Electromagnetic Gripper

Abstract

A gripper mechanism (1) including a stator (2) and a pair of movable grippers movably coupled to the stator (2) via linear bearings (8), wherein the stator includes a housing (4) and a permanent magnet (3) mounted within the housing (4), and the movable gripper (19) includes a support portion (9), a coil (11) mounted on the support portion (9), and gripper fingers (10) coupled to the support portion (9), wherein the coil (11) is configured to generate a magnetic field opposite to the magnetic field of the permanent magnet (3) to actuate displacement of the movable gripper relative to the stator. The gripper mechanism includes at least a first permanent magnet (18) mounted on one of the stator (2) and the movable gripper (19) and a complementary soft magnetic material portion or magnet mounted on the other of the stator (2) and the movable gripper (19), and further includes at least one magnetic locking mechanism (7) configured to magnetically lock the movable gripper in at least one of a closed position or an open position of the movable gripper (19).

Description

Content of disclosure

[0001] The present invention relates to a machine mounted gripper mechanism for gripping and releasing components. The gripper may be mounted in particular to a robot arm or other moving platform for picking and placing objects, for example for the assembly of components.

[0002] Gripper mechanisms are widely used in automated manufacturing processes for the assembly of components, typically mounted on a moving robot arm or a moving table or slide to pick and place components. Both pneumatic and electromagnetic gripper mechanisms are known. In many applications it is desirable to have a compact and lightweight gripper mechanism to allow high accelerations and decelerations of the moving machine elements carrying the gripper. Reliability and durability of the gripper mechanism are also important in view of the high costs of machine downtime in the manufacturing process. Ease of installation and maintenance are also key factors ensuring cost reduction. The performance of the gripper mechanism is also important, especially the operating speed. There is also an increasing demand for reduced energy consumption.

[0003] Many gripper mechanisms are based on pneumatic actuation, but these generally lack flexibility, especially when changes in the manufacturing process are required, for example to accommodate different parts. In this regard, electromagnetically actuated gripper systems are generally more convenient, but conventional electromagnetic gripper mechanisms do not meet the desired requirements, in particular the combination of low weight, compactness, reliability, ease of control and maintenance, and low energy consumption. In many electromagnetic gripper systems, one of the drawbacks is the heating of the gripper motor during operation and the need for a cooling system for high performance applications.

[0004] In view of the foregoing, it is an object of the present invention to provide a gripper mechanism that is reliable, has a high operating speed and low energy consumption.

[0005] It would be advantageous to provide a gripper mechanism that is lightweight and compact.

[0006] It would be advantageous to provide a gripper mechanism that is easy to control.

[0007] It would be advantageous to provide a gripper mechanism that is easy to install and maintain.

[0008] The object of the present invention is achieved by providing a gripper mechanism as claimed in claim 1. The dependent claims set out various advantageous features of the invention.

[0009] Disclosed herein is a gripper mechanism including a stator and a pair of movable grippers movably coupled to the stator via linear bearings. The stator includes a housing and a permanent magnet mounted within the housing. The movable gripper includes a support portion, a coil mounted to the support portion, and a gripper finger coupled to the support portion. The coil is configured to generate a magnetic field opposing a magnetic field of the permanent magnet to actuate displacement of the movable gripper relative to the stator. The gripper mechanism includes at least a first permanent magnet mounted to one of the stator and the movable gripper, and a complementary soft magnetic material portion or magnet mounted to the other of the stator and the movable gripper, and further includes at least one magnetic locking mechanism configured to magnetically lock the movable gripper in at least one of a closed position or an open position of the movable gripper.

[0010] In an advantageous embodiment, the magnetic locking mechanism includes at least a second permanent magnet positioned such that the moveable gripper is held by magnetic force in both the open and closed positions.

[0011] In an advantageous embodiment, the movable gripper and the stator include a spacer element at the position of abutment between the movable gripper and the stator in the open and / or closed position, which spacer element is configured to maintain a magnetic field gap between the permanent magnet and the complementary soft magnetic material part or magnet.

[0012] In an advantageous embodiment, the at least one permanent magnet of the magnetic locking mechanism is positioned substantially centrally between the moveable grippers in the direction of translational movement of the moveable grippers.

[0013] In an advantageous embodiment, the movable gripper includes a ferromagnetic portion complementary to a permanent magnet mounted on the stator housing.

[0014] In an advantageous embodiment, at least one said permanent magnet is mounted on one of the housing outer part of the stator and the end of the movable gripper, and at least one said complementary soft magnetic material part or magnet is mounted on the other of the housing outer part and the end of the movable gripper.

[0015] In an advantageous embodiment, the permanent magnet has a substantially flat shape with a thickness that is substantially smaller than the length or height of the permanent magnet as viewed in a plane parallel to the displacement direction of the movable gripper, this thickness being measured in a direction perpendicular to said plane parallel to the displacement direction.

[0016] In an advantageous embodiment, the permanent magnet has a substantially flat and substantially rectangular shape.

[0017] In an advantageous embodiment, the stator includes a magnetic circuit armature including an armature outer portion of soft magnetic material mounted on an outer facing side of the housing, with an air gap being formed between the armature outer portion and the permanent magnets, within which the coils of the movable gripper are positioned and displaced.

[0018] In an advantageous embodiment, the armature outer portion has a surface area facing the permanent magnets that is substantially the same size or larger.

[0019] In an advantageous embodiment, the armature outer portion is a substantially flat, thin portion.

[0020] In an advantageous embodiment, the permanent magnet is positioned centrally between the coils of the pair of movable grippers, with the poles NS of the permanent magnet oriented substantially parallel to the direction of translational movement of the movable grippers.

[0021] In an advantageous embodiment, the coil of each movable gripper has a length in the direction of translational motion of the movable gripper that is shorter than the length of the permanent magnet in the direction of translational motion of the movable gripper, preferably less than 70%, more preferably less than 60%.

[0022] Further objects and advantageous features of the present invention will be apparent from the appended claims and the following detailed description of embodiments of the invention taken in conjunction with the accompanying drawings. [Brief description of the drawings]

[0023] [Figure 1a] FIG. 2 is a perspective view of a gripper mechanism according to one embodiment of the present invention; [Figure 1b] FIG. 2 is a perspective cross-sectional view of a gripper mechanism according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a perspective exploded view of a gripper mechanism according to one embodiment of the present invention; [Figure 3a] FIG. 2 is a perspective, partially exploded view of a gripper mechanism according to one embodiment of the present invention shown in an open position; [Figure 3b] FIG. 2 is a perspective, partially exploded view of a gripper mechanism according to one embodiment of the present invention shown in a closed position; [Figure 4a] FIG. 2 is a perspective view of a gripper mechanism according to one embodiment of the present invention with outer housing portions separated; [Figure 4b] FIG. 2 is a perspective view of a gripper mechanism according to one embodiment of the present invention with an outer housing portion and one side of the moveable gripper removed. [Figure 4c] FIG. 2 is a perspective view of a movable gripper of a gripper mechanism according to one embodiment of the present invention; [Figure 5a] FIG. 13 is a simplified schematic diagram of a locking mechanism for a gripper mechanism according to one embodiment of the present invention. [Figure 5b] FIG. 13 is a simplified schematic diagram of a locking mechanism for a gripper mechanism according to one embodiment of the present invention. [Figure 6] FIG. 13 is a perspective view of a locking mechanism for a gripper mechanism according to one embodiment of the present invention.

[0024] Referring to the drawings, a gripper mechanism 1 according to an embodiment of the present invention includes a stator 2 and a pair of movable grippers 19 movably mounted on the stator 2 via bearings 8. The bearings 8 allow the movable grippers 19 to move in a translational manner relative to the stator 2. direction D. In an advantageous embodiment, the bearing 8 may be in the form of a linear ball or roller bearing. However, in other embodiments, the bearing may be a sliding bearing comprising a low friction material, or a lubricated sliding bearing. The term "bearing" is intended to mean one or more bearings, which may be arranged in close or spaced relationship to one another, depending on the size of the gripper mechanism and the required gripping force, since the higher the gripping force, the larger the gripper mechanism should typically be.

[0025] The stator 2 includes a permanent magnet 3 disposed between a pair of movable grippers 19. The permanent magnet 3 forms a stator component common to both grippers. The movable grippers 19 move linearly in opposite directions between open and closed positions.

[0026] The permanent magnet may advantageously be in the form of an essentially flat plate, for example a rectangular plate, with a thickness W substantially smaller than the length L1 or height H1 of the plate shape, the thickness W being seen in a direction T perpendicular to the translation or movement direction D of the mobile gripper 19 relative to the stator 2. The permanent magnet 3 has a pair of poles NS generating a magnetic flux perpendicular to the movement direction D of the mobile gripper. The permanent magnet 3 is thus substantially centrally arranged between the mobile grippers 19, the main plane of the permanent magnet 3 being substantially parallel to the movement direction D of the mobile gripper.

[0027] The stator 2 includes a housing 4 including a central portion 4b supporting a permanent magnet 3 and an outer portion 4a surrounding a movable gripper 19 positioned on either side of the central portion 4b. A support frame portion 16 of the central portion 4b mechanically supports the permanent magnet 3. The housing 4 further includes a bearing portion 17 for supporting a stationary portion 8a of a bearing 8. The housing central portion 4b and the outer portion 4a may be made of a non-magnetic material.

[0028] The stator 2 further comprises a magnetic circuit armature 5 made of ferromagnetic or soft magnetic material, which has at least a pair of armature lateral outer parts 5a mounted on the housing outer part 4a laterally outside the mobile gripper 19, the armature outer parts 5 forming, together with the permanent magnets 3, a magnetic circuit having magnetic field lines passing through the support part 9 of the mobile gripper 19. The armature outer parts 5a may have a size substantially the same as or larger than the permanent magnets. The armature outer parts 5a face the permanent magnets 3 and form an air gap between them, through which the active part of the mobile gripper 19 is displaced.

[0029] The mobile gripper 19 includes a coil 11, which is configured to be connected to an electric circuit (not shown) that drives a current through the coil during operation of the gripper mechanism 1 to move the mobile gripper 19. The coil 11 is mounted on the support part 9 of the mobile gripper, for example, in particular in a coil mounting cavity 13 of the support part 9. The support part 9 can form a substantially flat plate-shaped component that is translatable in the air gap formed between the permanent magnet 3 and the magnetic circuit armature 5, the coil in the support part 9 having a height H that can be comparable or approximately equal to the height H1 of the permanent magnet. The length L of the support part 9 (seen in the translation direction D) is less than the length L1 of the permanent magnet 3, but is greater than half the length of the permanent magnet 3. The length L of the support part preferably corresponds to the length of the permanent magnet 3 minus the amplitude of the maximum displacement of the mobile gripper. This ensures that the coil 11 is always in the air gap formed between the permanent magnet 3 and the magnetic circuit armature 5 and always overlaps with the permanent magnet 3 to ensure maximum magnetic force interaction.

[0030] The mobile gripper further comprises gripper fingers 10, or fixtures for gripper fingers (hereinafter generally referred to as "gripper fingers"), which are either formed integrally with the support part 9 or formed separately and assembled to the support part 9. The gripper fingers may in particular project below the lower end of the housing 4, although in certain applications it is also possible to have the gripper fingers within a housing or protective cage having an orifice allowing access to the component held by the gripper. The support part 9 further comprises a bearing part 14 for supporting the mobile part 8b of the bearing 8.

[0031] The gripper fingers 10 are shown in the illustrated embodiment diagrammatically as substantially rectangular blocks, but the shape of the gripper fingers in the actual application shall be adapted to the shape of the components to be selected. In an advantageous embodiment, the gripper fingers or parts of the gripper fingers are removable and replaceable by other gripper fingers or parts in order to change the shape or material depending on the object to be handled. This also allows modifying different surface materials for the relevant application, for example with regard to hardness, friction and other surface properties. It should be noted that in the illustrated embodiment the gripper fingers are shown in the closed position as being in direct contact with each other, but depending on the application the gripper fingers in the "closed" position may be separated by a gap. In many applications the gripping function is achieved by moving the gripper fingers towards each other to clamp the object to be handled between them. However, within the scope of the present invention it is possible for the gripper fingers to grip the object by moving them apart, for example by inserting them into an orifice or cavity of the object.

[0032] To displace the mobile gripper 19 between the open and closed positions, a current is injected into the coil 11 to generate a magnetic field which is opposite to the magnetic field in the air gap at the position the gripper is in during actuation. The magnetic field across the coil 11 when the gripper is in the open position is generally opposite to the magnetic field across the coil when the gripper is in the closed position due to the position of the mobile gripper support part 9 and the arrangement of the poles N-S of the substantially plate-shaped permanent magnet 3 relative to the air gap. To move the gripper from the open position to the closed position and back from the closed position, the direction of the current in the coil is reversed.

[0033] The movable grippers 19 may be interconnected by a mechanical system such as a rack and pinion, or by link arms configured to ensure that the movable grippers move in opposite directions with equal amplitude and speed.

[0034] However, in an alternative embodiment, the movable grippers may be independently controlled without a mechanical linkage between them.

[0035] A gripper mechanism according to an embodiment of the invention further comprises at least one magnetic lock 7, 7a, 7b arranged to maintain the movable gripper 19 in a gripping position (e.g. closed position) for handling an object. In an advantageous embodiment, the gripper mechanism comprises at least a second magnetic lock 7a, 7b such that the movable gripper is passively held by magnetic forces in both the open and closed positions, whereby displacement of the movable gripper from the open position to the closed position and vice versa is performed by injecting a current into the coil 11 in one or the other direction depending on the direction of the generated magnetic field.

[0036] In the open and / or closed position, the current in the coil is switched off or set to a lower amplitude than during actuation.

[0037] This advantageously provides a gripper mechanism which has low energy consumption and in particular generates low heat losses, so that a particularly compact and lightweight gripper mechanism can be provided.

[0038] In handling functions requiring high gripping forces, the passive magnetic force of the magnetic lock acting on the gripper elements can be supplemented by an electromagnetic force provided by injecting a current into a coil of the mobile gripper 19.

[0039] The magnetic lock is advantageously a permanent magnet attached to the housing 4 or to the mobile gripper 19. 18 whereby the other of the movable gripper or the housing comprises a ferromagnetic or soft magnetic material 15a, 15b or in a variant a permanent magnet.

[0040] In the first embodiment, the magnetic lock 7a includes a permanent magnet 18 mounted in the central portion 4b of the housing 4 and a movable gripper support portion 4b of the housing 4, as shown in Figs. 2 and 6. 9 and an associated ferromagnetic material 15a mounted on the

[0041] In a second embodiment, the magnetic lock 7b comprises a permanent magnet 18 and associated ferromagnetic material 15b mounted on the outer part 4a of the housing or on the outer edge of the movable gripper support part 9, as shown in Figures 2, 3a, 3b and 4a-4c. In a variant, the magnetic locking mechanism may comprise a combination of the above, such that there are magnetic locks located on both the outer part 4a and the inner part 4b of the housing, depending on the magnetic forces generated by the locking mechanism.

[0042] In an advantageous embodiment, the stator and the mobile gripper comprise spacer elements 21, which ensure that in the fully locked and / or fully released position a gap is formed between the permanent magnet 18 and the soft magnetic material 15a, 15b of the magnetic locking mechanism 7. The gap can be formed by an air gap or by an insert 21 of non-magnetic material, as shown diagrammatically in Fig. 5a. In a variant, the spacer elements can be protrusions 21 made of magnetic material contacting the permanent magnet 18, as shown for example in Fig. 6, whereby a reduced contact surface area reduces the magnetic force between the gripper and the permanent magnet.

[0043] The spacer elements 21 advantageously make it possible to avoid excessively high peak magnetic locking forces that would require high electromagnetic forces to overcome.

[0044] As previously mentioned, the magnetic locking feature can be configured to passively maintain the movable grippers in an open (spaced apart) position rather than closed relative to one another. In a variant, the magnetic locking feature can be configured to passively maintain the movable grippers in both the open and closed positions, as shown diagrammatically in Figures 5a-b.

[0045] In the embodiment shown in Figures 2 to 4c, the permanent magnets 18 and associated ferromagnetic elements 15b forming the magnetic locks 7, 7a, 7b mounted on the housing outer part 4a or on the outer end of the mobile gripper 19 are configured such that the magnetic elements 18, 15b are in contact with each other, either directly or via a spacer element 21, when the mobile gripper is in the closed position.

[0046] In one embodiment having magnetic locking capability in both the fully open and fully closed positions, the displacement movement of the movable gripper 19 is configured such that when the movable gripper is in the closed position, the magnetic elements 18, 15b contact each other at one end of the housing, and when the movable gripper is in the open position, the magnetic elements 18, 15b contact each other, either directly or via a spacer element 21, at the opposite end of the housing.

[0047] In one embodiment, the permanent magnet 18 is mounted in a ferromagnetic cap or U-shaped ferromagnetic element 22 mounted on the end wall of the outer housing portion 4a. The role of the U-shaped ferromagnetic element is to increase and create a more continuous magnetic force around the locking element.

[0048] [List of reference numbers] Gripper mechanism 1 Stator 2 Permanent Magnet 3 Magnetic circuit armature 5 Armature lateral (outside) part 5a Housing 4 Outer part 4a Center (inside) part 4b Support frame part 16 Bearing part 17 Bearing 8 Fixed part 8a Movable part 8b Movable Gripper 19 Support part 9 Coil mounting cavity 13 Coil 11 Gripper Fingers 10 Gripper inserts (replaceable, not shown) Bearing part 14 Magnetic Lock 7 Central magnetic lock 7a Outer magnetic lock 7b Permanent Magnets 18 Complementary magnetic locking portions 15a, 15b (ferromagnetic / soft magnetic material or permanent magnets) Spacer element 21 Ferromagnetic armature 22

[0049] [Embodiment] (1) A gripper mechanism (1) including a stator (2) and a pair of movable grippers movably coupled to the stator (2) via linear bearings (8), the stator including a housing (4) and a permanent magnet (3) mounted within the housing (4), each movable gripper (19) including a support portion (9), a coil (11) mounted on the support portion (9), and a gripper finger (10) coupled to the support portion (9), the coil (11) being configured to rotate a magnet for actuating displacement of the movable gripper relative to the stator. , configured to generate a magnetic field opposite to the magnetic field of the permanent magnet (3), the gripper mechanism including a first permanent magnet (18) mounted on one of the stator (2) and the movable gripper (19) and a complementary soft magnetic material portion or magnet mounted on the other of the stator (2) and the movable gripper (19), and further including at least one magnetic locking mechanism (7) configured to magnetically lock the movable gripper in at least one of a closed position or an open position of the movable gripper (19). (2) A gripper mechanism as described in embodiment 1, wherein the magnetic locking mechanism (7) includes a second permanent magnet (18) arranged so that the movable gripper is held by magnetic force in both the open and closed positions. (3) The gripper mechanism of embodiment 1, wherein the movable gripper and the stator include a spacer element at a position of abutment between the movable gripper and the stator in the open position and / or the closed position, the spacer element being configured to maintain a magnetic field gap between the permanent magnet and the complementary soft magnetic material portion or magnet. (4) A gripper mechanism as described in embodiment 1, wherein the first permanent magnet (18) of the magnetic locking mechanism is positioned substantially centered between the movable grippers (19) in the direction of translational motion (D) of the movable grippers. (5) The gripper mechanism of claim 4, wherein the movable gripper (19) includes ferromagnetic portions (15a, 15b) complementary to the permanent magnets mounted on the stator housing (4, 4a, 4b).

[0050] (6) The gripper mechanism of embodiment 1, wherein the stator (2) includes a housing outer portion (4a), and at least one of the second permanent magnets (18) is mounted on one of the housing outer portion (4a) of the stator (2) and the end of the movable gripper, and at least one of the complementary soft magnetic material portions or magnets is mounted on the other of the housing outer portion (4a) and the end of the movable gripper. (7) A gripper mechanism as described in embodiment 1, wherein the permanent magnet (3) has a substantially flat shape having a thickness (W) substantially smaller than the length or height of the permanent magnet as viewed in a plane parallel to the displacement direction of the movable gripper, the thickness being measured in a direction (T) perpendicular to the plane parallel to the displacement direction (D). (8) A gripper mechanism as described in embodiment 7, wherein the permanent magnet (3) has a substantially flat rectangular shape. (9) The gripper mechanism of embodiment 1, wherein the stator (2) includes a magnetic circuit armature (5) including an armature outer portion (5a) of soft magnetic material mounted on an outer facing side of the housing, and an air gap is formed between the armature outer portion and the permanent magnet (3), within which the coil (11) of the movable gripper is positioned and displaced. (10) The gripper mechanism of claim 9, wherein the armature outer portion has a surface area facing the permanent magnet that is substantially the same size or larger.

[0051] (11) The gripper mechanism of claim 10, wherein the armature outer portion is a substantially flat, thin portion. (12) A gripper mechanism as described in embodiment 1, wherein the permanent magnet is positioned centrally between the coils (11) of the pair of movable grippers (19) and the poles NS of the permanent magnet are arranged in a direction substantially parallel to the translational motion direction (D) of the movable gripper. (13) A gripper mechanism as described in embodiment 1, wherein the coil (11) of each movable gripper has a length (L1) in the translational direction of the movable gripper which is shorter than the length of the permanent magnet (3) in the translational direction of the movable gripper, and preferably the length (L1) in the translational direction of the movable gripper is less than 70%, more preferably less than 60%, of the length of the permanent magnet (3) in the translational direction of the movable gripper.

Claims

1. A gripper mechanism (1) including a stator (2) and a pair of movable grippers movably coupled to the stator (2) via linear bearings (8), the stator including a housing (4) and a permanent magnet (3) mounted within the housing (4), each movable gripper (19) including a support portion (9), a coil (11) mounted on the support portion (9), and a gripper finger (10) coupled to the support portion (9), the coil (11) being configured to rotate the gripper finger (10) relative to the stator for actuating the displacement of the movable gripper relative to the stator. , configured to generate a magnetic field opposite to the magnetic field of the permanent magnet (3), the gripper mechanism including a first permanent magnet (18) mounted on one of the stator (2) and the movable gripper (19), and a complementary soft magnetic material portion or magnet mounted on the other of the stator (2) and the movable gripper (19), and further including at least one magnetic locking mechanism (7) configured to magnetically lock the movable gripper in at least one of a closed position or an open position of the movable gripper (19).

2. 2. The gripper mechanism of claim 1, wherein the magnetic locking mechanism (7) includes a second permanent magnet (18) arranged such that the movable gripper is held by magnetic force in both the open and closed positions.

3. 2. The gripper mechanism of claim 1, wherein the movable gripper and the stator include a spacer element at a position of abutment between the movable gripper and the stator in the open position and / or the closed position, the spacer element being configured to maintain a magnetic field gap between the first permanent magnet (18) and the complementary soft magnetic material portion or magnet.

4. 2. The gripper mechanism of claim 1, wherein the first permanent magnet (18) of the magnetic locking mechanism is positioned substantially centrally between the movable grippers (19) in a translational direction (D) of the movable grippers.

5. 5. The gripper mechanism according to claim 4, wherein the movable gripper (19) comprises ferromagnetic portions (15a, 15b) complementary to the first permanent magnet (18) mounted on the housing (4) of the stator.

6. 3. The gripper mechanism of claim 2, wherein the stator (2) comprises a housing outer portion (4a), and wherein at least one of the second permanent magnets (18) is mounted on one of the housing outer portion (4a) of the stator (2) and the end of the movable gripper, and at least one of the complementary soft magnetic material parts or magnets is mounted on the other of the housing outer portion (4a) and the end of the movable gripper.

7. 2. The gripper mechanism according to claim 1, wherein the permanent magnet (3) has a substantially flat shape with a thickness (W) substantially smaller than a length or height of the permanent magnet (3) as viewed in a plane parallel to the displacement direction of the movable gripper, the thickness being measured in a direction (T) perpendicular to the plane parallel to the displacement direction (D).

8. The gripper mechanism according to claim 7, wherein the permanent magnet (3) has a substantially flat rectangular shape.

9. 2. The gripper mechanism according to claim 1, wherein the stator (2) comprises a magnetic circuit armature (5) including an armature outer part (5a) of soft magnetic material mounted on an outer facing side of the housing, an air gap being formed between the armature outer part and the permanent magnets (3), within which the coils (11) of the movable gripper are positioned and displaced.

10. The gripper mechanism of claim 9, wherein the armature outer portion has a surface facing the permanent magnet (3).

11. The gripper mechanism of claim 10 , wherein the armature outer portion is a substantially flat, thin portion.

12. 2. The gripper mechanism of claim 1, wherein the permanent magnet (3) is positioned centrally between the coils (11) of the pair of movable grippers (19) and the poles N-S of the permanent magnet are oriented substantially parallel to the translational direction (D) of the movable gripper.

13. 2. The gripper mechanism of claim 1, wherein the coil (11) of each movable gripper has a length (L1) in the translational direction of the movable gripper, which is shorter than a length of the permanent magnet (3) in the translational direction of the movable gripper.

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