manner that in a first direction the movement of the slide is stopped by the engagement of the second pole piece with the first section, and in a second direction by the engagement of the second pole piece with the second section. Thus, at the first position of the slide the second pole piece is in contact with the first section, and at the second position of the slide the second pole piece is in contact with the second section. The contact surface between the second pole piece and the first section may have different shapes and orientations. The contact surface can have, for example, the shape of an annulus or a truncated cone. 
     The movement of the slide is mechanically supported by the wall of the hole. Preferably, the slide stays in contact with the wall of the hole in all positions of the slide. 
     The first end of the hole opens into the cavity of the body. A second end of the hole can be closed or open. The second end of the hole can be closed by the first section or a fourth section that is attached to the first section. 
     The sections of the body may have various shapes and sizes, and they can be formed of one or more parts. The first, the second and the third section 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. The second section can be a cylinder that is arranged inside the third section. The first and the second section 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 first section and the second section are preferably made of the same material. The third section is made of non-magnetic material, which can be paramagnetic 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 pole piece 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 magnet according to the invention is suitable for moving objects and can thus be used as a lifting magnet. The magnet can be used as follows to move an object from one location to another. First, the magnet is arranged in contact with an object in such a manner that the object is in contact with the attachment surfaces of the first and the second section. Then, the slide is moved to the second position as a result of which the magnet is attached to the object. Next, the object is moved with the magnet to a desired location and the magnet is detached from the object by moving the slide to the first position. 
     According to an embodiment of the invention the diameter of the second pole piece is larger than the diameter of the hole. Because the diameter of the second pole piece is larger than the diameter of the hole, the second pole piece stays outside the hole and thus inside the cavity in all positions of the slide. 
     According to an embodiment of the invention the hole and the first pole piece are cylindrical. Preferably, the diameter of the first pole piece is only slightly smaller than the diameter of the hole, whereby the wall of the hole can efficiently support the slide while the slide 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 first pole piece can be, for example, less than 2 mm, less than 1 mm, less than 0.5 mm, less than 0.1 mm, less than 0.01 mm or 0.005-0.5 mm smaller than the diameter of the hole. 
     According to an embodiment of the invention the second pole piece is cylindrical. The diameter of the second pole piece is larger than the diameter of the first pole piece and the diameter of the hole. 
     According to an embodiment of the invention the cavity is cylindrical. The diameter of the cavity is larger than the diameter of the hole of the first section. The diameter of the cavity can be, for example, 1-5 mm, 2-10 mm, 5-50 mm or 25-300 mm larger than the diameter of the hole. 
     According to an embodiment of the invention the height of the slide is larger than the height of the hole. The height of the slide can be, for example, less than 3 mm, 3-10 mm, 10-100 mm or 100-500 mm. The height of the slide can be, for example, 1-5 mm, 2-10 mm, 5-50 mm or 25-100 mm larger than the height of the hole. 
     According to an embodiment of the invention the permanent magnet comprises a plurality of 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 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. 
     According to an embodiment of the invention the permanent magnet is cylindrical and the diameter of the permanent magnet is smaller than the diameter of the first pole piece. The smaller diameter of the permanent magnet prevents the permanent magnet from touching the wall of the hole. 
     According to an embodiment of the invention the first section defines the wall of the cavity. 
     According to an embodiment of the invention the third section is a hollow cylinder attached inside the first section, and the second section is attached inside the third section. The second section is preferably cylindrical. 
     According to an embodiment of the invention the third section defines the wall of the cavity. 
     According to an embodiment of the invention the first and the second section are plates arranged in parallel to each other. An object to be attached is meant to be in contact with a side of the first plate and a side of the second plate. The slide is arranged to move in a direction perpendicular to the plane of the plates. Preferably, the first and the second plate are rectangular. The thickness of the first and the second plate can be, for example, less than 3 mm, 3-10 mm, 10-100 mm or 100-500 mm. A side of the first and the second plate that is meant to be in contact with an object can be straight, curved or bevelled. Preferably, the side of the first and the second plate that is meant to be in contact with an object is shaped to correspond to the shape of the object. 
     According to an embodiment of the invention the third section comprises a plurality of connecting members attached between the first and the second section. The number of the connecting members can vary depending on the application. In a case where the first and the second section are plates, a connecting member is preferably attached to each corner of the first and the second plate, whereby if the first and the second plate are rectangular, the body comprises four connecting members. Preferably, the connecting members are rods or bars, whose first ends are attached to the first section, and whose second ends are attached to the secand section. The connecting members can be made of paramagnetic material, such as resin, brass or aluminium, or diamagnetic material, such as acid-proof steel or stainless steel. 
     According to an embodiment of the invention the cavity is defined by the first, the second and the third section. The cavity can be, for example, cylindrical. The third section can comprise a cylindrical through hole that defines the wall of the cavity. 
     According to an embodiment of the invention a second end of the hole is closed by a fourth section of the body. The fourth section is attached to the first section and it may define a cavity into which the second end of the hole opens. The fourth section can be made of magnetic or non-magnetic material. The fourth section can be made of the same material as the first section. The first section and the fourth section may form an integral structure. This structure can be manufactured from one piece of magnetic material by machining. 
     According to an embodiment of the invention the magnet comprises means for transferring a medium into and out of the body in order to move the slide. The slide is moved relative to the body by transferring medium that can be gas or liquid into and out of the hole and the cavity. The slide can be moved towards the first position by transferring medium into the cavity and out of the hole. The slide can be moved towards the second position by transferring medium into the hole and out of the cavity. The holding force of the magnet depends on the position of the slide. Because the slide can be positioned to any position between the first and the second position, the holding force of the magnet can be adjusted steplessly. The magnet consumes very little energy for maintaining the holding force constant. The holding force is at its minimum when the slide is at the first position, and at its maximum when the slide is at the second position. 
     The cavity and the hole can be connected together through the means for transferring the medium, whereby the medium can be transferred from the cavity to the hole, and vice versa. Alternatively, the means for transferring the medium can be arranged to separately control the medium in the cavity and the hole. In this case, the means for transferring the medium can comprise containers for both the cavity and the hole for receiving the medium that has been transferred out of the body. 
     According to an embodiment of the invention the means for transferring a medium into and out of the body comprises a first and a second conduit integrated into the body, a first end of the first conduit being in communication with the cavity and a first end of the second conduit being in communication with the hole. Medium can be transferred into and out of the cavity and the hole through the first conduit and the second conduit, respectively. Preferably, the first end of the first conduit is arranged to the bottom of the cavity, and the first end of the second conduit is arranged to the second end of the hole. Second ends of the conduits may be in communication with each other so that the medium that has been transferred out of the cavity can be transferred into the hole, and vice versa. 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 a medium into and out of the body 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 a medium into and out of the body comprises a pump coupled to the second end of the first conduit and the second end of the second conduit. The pump can be configured to transfer medium in two directions, whereby the medium can be transferred from the cavity into the hole, and vice versa. The means for transferring a medium into and out of the body 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 body. 
     According to an embodiment of the invention the means for transferring a medium into and out of the body 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 magnet comprises a medium that 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 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 output voltage and/or output 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 the output 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 the output current of the magnetic flux sensor, an output signal identifying whether the slide is in 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, for example, arranged inside the first section, or attached on its surface. 
     The magnet may comprise a plurality of magnetic flux sensors, which are configured to measure magnetic flux densities in different spatial locations and/or directions. The magnetic flux sensors can be configured to measure magnetic flux densities, for example, 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 a reed relay. 
     According to an embodiment of the invention the slide comprises a guiding rod extending to a bore in the second section. The guiding rod, which extends in the longitudinal direction of the hole, may be arranged to extend partly or completely 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 diamagnetic 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 and configured to generate a magnetic force for moving the slide towards the first or the second position depending on the direction of electric current that is supplied to the coil. The coil can be 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. 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 on 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 the electric current pulse is 30-300 ms. The coil may also be used to increase or decrease the holding force of the magnet depending on the direction of electric current that is supplied to the coil. Preferably, the coil is used to adjust the holding force of the magnet. 
     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 magnet comprises a sealing ring installed around the slide or attached to the wall of the hole. Preferably, the sealing ring is installed around the first pole piece. The sealing ring divides an airtight space formed 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, ethylene-propylene, polyurethane, nitrilebutadiene 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 in 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 arranged close to the first pole piece. The groove may be located 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 slide comprises grooves extending in the longitudinal direction of the slide. 
     An advantage of the magnet according to the invention is that the magnet can be made low because the length of the movement of the slide is small. 
     Another advantage of the magnet according to the invention is that the magnet does not consume energy when the slide is at either of its two positions. Also the magnet does not have any complex structure for keeping the slide at either of its two positions. In fact, the slide stays in place at both of its two positions with the help of the magnetic field produced by the permanent magnet. Another advantage of the magnet is that its magnetic state can be changed easily. 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 it provides a large holding force with a small size. 
     Still another advantage of the magnet is that the demagnetization of the permanent magnet can be greatly reduced because the magnetic circuit is closed at the first position of the slide. Still another advantage of the magnet is that its holding force is minimal when the magnet is in the OFF state. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional view of a magnet according to a first embodiment of the invention, 
         FIG. 2  illustrates the magnetic field generated by the magnet of  FIG. 1 , when the slide is at the first position, 
         FIG. 3  illustrates the magnetic field generated by the magnet of  FIG. 1 , when the slide is at the second position, 
         FIG. 4  illustrates a cross-sectional view of a magnet according to a second embodiment of the invention, 
         FIG. 5  illustrates the magnetic field generated by the magnet of  FIG. 4 , when the slide is at the first position, and 
         FIG. 6  illustrates the magnetic field generated by the magnet of  FIG. 4 , when the slide is at the second position. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The same reference signs are used of the same or like components in different embodiments. 
       FIG. 1  illustrates a cross-sectional view of a magnet according to a first embodiment of the invention. The magnet comprises a body  101  and a slide  102  that is arranged to be movable relative to the body  101  between a first and a second position. The magnet comprises at its one end an attachment surface  103 , which is meant to be arranged in contact with an object to be attached (not shown in  FIG. 1 ). 
     The body  101  comprises a first and a second section  104 ,  105  that are made of magnetic material for conveying magnetic flux to an object to be attached. The first and the second section  104 ,  105  are attached together with a third section  106  of the body  101 , which third section  106  is made of non-magnetic material. The third section  106  is a ring-shaped section inside which the cylindrical second section  105  is attached. 
     The slide  102  comprises a permanent magnet  107 , and a first and a second cylindrical pole piece  108 ,  109  that are attached to opposite magnetic pole surfaces of the permanent magnet  107 . The slide  102  also comprises a sealing ring  110  that is mounted in a groove in the first pole piece  108 . 
     The first section  104  comprises a cylindrical hole  111  that opens at its first end into a cavity  112  of the body  101 , which cavity  112  is defined by the first, the second and the third section  104 ,  105 ,  106 . A second end of the cylindrical hole  111  is closed by a fourth section  113  of the body  101 , which fourth section  113  is made of non-magnetic material. 
     The slide  102  is movably arranged in the hole  111  in such a manner that part of the first pole piece  108  always stays inside the hole  111 , to which part the sealing ring  110  is attached. The second pole piece  109  is directed towards the bottom of the cavity  112 , which bottom is defined partly by the second section  105 . The diameter of the second pole piece  109  is larger than the diameter of the hole  111 . The movement of the slide  102  is mechanically limited by the first and the second section  104 ,  105  in such a manner that at the first position of the slide  102 , the second pole piece  109  engages with the first section  104 , and at the second position of the slide  102 , the second pole piece  109  engages with the second section  105 . 
     The slide  102  is moved relative to the body  101  by conveying liquid into and out of the hole  111  and the cavity  112 . When liquid is supplied into the cavity  112  and exhausted from the hole  111 , the slide  102  moves towards the first position. When liquid is supplied into the hole  111  and exhausted from the cavity  112 , the slide  102  moves towards the second position. 
     The liquid is conveyed into and out of the hole  111  and the cavity  112  using a hydraulic system  114  that is coupled via a first and a second pipe  115 ,  116  to a first and a second conduit  117 ,  118  that are integrated into the body  101 . The first end of the first conduit  117  is arranged in communication with the hole  111  through the fourth section  113  that closes the second end of the hole  111 . The first end of the second conduit  118  is arranged in communication with the cavity  112  through the wall of the cavity  112 . The position of the slide  102  is determined with a magnetic flux sensor  119  that is arranged inside the first section  104 , close to the hole  111 . 
       FIG. 2  illustrates the magnetic field generated by the magnet of  FIG. 1 , when the slide  102  is at the first position. At this position, the first pole piece  108  and the permanent magnet  107  are located inside the hole  111 , and the second pole piece  109  is located inside the cavity  112  and in contact with the first section  104 . Essentially all of the magnetic flux (illustrated with magnetic field lines) generated by the permanent magnet  107  passes from the first pole piece  108  to the second pole piece  109  through a portion of the first section  104  that surrounds the hole  111  and through a portion of the first section  104  that is in contact with the second pole piece  109 . The first section  104  thus short-circuits the magnetic flux generated by the permanent magnet  107 . As a result of this, the holding force of the magnet is very small, and therefore the magnet cannot attach to a metal sheet  201 . 
       FIG. 3  illustrates the magnetic field generated by the magnet of  FIG. 1 , when the slide  102  is at the second position. At this position, the second pole piece  109  is in contact with the second section  105  and only part of the first pole piece  108  is located inside the hole  111 , whereby essentially all of the magnetic flux (illustrated with magnetic field lines) generated by the permanent magnet  107  passes from the first pole piece  108  through the first section  104  to the metal sheet  201  and therefrom through the second section  105  to the second pole piece  109 . As a result of this, the holding force of the magnet is large, and therefore the magnet is tightly attached to the metal sheet  201 . 
       FIG. 4  illustrates a cross-sectional view of a magnet according to a second embodiment of the invention. The magnet comprises a slide  102  that is arranged to be movable relative to a body  101  between a first and a second position. The magnet is in an OFF state, when the slide  102  is at the first position, and in an ON state, when the slide  102  is at the second position.  FIG. 4  shows the magnet in the ON state. 
     The slide  102  comprises a permanent magnet  107 , and a first and a second cylindrical pole piece  108 ,  109 , which are attached to opposite magnetic pole surfaces of the permanent magnet  107 . The slide  102  also comprises a sealing ring  110  that is attached around the first pole piece  108 . The body  101  comprises a first and a second section  104 ,  105 , which are made of magnetic material and attached to a third section  106  that is made of non-magnetic material. An object to be attached (not shown in  FIG. 4 ) is meant to be in contact with the bottom sides of the first and the second section  104 ,  105 . 
     The first section  104  comprises a cylindrical hole  111  that opens at its first end into a cavity  112  defined by the first, the second and the third section  104 ,  105 ,  106 . A second end of the hole is closed by the first section  104 . The slide  102  is movably arranged in the hole  111  in such a manner that part of the first pole piece  108  always stays inside the hole  111 . The sealing ring  110  is attached to the part that always stays inside the hole  111 . The second pole piece  109  is directed towards the bottom of the cavity  112 , which bottom is defined by the second section  105 . The diameter of the second pole piece  109  is larger than the diameter of the hole  111 , so that the second pole piece  109  cannot enter the hole  111 . The movement of the slide  102  is mechanically limited by the first and the second section  104 ,  105  in such a manner that at the first position of the slide  102 , the second pole piece  109  is engaged with the first section  104 , and at the second position of the slide  102 , the second pole piece  109  is engaged with the second section  105 . When the slide  102  is at the first position, the first pole piece  108  and the permanent magnet  107  are located inside the hole  111 . When the slide  102  is at the second position, only part of the first pole piece  108  is located inside the hole  111 . The slide  102  is moved relative to the body  101  by conveying air into and out of an airtight space that is formed by the cavity  112  and the hole  111 . When air is supplied into the hole  111  and exhausted from the cavity  112 , the slide  102  moves towards the second position. When air is supplied into the cavity  112  and exhausted from the hole  111 , the slide  102  moves towards the first position. 
     The air is conveyed into and out of the hole  111  and the cavity  112  using a pneumatic system  401  that is coupled via a first and a second pipe  115 ,  116  to a first and a second conduit  117 ,  118  that are integrated into the body  101 . The first end of the first conduit  117  is arranged in communication with the hole  111  through the second end of the hole  111 . The first end of the second conduit  118  is arranged in communication with the cavity  112  through the portion of the second section  105  that is in contact with the slide  102 , when the slide  102  is at the second position. 
       FIG. 5  illustrates the magnetic field generated by the magnet of  FIG. 4 , when the slide  102  is at the first position. The magnetic field is represented by magnetic field lines. At the first position of the slide  102 , the first pole piece  108  and the permanent magnet  107  are located inside the hole  111 , and the second pole piece  109  is located inside the cavity  112  and in contact with the first section  104 . The magnetic flux generated by the permanent magnet  107  passes from the first pole piece  108  to the second pole piece  109  through a portion of the first section  104  that surrounds the hole  111  and through a portion of the first section  104  that is in contact with the second pole piece  109 . Because the magnetic flux generated by the permanent magnet  107  is short-circuited by the first section  104 , the holding force of the magnet is negligible, and therefore the magnet cannot attach to a metal sheet  201 . 
       FIG. 6  illustrates the magnetic field generated by the magnet of  FIG. 4 , when the slide  102  is at the second position. The magnetic field is represented by magnetic field lines. At the second position of the slide  102 , the second pole piece  109  is in contact with the second section  105  and only part of the first pole piece  108  is located inside the hole  111 , whereby essentially all of the magnetic flux generated by the permanent magnet  107  passes from the first pole piece  108  through the first section  104  to the metal sheet  201  and therefrom through the second section  105  to the second pole piece  109 . The metal sheet  201  that is arranged in contact with both the first and the second section  104 ,  105  closes the magnetic circuit. The magnet is thus attached to the metal sheet  201 . 
     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 restrict the scope of protection of the invention as such.