Abstract: The present invention is directed to a coil unit of a single-acting linear engine. The coil unit includes an iron core with coils that are linked with the iron core by way of a cast compound thereby forming a molded part. The molded part is enclosed by a housing having a narrow circumferential gap formed between the housing and the molded part. Through the gap, cooling air may flow or be blasted. The air extensively cools the iron core with the coils and thermally insulates the housing from the iron core and the coils.

Abstract: A magnet assembly includes a back iron and an array of magnets. The back iron is in the form of a plate having opposed surfaces. The magnets are arranged along one of the surfaces, with the other surface being dimensioned and configured according to the magnetic field distribution associated with the magnets. The back iron geometry provides for reduced mass, reduced leakage flux, and high flux densities to improve performance of a linear motor that employs such a magnet assembly.

Abstract: A linear motor is provided which includes members for engaging a magnet rail to prevent bending of the magnet rail. Such members may include a wheel or sliding block, for example. The members exert a force proximate an upper edge of the magnet rail to counter potential bending of the magnet rail during operation of the motor, and thereby avoid substantial variations in the air gaps. Furthermore, an armature yoke is provided which includes at least one partial tooth at one end of the armature yoke for reducing an end effect cogging of the linear motor armature in relation to a magnet rail along which the armature will run. In addition, an armature yoke is provided which includes multiple segments which are held end-to-end via fasteners. Such fasteners may serve as part of the magnetic flux path. A support strip may be provided to reduce bowing.

Abstract: A sliding means with built-in moving-magnet linear motor is provided, realizing high-speed operation and much response ability of a table to a stationary bed, and also accurate position control of the table to the bed. With the sliding means of this invention, armature windings carry a three-phase current while a driving circuit is transferred to the external driver to make the bed slim in construction. Thus, the sliding means is reduced in overall height. A field magnet of rare earth permanent magnet is effective in raising flux density, thereby providing high propulsion for the table. An encoder to monitor a position of the table is an optical encoder having an optical linear scale, which contributes to improvement in accurate monitoring. The construction in which the armature windings connected to cords, lines, and so on are placed on the stator side has no fear of causing dust and dirt, thus realizing clean environment.

Abstract: A linear motor having a reduced size, and capable of being manufactured at reduced cost and having improved efficiency is provided. The linear motor includes a bobbin, a plurality of coil groups wound on the bobbin, the coil groups have different coil thickness, and a magnet linearly movable due to a magnetic field generated by the coil groups.

Abstract: An actuator is provided in which one or more separate compensating coils are employed to alter the distribution of the magnetic flux density in the air gap to produce a substantially constant force throughout the stroke; and in which current in the separate compensating coils can be adjusted with the stroke using separate power supplies and position feedback.

Abstract: A reciprocating motor is disclosed having a magnet frame (10) connected between a magnet (8) and an element to be reciprocally moved to transmit a reciprocal movement of the magnet (8) to the element, the magnet consisting of a first mounting unit (12) at which the magnet (8) is fixed and a second mounting unit (14) made of a different material with the first mounting unit (12), being connected to the first mounting (12), and with which the element is engaged. A portion of the magnet frame (10) where the magnet is fixed is made of a nonmetallic material to minimize a loss of flux generated from a winding coil (6).

Abstract: A magnetic actuator for adjusting a force on a load is presented herein. The magnetic actuator comprises a first actuating part that includes a first magnetic element and a second magnetic element, a second actuating part that includes a third magnetic element and a piezoelectric element attached to the first and second magnetic elements to relatively displace the first and second magnetic elements. The first actuating part and said second actuating part are constructed and arranged to generate a magnetic force between both actuating parts along a first direction with a load being attached to one of the first and second actuating parts.

Abstract: A linear motor comprises a stator including a stator yoke and a plurality of permanent magnets arranged side by side on the stator yoke along a motor running direction in alternately reversed directions to produce alternating polarities, and a moving part including a plurality of magnetic teeth arranged along the motor running direction and coils wound around the individual magnetic teeth. Cutouts formed in end surfaces of yoke portions of the individual magnetic teeth opposite to their side facing the stator line up to form a groove-shaped channel running through the yoke portions of the successive magnetic teeth, and the multiple magnetic teeth are joined together into a single structure by fitting a connecting bar in the groove-shaped channel.

Abstract: A positioning device comprising a first part (1) which is movable relatively to a second part (2) in an X-direction and a Y-direction, said first part (1) comprising a carrier (5) on which a system of magnets (3) is arranged according to a pattern of rows (7) and columns (8) extending parallel to the X-direction and the Y-direction, respectively. The magnets in each row and column are arranged according to a Halbach array, i.e. the magnetic orientation of successive magnets in each row (7) and each column (8) rotates 90° counter-clockwise. The second part (2) comprises an electric coil system (4) with two types of electric coils (C1, C2), one type having an angular offset of +45°, and the other type having an offset of ?45° with respect to the X-direction. The magnet configuration causes a very strong magnetic field.

Abstract: A linear motor includes an outer yoke formed of a core made of thin magnetic plates laminated and a coil, an inner yoke formed of a core made of thin magnetic plates laminated and permanent magnets rigidly stuck on this laminated core, and an output shaft fixed to the inner yoke.

Abstract: A resonant electrical generation system includes a resonator, an energy source, and an electrical generator. The resonator provides resonating movement in a resonating element. The resonator can include a mass and spring element. The energy source is operatively coupled to the resonator to maintain resonating movement of the resonating element. The energy source can be a pulsatile linear combustor. The electrical generator is operatively coupled to and driven by the resonator to generate electrical power from the resonating movement.

Abstract: A voice coil actuator is disclosed which provides a proportional solenoid type characteristic and in which first and second magnets are positioned at different ends of a first soft magnetic pole piece and magnetized in opposite directions, the first soft magnetic pole piece having a length greater than the length of the first magnet and the length of the second magnet. A coil having a length less than the lengths of the first soft magnetic pole piece is positioned for travel along an axis common to the first soft magnetic pole piece and first and second magnets, and a structure provides a magnetic path between free ends of the first and second magnets.

Abstract: A low cost, highly reliable linear oscillatory actuator comprising a yoke formed of magnetic material; an excitation coil wound on the yoke; and two permanent ring magnets arranged around the yoke and integrably set face-to-face on a common magnetic pole along the axial direction.

Abstract: Positioning device having a first part (1) which is movable relative to a second part (2) in a X and Y-direction, said first part (1) comprising a carrier (5) on which a system of magnets (3) is arranged in a pattern of rows (7) and columns (8) parallel to the X- and Y-direction, respectively. The magnets in each row and column are arranged in a Halbach array, i.e. the magnetic orientation of each successive magnet in each row (7) and each column (8) is rotated through 90° counter-clockwise. The second part (2) comprises an electric coil system (4) with two types of electric coils (C1, C2), one type having an angular offset of +45° and the other type having an offset of ?45° with respect to the X-direction. The magnet configuration causes a very strong magnetic field.

Abstract: Some of linear motors which constitute an exposure apparatus have coils each obtained by winding a copper foil wire as a foil-like conductor having an insulating layer around a core member in a multilayered structure. The coil is fixed to a positioning portion serving as the coil fixing portion of the linear motor by using the core member. The core member is formed from the same insulating material with low thermal conductivity as that of the positioning portion of a linear motor housing or a material with almost the same linear expansion coefficient.

Abstract: A permanent magnet synchronous linear motor comprising a field yoke (1) having a plurality of permanent magnets (2) the polarity of which is alternated and which are arranged linearly and an armature (3) facing the array of the permanent magnets (2) through a magnetic air gap, wherein the armature (3) comprises an armature core (4) having main teeth (4b) and slots (4a), an armature winding (5) wound in the slots 4a of the armature core (4), and auxiliary teeth (6) provided at both the ends of the armature core (4). The distance &tgr;p between the centers of the auxiliary teeth (6) and the pitch &tgr;m of the field poles satisfy a relation &tgr;p=(2n−1)×&tgr;m/2 (n is a positive integer) and the length O of the auxiliary teeth (6) is in a relation of O<Hd<Ht where Hd is the length of the auxiliary teeth (6) in the direction orthogonal to the array of permanent magnets and Ht is the length of the main teeth (4b) in the direction orthogonal to the array of permanent magnets.

Abstract: A gimbal for supporting a moveable mirror includes an attachment section, first and second pairs of beams extending along the x-axis and connected to the attachment section, and third and fourth pairs of beams extending along the y-axis connected with the first and second pair of beams. The connections are such that the mirror can tilt or rotate about both the x-axis and y-axis. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Abstract: A stator fastening structure of a reciprocating motor is provided. In the stator fastening structure of a reciprocating motor, a frame which includes an outer core fixing member, to which an outer core is fixed, and an inner core fixing member, to which an inner core is fixed, wherein the outer core fixing member of the frame and the inner core fixing member of the frame are formed of different material from each other. It is possible to prevent leakage of flux to a frame, in which the reciprocating motor is loaded, to thus minimize the loss of the flux and to reduce material cost and production cost.

Abstract: A free piston gas compressor comprising a cylinder, a piston reciprocable within the cylinder and a reciprocating linear electric motor derivably coupled to the piston having at least one excitation winding. A measure of the reciprocation time of the piston is obtained, any change in the reciprocation time is detected and the power input to said excitation winding is adjusted in response to any detected change in reciprocation time.

Abstract: A high-precision, low backlash linear actuator drive mechanism characterized by minimal positional repeatability error irrespective of the direction from which a selected or target position is approached may comprise a bellows coupling operative to transmit rotational motion from a motor to a drive spindle. Rotation of the drive spindle causes linear translation thereof in an axial direction.

Abstract: The integrated magnet body according to the invention is an integrated magnet body formed by laminating and securing a plurality of pieces of magnet through an insulating film between pieces of magnet, characterized in that the insulating film has a film thickness of 0.01 &mgr;m or more, and a ratio of a total sum of the thickness of the insulating films to an overall length in the laminating direction of the integrated magnet body is in a range of from 0.0005 to 3%. The integrated magnet body according to the invention is excellent in insulating property and effective volume ratio and thus can attain high efficiency of a motor by installing the integrated magnet body in the motor.

Abstract: A linear motion device is disclosed which includes an armature assembly, and a field assembly which includes a plurality of permanent magnets each having a length, and a plurality of pole pieces each having a length. The ratio between the length of the plurality of permanent magnets and the length of the plurality of pole pieces is tailored to achieve a constant force versus stroke characteristic, when sinusoidal commutation is used.

Abstract: The structure for symmetrically disposed linear motor operated tool machine comprises two parallelly erected sustaining walls, a movable gantry, a sliding saddle, a tool spindle, and a weight compensation device. At both sides of the movable gantry between the sustaining walls, between the gantry and the sliding saddle, and between the tool spindle and the sliding saddle, a pair of linear motors is symmetrically disposed such that the armature reaction between the winding and magnet is compensated each other by the opposite one therefore any possible deformation force to the tool machine structure is eliminated.

Abstract: A rotary-linear actuator system, method of manufacturing and method of using a linear actuator are provided. An aspect of the invention includes a rotary-linear actuator having a spindle assembly and a housing assembly. The spindle assembly includes a plurality of annular magnets and a plurality of bar magnets affixed to a rotor. The plurality of annular magnets can be arranged with alternating polarity. The plurality of bar magnets can likewise be arranged with alternating polarity. The housing assembly includes a first set of coil(s) and a second set of coil(s). The spindle assembly can be movable and/or rotatable along a longitudinal axis extending through the spindle assembly. The first set of coil(s) can be arranged to, when energized, interact with at least one of the plurality of annular magnets to move the spindle assembly in a linear mode.

Abstract: In varying embodiments, the fault tolerant linear actuator of the present invention is a new and improved linear actuator with fault tolerance and positional control that may incorporate velocity summing, force summing, or a combination of the two. In one embodiment, the invention offers a velocity summing arrangement with a differential gear between two prime movers driving a cage, which then drives a linear spindle screw transmission. Other embodiments feature two prime movers driving separate linear spindle screw transmissions, one internal and one external, in a totally concentric and compact integrated module.

Abstract: In a linear motor coil assembly (12), and a method for manufacturing the same, a plurality of coils (14) are arranged in a line in a direction of movement and have respective coil axes perpendicular to the direction of movement of the motor. A flat cooling tube (20) is arranged to meander inside the plurality of coils. The cooling tube has a cross section elongated in a direction parallel to the coil axes, and a plurality of clearance holes (25) through which coolant flows are formed inside the cooling tube. The cooling tube has interleaved folds at least equal in number to the number of coils. The coils being fitted into these folds. At the time of manufacture of the coil assembly, the coils are wound around cores that are divided for each coil, and the cores are inserted into the folds of the cooling tube.

Abstract: The invention relates to an electrical apparatus having an actuator including at least two permanent magnets (1, 1a) and at least one electrical coil (2) which is movably supported by means of a swing arm (3), which coil is arranged to be traversed by magnetic fields of the permanent magnets (1, 1a), the actuator having a cage (4), which encloses the coil (2) and the permanent magnets (1, 1a), as a closed magnetic return path.

Abstract: A motor controller for an image reading apparatus including a carriage driven with the use of a DC motor is provided, which controller comprises a scale for position detection disposed along a direction, in which said carriage is driven, a sensor mounted to said carriage for detecting said scale for position detection, and a control part for enabling said DC motor to be driven based on a detection signal resulting from said sensor. Specifically, in the case where the DC motor comprises a linear motor, triangular pulses are obtained from the photosensor while the carriage runs. At a predetermined reference position, the pulse is inverted, and the inverted pulse is fed back to the motor inversely to apply the brake thereon.

Abstract: An electron beam lithography apparatus of the present invention prevents the electron beam trajectory from being affected by a leakage magnetic field from a permanent magnet which is used as a sample stage guide/driving mechanism. In this electron beam lithography apparatus, an air bearing guide is used as a sample stage guide mechanism, and the stage posture is held by attracting the stage floating on a surface plate to the surface plate side by the permanent magnet. To avoid the leakage magnetic field from the permanent magnet from affecting the electron beam irradiation position on the sample, the permanent magnet is magnetically shielded by a shield member. In addition, to reduce variations in magnetic field above the sample, which are generated when the shield member moves in a leakage magnetic field from the electron lens, another shield member is arranged under the electron lens.

Abstract: An actuator for tilting a moveable object such as a mirror includes a base and a coil-object assembly that includes first and second pairs of coils each of which is attached to the object, the first pair of coils being arranged along a longitudinal axis, and the second pair of coils being arranged along a transverse axis substantially orthogonal to the longitudinal axis. A gimbal has an attachment section attached to the object, and mounting sections connected via a plurality of beams to the attachment section, the mounting sections being attached to the base. A permanent magnet is positioned adjacent a corresponding one of each of the coils such that when current flows through the coils a rotational force is generated that causes the coil-object assembly to rotate about an axis. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure.

Abstract: A linear motor includes an outer yoke formed of a core made of thin magnetic plates laminated and a coil, an inner yoke formed of a core made of thin magnetic plates laminated and permanent magnets rigidly stuck on this laminated core, and an output shaft fixed to the inner yoke.

Abstract: A position-control stage is disclosed which makes it possible to extend largely an angular movement in circular direction for position control, rendering the stage itself compact in construction, allowing to work with high precision in clean environment and further produce the stage with inexpensive cost. The position-control stage has an onboard linear motor composed of armature windings of primary side and field magnets of secondary side. The armature windings are each made in a hollow rectangle where winding turns are wound in the form of flat loop, and are circularly arranged on a disc surface of the bed along a preselected curvature in such a way lying in radial juxtaposition in circumferential direction. The field magnets are made in a flat shape and arranged underneath the turntable in such a pattern lying radially on a disc surface of the preselected curvature in opposition to the armature windings, with their unlike poles alternating in polarity along a circular direction.

Abstract: A linear motor apparatus which drives a mover of a linear motor in a stable manner, by using only information on change of relative position, without using an absolute value sensor. The apparatus has a coil array of plural phases of coils and a magnet relatively movable to the coil array, plural drivers, at least one of which being provided for each phase, to feed a current to all the same phase coils of the coil array, a sensor for measuring a change of relative position of the magnet moved by the current to the coil array, and a controller to determine the polarity of the current applied to the same phase coils based on the change of the relative position measured by the sensor, and apply a drive current to drive the magnet in a desired direction to the same phase coils.

Abstract: A linear electric machine includes a plurality of teeth spaced apart from each other, a yoke magnetically connected to the teeth, a plurality of coils mounted on the teeth and a movable core disposed opposite the teeth to reciprocate relative to the teeth along an axis of the movable core. The movable core has a plurality of permanent magnets at axially opposite ends thereof for supplying magnetic flux to the teeth, and a member for suppressing generation of a magnetic field disposed at an axial center of the movable core to magnetically separate the permanent magnets.

Abstract: An electromechanical linear actuator assembly having in-line axial load support of its screw drive shaft is provided. The bearing support structure provides a single in-line ball bearing accommodated within a hardened end fitting and a screw pivot recess in the axial screw drive shaft. For axial loading in an opposite direction, a number of smaller ball bearings are provided around the outer periphery of the screw shaft in a groove, and are retained within the hardened end fitting by a bearing retainer. End rod support bearings providing heat compensation and lubrication fluid transfer are also provided. A braking mechanism having increased life and more consistent load engagement is also provided. Further, integrated electronics may be provided for position sensing and power efficiency control.

Abstract: An actuator is provided in which one or more separate compensating coils are employed to alter the distribution of the magnetic flux density in the air gap to produce a substantially constant force throughout the stroke; and in which current in the separate compensating coils can be adjusted with the stroke using separate power supplies and position feedback.

Abstract: The invention provides a motor system and a method for positioning a load. The system comprises support means such as an XY table for supporting the load. A first motor and a second motor cooperate to drive the load, whereby the first motor is adapted to provide higher thrust than the second motor and the second motor is adapted to deliver higher positioning accuracy than the first motor during a positioning sequence to position the load.

Abstract: A linear motor includes a secondary part (1) having several permanent magnets (4) disposed behind one another with alternating orientation of the polarity, or excitation windings. These permanent magnets (4) or excitation windings are wound helically around a rotation axis. Furthermore, there is provided a primary part (2) having a current-carrying winding disposed in parallel relationship to the permanent magnets (4) or excitation windings and arranged about the rotation axis such that a magnetic traveling field travels helically about the rotation axis.

Abstract: A linear actuator includes a moving part which has a facing surface and which is provided such that the facing surface of the moving part faces a facing surface of a stator. The moving part is lonearly movable. A permanent magnet is provided in one of the stator or the moving part and has a magnet surface facing the facing surface of another of the stator or the moving part. A cross-sectional area of the permanent magnet along a first plane substantially in parallel with the facing surface of the one of the stator or the moving part reduces toward the magnet surface.

Abstract: A direct electrodynamic linear drive comprising a drive coil system composed of coils arranged in a row alongside each other on an elongated ferromagnetic core, which coil is able to be supplied with a switched exciting voltage. The drive coil system has a ferromagnetic tube fitting around it, on whose inner side a plurality of permanent magnets are arranged in a row in the longitudinal direction of the tube. The core provided with the drive coil system is adapted to function as a stator and the tube having the permanent magnets as an armature. In this arrangement the ferromagnetic tube serves as a guide tube, as a magnetic return member and as a holder means for the permanent magnets, the possibility of a simple and accurate longitudinal guiding of the tube meaning that elongated units with a small cross section may be manufactured.

Abstract: Apparatus for commutating turns of a stator coil for a helical coil launcher. An annular brush assembly includes an annular support and a plurality of electrical contacts disposed circumferentially about the annular support for contacting the stator coil. A projectile for a helical coil launcher is also disclosed having an armature coil and a body assembly containing at least one annular brush assembly for contacting a stator coil that is slidably engaged with the projectile.

Abstract: A small and high rigid linear motor (1), wherein a tabular heat pipe (7) is disposed between a plurality of coil rows forming an armature coil (6) along the longitudinal direction of an armature (5) with a part of the heat absorbing part of the heat pipe (7) fixed to the armature coil (6) and the heat radiating part thereof inserted into a refrigerant passage (11), a block-shaped locking member (8) for fixing the heat pipe (7) to a frame (10) is installed between the portion of the heat pipe (7) for fixing the armature coil thereto and the portion thereof inserted into the refrigerant passage (11), and a hollow recessed part (10a) forming the refrigerant passage (11) and allowing die insertion of the heat radiating part of the heat pipe (7) therein and a locking step part (10c) allowing the locking of the frame (10) to the locking member (8) installed on the heat pipe are provided in the frame (10), whereby the heat of the armature coil can be efficiently transferred to tie frame, and the thrust of a rotor ca

Abstract: A plurality of permanent magnets are embedded at predetermined pitches &tgr;p in a longitudinal direction in a field yoke composed of a magnetic substance so that adjacent magnet poles have opposite polarity, and a plurality of electromagnets, which have magnetic pole pitches &tgr;e≈(2n+1)&tgr;p of preset intervals provided that n is 0, 1, 2, . . . , which are disposed at predetermined pitches &tgr;g=(2s+1/m)&tgr;e in the longitudinal direction of the field yoke provided that m is a number of exciting phase and s is 1, 2, 3, . . . , and which are separated from the field yoke by a predetermined gap while opposing to the field yoke, are provided. The field yoke is driven by successively exciting the electromagnets.

Abstract: A transport system includes at least one product carrier which is adapted to be moved along a slide way by means of a linear motor drive unit comprising a primary part and a secondary part. In order to improve the transport system in such a way that basically any number of product carriers can be moved along the slide way independently of one another and can be used in a versatile manner for transporting different types of products or workpieces, the product carrier is provided with a substantially flat base plate for transporting objects, the base plate having associated therewith the primary part, and the slide way having associated therewith the secondary part.

Abstract: Positioning device having a first part (1) which is movable relative to a second part (2) in a X and Y-direction, said first part (1) having a carrier (5) on which a system of magnets (3) is arranged in a pattern of rows (7) and columns (8) parallel to the X- and Y-direction, respectively. The magnets in each row and column are arranged in a Halbach array, i.e. the magnetic orientation of each successive magnet in each row (7) and each column (8) is rotated through 90° counter-clockwise. The second part (2) has an electric coil system (4) with two types of electric coils (C1, C2), one type having an angular offset of +45° and the other type having an offset of −45° with respect to the X-direction. The magnet configuration causes a very strong magnetic field.

Abstract: In a linear motor coil assembly (12), and a method for manufacturing the same, a plurality of coils (14) are arranged in a line in a direction of movement and have respective coil axes perpendicular to the direction of movement of the motor. A flat cooling tube (20) is arranged to meander inside the plurality of coils. The cooling tube has a cross section elongated in a direction parallel to the coil axes, and a plurality of clearance holes (25) through which coolant flows are formed inside the cooling tube. The cooling tube has interleaved folds at least equal in number to the number of coils. The coils being fitted into these folds. At the time of manufacture of the coil assembly, the coils are wound around cores that are divided for each coil, and the cores are inserted into the folds of the cooling tube.

Abstract: A linear motor has a stationary member shaped as a channel and a moving member therein, with a magnet and an armature producing force for displacing the moving member along the channel. A first air flow path for cooling is defined along the path of the moving member in the channel. The moving member can have channels allowing cooling air to flow through. The channel can be closed over by a cover band passing through a slit in the moving member and closed or lifted by rollers on the moving member. The channel can be supplied with forced air flow by a blower and/or an exhaust fan, and preferably includes a filter for excluding metallic particles from the first air flow path.

Abstract: A two-phase excitation linear motor is provided, having a compact size and a large thrust force simultaneously. Each of two single coils is formed to have an almost rectangular ring-like shape where two sides opposing to each other function as a pair of effective conductors, and the other opposing two sides function as a pair of connecting conductors for connecting between the effective conductors. The connecting conductors are offset at an almost right angle. The two single coils are integrated such that the one effective conductor of the pair of effective conductors of the one single coil is interposed between the pair of effective conductors of the other single coil while the single coils are being combined such that the offset directions of the connecting conductors of the individual single coils are opposite to each other in a direction perpendicular to a traveling direction.

Abstract: Provided as a voice coil-type linear motor is a linear motor which achieves relatively high output power, small size and light weight, high thrust, and high efficiency, such as to be available as an industrial linear motor. Ends of inside yokes are coupled by means of auxiliary yokes, outside yokes are configured such that inner peripheral surfaces of magnets are of different magnetic poles, a closed magnetic path is formed by the inside yokes, auxiliary yokes, outside yokes, and magnets. By feeding current to coils, magnetic action is caused to occur between a magnetic field generated by the closed magnetic path and the coils, so that the outside yokes and the inside yokes may move relative to each other.