Apparatus for producing linear motion

Sealed telescoping tubes are electromagnetically and fluidly actuated to provide linear motion and withstand loading. One of the tubes may be connected to a pressure or vacuum source, and the other sealed to a fixture to provide variable volume load augmentation in addition to electromagnetic actuation. Alternatively, both tubes may be sealed to provide a pneumatic or hydraulic spring with electromagnetic actuation, and the sealed tubes may also be connected to a pressure or vacuum source for load augmentation. A bearing may be used in conjunction with the seal between the tubes to provide resistance to traversal loads. The electromagnetic actuation includes a helical arrangement of coils or permanent magnets to provide linear motion or resistance. The coils can be arranged to be connected to a conventional multi-phase motor drive.

BACKGROUND OF THE INVENTION
 The present invention relates to electromagnetic apparatus for producing
 linear motion. One means of producing linear motion directly by
 electromagnetic means is the linear electric motor devised by Professor
 Laithwaite of Imperial College, London. This device can be considered as a
 conventional rotary electric motor in which the stator and armature have
 been split axially and unrolled. Although the device is very effective, it
 does have the disadvantage that considerable unbalanced attractive forces
 exist between the stator and moving armature, so that some means such as a
 track or wheeled carriage has to be used to maintain a separation between
 the stator and the armature.
 A second form of linear motor has a cylindrical topology like a
 conventional electric motor but with the magnetic field-producing elements
 disposed transversely around the longitudinal axis so that the magnetic
 fields alternate in polarity along the axis of the motor.
 Examples of such linear motors are shown in OLS 2,229,711; EP 0 221 228;
 U.S. Pat. No. 4,714,300; EP 0 280 743; EP 0 314 493 and WO 93/01646.
 OLS 2,229,711 discloses a DC motor in the form of a piston with a single
 coil which moves in a cylinder under the action of magnetic fields which
 are generated by the successively energizing and de-energizing ring-shaped
 coils spaced axially along the inner surface of the cylinder surrounding
 the piston.
 EP 0 221 228 discloses an arrangement in which both linear and rotary
 motion of a shaft are produced, the linear motion of the shaft being
 produced by circumferential coils and the rotary motion by axial
 peripheral coils.
 U.S. Pat. No. 4,714,300 discloses a pressure modulator for use in anti-skid
 brake systems in which a plunger is made to oscillate longitudinally in a
 chamber under the influence ofa longitudinally-varying magnetic field
 pattern generated between a winding on the inside of an outer portion of
 the chamber and a number of magnets disposed around the periphery of the
 plunger, or vice versa.
 EP 0 280 743 discloses a screw for an injection molding machine which is
 moved linearly by the interaction between magnets on a shaft forming part
 of the injection screw and a number of coils surrounding the shaft and to
 which a three-phase alternating current is applied.
 EP 0 314 493 discloses another arrangement in which a shuttle is caused to
 oscillate in a chamber by the interaction between two magnetic fields, one
 generated by a winding on the shuttle and the other by a winding forming
 part of the chamber wall.
 WO 93/01646 discloses another arrangement in which a piston is driven
 longitudinally in a cylinder by means of the interaction between two
 radial magnetic fields, one generated by a series of circumferential coils
 forming part of the cylinder wall and the other by a plurality of magnets
 forming part of the piston, or vice versa. For many purposes, the
 arrangements disclosed in WO 93/01646 are quite satisfactory. However, if
 the invention of WO 93/01646 is to be used as a combined
 fluid/electromagnetic actuator, the need to provide fluid-tight seals both
 on the piston and where the piston rod emerges through an end wall of the
 cylinder is a disadvantage, particularly as heavy and off-axis loads will
 tend to bend the piston rod out of true. Also, the solid construction of
 the piston and piston rod means that larger sizes of the device will
 become very heavy.
 OBJECTS AND SUMMARY OF THE INVENTION
 It is an object of the present invention to overcome the drawbacks of the
 prior art.
 It is another object of the present invention to provide an improved
 electromagnetic apparatus for producing linear motion which is lighter in
 construction and which has increased transverse stiffness.
 Briefly stated, the present invention provides sealed telescoping tubes
 that are electromagnetically and fluidly actuated to provide linear motion
 and withstand loading. One of the tubes may be connected to a pressure or
 vacuum source, and the other sealed to a fixture to provide variable
 volume load augmentation in addition to electromagnetic actuation.
 Alternatively, both tubes may be sealed to provide a pneumatic or
 hydraulic spring with electromagnetic actuation, and the sealed tubes may
 also be connected to a pressure or vacuum source for load augmentation. A
 bearing may be used in conjunction with the seal between the tubes to
 provide resistance to traversal loads. The electromagnetic actuation
 includes a helical arrangement of coils or permanent magnets to provide
 linear motion or resistance. The coils can be arranged to be connected to
 a conventional multi-phase motor drive.
 According to an embodiment of the present invention there is provided an
 electromagnetic apparatus for producing a linearly-directed force,
 comprising at least two tubular members adapted to slide telescopically,
 one within the other, means for producing a first magnetic field, the
 polarity of which alternates along at least a portion of the axial length
 of the inner tubular member, means for producing a second magnetic field
 the polarity of which alternates along at least a portion of the axial
 length of the outer tubular member, and means for varying at least one of
 the magnetic fields so as to cause the magnetic fields to interact to
 provide an axial force tending to cause relative motion between the
 tubular members.
 Preferably there is provided at least one seal between the inner surface of
 the outer tubular member and the outer surface of the inner tubular member
 so as to prevent the access of harmful environmental components to those
 parts of the device where relative motion occurs.
 The outer ends of the tubular members may include flanges by means of which
 they can be attached to components of another structure.
 The device may be so constructed that the inner tubular member carries an
 array of permanent magnets and the outer tubular member an assembly of
 coils which are selectively energized to vary one of the magnetic fields,
 or vice versa. Other arrangements are possible, for example, both tubular
 members may carry electrical coils, or the permanent magnets may be
 replaced by low-reluctance magnetic elements or by a conducting sleeve in
 which a pattern of eddy currents is induced.
 According to a particular aspect of the present invention there is provided
 an electromagnetic apparatus for producing a linearly-directed force
 comprising two cup-shaped members, one fitted within the other and adapted
 to allow relative motion therebetween, means for producing a first
 magnetic field, the polarity of which alternates along at least a part of
 the axial length of one of the cup-shaped members, means for producing a
 second magnetic field the polarity of which alternates along at least a
 part of the axial length of the other cup-shaped member, and means for
 varying at least one of the magnetic fields so as to cause the magnetic
 fields to interact to provide an axial force tending to cause relative
 motion between the cup-shaped members.
 Preferably, there are seals interposed between the cup-shaped members so
 that they form an enclosed space and one of the cup-shaped members has an
 opening therein so as to enable fluid to pass into and out of the enclosed
 space.
 The device may be constructed as a moving-coil or moving-magnet device as
 desired, but in a preferred arrangement the outer cup-shaped member has an
 electromagnetic element consisting of a plurality of annular coils
 separated by ferromagnetic spacers which are arranged to present a
 constant reluctance to a radially-directed magnetic field originating from
 a magnetic field-generating element associated with the inner cup-shaped
 element. Preferably the magnetic field generating element associated with
 the inner cup-shaped element is a plurality of permanent magnets.
 According to another embodiment of the present invention, there is provided
 an apparatus for producing linearly-directed force, comprising: a
 plurality of tubular members, at least two of the tubular members are
 fittable one within another and movable relative to each other in an axial
 direction, means for producing a first magnetic field alternating in
 polarity along at least a portion of one of the at least two tubular
 members, means for producing a second magnetic field alternating in
 polarity along at least a portion of another of the at least two tubular
 members, and the first and second magnetic fields interact to urge the at
 least two tubular members in an axial direction relative to each other.
 According to a further embodiment of the present invention, there is
 provided an apparatus for producing linear motion, comprising: a plurality
 of cup-shaped members, at least two of the cup-shaped members are fittable
 one within another and movable relative to each other, means for producing
 a first magnetic field alternating in polarity along at least a portion of
 one of the at least two cup-shaped members, means for producing a second
 magnetic field alternating in polarity along at least a portion of another
 of the at least two cup-shaped members, and the first and second magnetic
 fields interact to urge the at least two cup-shaped members in a direction
 relative to each other.
 The above, and other objects, features and advantages of the present
 invention will become apparent from the following description read in
 conjunction with the accompanying drawings, in which like reference
 numerals designate the same elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to FIG. 1 of the drawings, an apparatus 100 for producing linear
 motion electromagnetically consists of an outer tubular member 101 which
 has a mounting flange 102 at one end. Inside the outer tubular member 101
 is an axial array of coils 103 which are mounted on a polished steel liner
 104. An inner tubular member 105 is arranged to be a good sliding fit in
 the liner 104 of the outer tubular member 101. The inner tubular member
 105 also has a mounting flange 106 at its outer end. A portion 107 of the
 inner tubular member 105 is provided with a series of annular magnets 108
 arranged to produce a magnetic field pattern which alternates in the axial
 direction and interacts with the magnetic field pattern produced by the
 coils 103. The outside diameter of the annular magnets 108 is
 approximately the same as that of the remainder of the inner tubular
 member 105. A seal 109 is provided where the inner tubular member 105
 projects from the outer tubular element 101. A bearing and wiper ring 110
 is fitted to the end of the inner tubular member 105 which works inside
 the outer tubular member 101. The bearing and wiper ring 110 acts to
 constrain any radial deflection of the inner tubular member 105 and to
 clean the inner surface of the liner 104 of the outer tubular member 101.
 Also included, but not shown in the drawing, is a sensor for determining
 the position in the outer tubular member 101 of the inner tubular member
 105. Signals from this sensor are used to control the action of a power
 supply 111, which is capable of generating electric currents of variable
 frequency, phase and amplitude, connected to the array of coils 103, so
 that the electric currents flowing in the coils 103 interact with the
 magnetic fields produced by the array of magnets 108 on the inner tubular
 member 105 to produce an axial electromagnetic force of a desired
 magnitude and direction.
 The construction of the device is such that it can withstand considerable
 transverse loads. Also, the presence of the seal 109 permits the device to
 be used in hazardous or difficult environments, providing that the open
 ends of the tubular members 101 and 105 are closed off.
 If so desired, the portion of the inner tubular member 105 which projects
 beyond the outer tubular member 101 when the inner tubular member 105 is
 at the inner limit of its travel may be of larger or smaller diameter than
 the remainder. Also, the mounting flanges 102 and 106 may incorporate
 O-ring seals so that when they are assembled to other structures the
 volume bounded by the outer faces of the mounting flanges 102 and 106 is
 sealed. The device may then be adapted to act as a pneumatic spring or
 actuator as well as providing an electromagnetically generated linear
 force.
 FIG. 2 shows another embodiment of the invention which is identical to that
 of FIG. 1 except that the liner 104 is replaced by a copper or aluminum
 tube 201, which may be plated with a hard metal such as chromium so that
 the tube 201 also acts as the liner 104. Alternatively, a separate liner
 can be used. Coils 202 in the inner tubular member 105 replace the magnets
 108 and generate eddy currents in the copper or aluminum tube 201, which
 interact with the magnetic field produced by the coils 202. Those
 components which are common to both embodiments have the same reference
 numerals.
 FIG. 3 shows a third embodiment of the invention in which the two tubular
 members are closed off.
 Referring to FIG. 3, an apparatus for electromagnetically producing a
 linearly-acting force consists of two hollow cup-shaped members 301, 302
 one of which (301) is adapted to move linearly within the other (302).
 Each of the cup-shaped members 301 and 302 has a tubular section 303,304,
 respectively, and end closure members 305 and 306 respectively, positioned
 at the outer ends of the tubular sections 303 and 304 to form an enclosure
 307. As before, an axial array of coils 308 is contained between the
 tubular section 304 of the cup-shaped member 302 and a polished steel
 liner 309. At the inner end of the cup-shaped member 301 there is an
 axially-extending annular array of magnets 310 which produces a magnetic
 field which alternates axially and interacts with the magnetic field
 produced by the coils 308 when they are energized by a suitable power
 supply 311. As before, there is included a sensor (not shown) for
 determining the position of the cup-shaped member 301 within the
 cup-shaped member 302 and producing signals which are used to control the
 action of the power supply 311, which again is capable of producing
 electric currents of variable phase, frequency and amplitude, so that the
 electric currents flowing in the coils 308 interact with the magnetic
 field produced by the array of magnets 310 on the cup-shaped member 301 to
 produce an axial electromagnetic force of a desired magnitude and
 direction.
 The cup-shaped member 302 is sealed to the cup-shaped member 301 by a
 sliding bearing and seal 312 and the inner end of the cup-shaped member
 301 is supported by another sliding bearing 313. In this form of the
 invention there is no need to provide another seal at the inner end of the
 cup-shaped member 301 because of the end-closures 305 and 306. As before,
 considerable lateral stiffness is imparted by the design of the device.
 The sealing of the enclosure 307 enables the device to act as a pneumatic
 spring because movement of one cup-shaped member 301, 302 relative to the
 other causes a change in the volume of the enclosure 307, and hence the
 pressure of a compressible medium in the enclosure 307. If one or other of
 the cup-shaped members has a suitable port in it, the device can be
 connected to an external reservoir and so be enabled to function as a
 pneumatic or hydraulic ram as well as an electromagnetic linear force
 producer. If the device is used as an actuator, then such an arrangement
 would enable a static or slowly-varying load to be accommodated without
 drawing an electric current from the power supply 311.
 FIG. 4 shows an arrangement of coils for use with any of the forms of the
 invention described above. Referring to FIG. 4, the array of magnetic
 field-producing coils associated with the outer members 102,302 previously
 described, is made up from a stack of circular coils 401 which are
 interposed between rings 402 made of a ferromagnetic material and external
 ferromagnetic spacer rings 403. The dimensions of the coils 401, rings 402
 and spacers 403, are such that the assembly forms a single cylindrical
 coil assembly 404. In order that the assembly 404 presents a constant
 reluctance to a radially-directed magnetic field produced by the member
 moving within the coil assembly 404, each module of the coil assembly 404
 is skewed by the axial length of one coil 401. In another arrangement
 which is not illustrated, the rings 402 are skewed back and forth over one
 complete rotation so as to balance any off-axis forces generated by the
 coil assembly 404.
 In the form of invention described with reference to FIGS. 1 and 2, more
 than one telescopic stage can be used and the terms "inner" and "outer"
 tubular members are to be interpreted accordingly, as referring to any
 particular stage of the apparatus.
 Having described preferred embodiments of the invention with reference to
 the accompanying drawings, it is to be understood that the invention is
 not limited to those precise embodiments, and that various changes and
 modifications may be effected therein by one skilled in the art without
 departing from the scope or spirit of the invention as defined in the
 appended claims.