Round linear actuator utilizing flat permanent magnets

[0002] The present invention relates to a linear actuator and, more specifically, to a linear actuator which incorporates flat magnets.

[0003] A typical linear actuator of the prior art consists of a housing which encloses a cylindrical coil and a cylindrical magnet. The coil and magnet are disposed about a shaft or armature which moves axially within the housing. When the coil is not energized, the shaft is in a first position. When the coil is energized, the shaft moves axially to a second position. The sealing force, the force maintaining the shaft in either the first or second position, is created by a magnet which passes magnetic flux through the shaft. The strength of the sealing force is a function of the amount of air gap between the shaft and the magnet. Sealing force decreases as the air gap between the magnet and the shaft increases. Magnetic flux is also transferred through elements of the housing. Accordingly, the sealing force also decreases as the total air gap between the magnet and the housing components increases.

[0004] Because the amount of force exerted on the shaft is a function of the air gap between the magnets and the shaft and/or the total air gap between the magnet and the components of the housing, it is desirable to minimize the air gap. As shown in FIG. 1, prior art linear actuators use cylindrical magnets 1 having a cylindrical hole therethrough. The cylindrical magnet is typically held on a magnet carrier 2 which is disposed within the cylindrical hole, between the magnet and the shaft 3. The magnet is further enclosed within a protective housing 4. Thus, there are three circular interfaces in linear actuators in the prior art. It is difficult, however, to machine components having tight tolerances with a cylindrical interface. As shown on FIG. 1, circular holes may be slightly elliptical or oversized, resulting in an air gap 5,6, 7. The sealing force created by the magnet could be increased if the air gap 5, 6, 7 between the shaft and magnet and/or the total air gap between the magnet and housing could be reduced. Additionally, production costs could be reduced if the cylindrical interface were not used.

[0005] Therefore, there is a need for a linear actuator that reduces the air gap between the magnet carrier and the shaft.

[0006] There is a further need for a linear actuator that reduces the total air gap between the magnet and the housing components.

[0007] There is a further need for a linear actuator that is easily manufactured with a minimal air gap.

[0008] These needs and others are satisfied by the present invention, which is directed to a linear actuator, which utilizes a plurality of flat magnets spaced about the linear actuator's shaft

[0009] As with prior art linear actuators, the invention includes a housing which encloses a coil and which has a shaft passing through the coil and housing. In place of the typical cylindrical magnet, however, is a plurality of flat magnets. The flat magnets are sandwiched between a magnet support ring, which forms a portion of the housing, and a magnet carrier which is adjacent to the shaft. Because the magnets are flat, both the magnet support ring and the magnet carrier are easily machined so that the average gap between the magnet and either the magnet support ring or the magnet carrier is less than 0.001 inch. Additionally, because the magnets are flat, the present invention, requires only one circular interface, unlike the prior art which includes three circular interfaces. With a smaller air gap between the flat magnets and their support structures, the actuator provides a greater sealing force.

[0010] A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:

[0011] FIG. 1, prior art linear actuators showing air gaps.

[0012] FIG. 2 is a top view of a linear actuator according to the present invention.

[0013] FIG. 3 is a cross sectional view taken along line 3—3 of FIG. 2.

[0014] FIG. 4 is a detail view of a section of the magnet support ring. Specifically, FIG. 4A shows a plurality of sections forming a ring, FIG. 4B shows an individual segment, and FIG. 4C shows an isometric view of a segment.

[0015] FIG. 5 is a cross sectional view of an alternate embodiment incorporating a spring.

[0016] A linear actuator 10 according to the present invention is shown in FIGS. 2 and 3. As shown on FIG. 3, the linear actuator 10 includes a housing 20 which is formed of a top plate 21, a top cylinder 22, a magnet support ring 24, a bottom cylinder 26, and a bottom plate 28. The top plate 21 is coupled to the top cylinder 22. The top cylinder 22 is coupled to the top plate 21 on one end, and to the magnet support ring 24 or the other end. The magnet support ring 24 is further coupled to the bottom cylinder 26. Bottom cylinder 26 is coupled on one end to the magnet support ring 24 and on the other end to bottom plate 28. In the preferred embodiment, the top plate 21 and bottom plate 28 are square. Top cylinder 22 and bottom cylinder 26 are hollow, forming upper and lower cavities 23 and 27 respectively. Top plate 21 and bottom plate 28 each has a medial opening passing therethrough. The medial openings are aligned. Shaft 30 is slidably disposed through both medial openings. Shaft 30 is preferably cylindrical. Shaft 30 may have an enlarged plunger body 32 disposed between the top plate 21 and bottom plate 28. The plunger body 32 is preferably cylindrical and disposed about shaft 30. The housing 20 and the magnet carrier 42 (described below) are made of magnetically permeable material, i.e. a material with low reluctance.

[0017] A coil assembly 35, which includes a coil support 36 and wire 37 is disposed within upper cavity 23. The coil support 36 is a torid having a U-shaped cross section. Wire 37, preferably 500 turns of 18-gauge magnet wire, is wrapped around coil support 36. Coil assembly 35 is coupled to an electric source (not shown). A medial opening passes through coil support 36. Shaft 30 and/or plunger body 32 are slidably disposed in the coil support 36 medial opening.

[0018] The magnet support ring 24 may be any shape, however, as shown on FIGS. 4A, 4B, and 4C the magnet support ring 24 preferably has a cylindrical outer surface 25 and an inner surface 41 forming a plurality of flat surfaces 50. In the preferred embodiment there are six flat surfaces 50 evenly spaced about the inner surface 41. The magnet support ring 24 may be formed of three 120° segments 24a, 24b, 24c.

[0019] A plurality of flat magnets 40 are disposed adjacent to and contacting each inner surface 41. Preferably flat magnets 40 are evenly spaced angularly about shaft 30. The flat magnets 40 are planar and rectangular. The flat magnets 40 are preferably made from a rare earth material such as neodymium iron boron (NdFeB), Alnico, Ferrite, Samarium Cobalt, or ceramic. Rare earth magnets tend to maintain their magnetism longer than other materials. The flat magnets 40 are sandwiched between the inner surface 41 of magnet support ring 24 and the outer surface 43 of magnet carrier 42. Preferably, flat magnets 40 are each perpendicular to a radius of shaft 30. The outer surface 43 of magnet carrier 42 has a plurality of flat surfaces 51 opposing each flat surface 50 on the magnet support ring. Magnet carrier 42 also has a cylindrical medial opening passing therethrough. Shaft 30 and/or plunger body 32 pass through the medial opening in magnet carrier 42. Flat magnets 40 are held between magnet support ring 24 and magnet carrier 42 by cylindrical caps 38, 44. Magnet cap 38 is disposed between coil assembly 35 and magnet support ring 24 and magnet carrier 42. Magnet cap 44 is disposed in lower cavity 27, contacting magnet support ring 24 and magnet carrier 42. Both magnet caps 38, 44 have medial openings. Shaft 30 and/or plunger body 32 is slidably disposed through the medial openings in magnet caps 38, 44.

[0020] Because the magnets 40 are flat, the inner surface 41 of magnet support ring 24 and outer surface 43 of magnet carrier 42 can be machined to have virtually no air gap between the magnets 40 and surfaces 50, 51. The average air gap between flat magnets 40 and magnet support ring 24 is approximately 0.001 inch. The average air gap between flat magnets 40 and magnet carrier 42 is approximately 0.001 inch. Additionally, the components of the housing 20, top plate 21, a top cylinder 22, a magnet support ring 24, a bottom cylinder 26, and a bottom plate 28, each have a flat interface between the respective adjacent parts. These flat surfaces can also be machined to have virtually no air gap. The average total air gap between the components of the housing 20 is approximately 0.001 inch or less. Because each of the surfaces 50, 51 that contact the flat magnets 40 are also flat, the cost of manufacturing the magnet support ring 24 and magnet carrier 42 to tight tolerances is reduced.

[0021] In operation, shaft 30 may be positioned in either a first position, with plunger body 32 adjacent to top plate 21, or a second position, with plunger body 32 adjacent to bottom plate 28. Coil assembly 35 my be energized by an electric source. By varying the polarity of the electric source, the current through the coil assembly 35 may travel in different directions. When the current through coil assembly 35 travels in a first direction, the coil assembly 35 creates a magnetic field which draws the shaft 30 into the first position. When the current through coil assembly 35 travels in a second direction, the coil assembly 35 creates a magnetic field which draws the shaft 30 into the second position. When the coil in not energized, the shaft 30 is maintained in either the first or second position by the sealing force created by flat magnets 40. The sealing force is created by magnetic flux passing through plunger body 32 and housing 20. The smaller the air gap between the magnets 40 and the housing 20 and the magnet carrier 42 the greater the sealing force.

[0022] In an alternate embodiment, shaft 30 is biased in a first position by an external force, such as a spring (not shown) or, as shown in FIG. 5, a spring 60 may be disposed in lower cavity 27. When coil assembly 35 is energized, a magnetic field is created which acts on the plunger body 32 with enough force to overcome the force of the external force or spring 60 and draw the plunger body 32 to a second position adjacent to bottom plate 28. The plunger body 32 is held in the second position by the sealing force created by the flat magnets 40.

[0023] While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.