Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F7/00—Magnets
  • H01F7/06—Electromagnets; Actuators including electromagnets
  • H01F7/08—Electromagnets; Actuators including electromagnets with armatures
  • H01F7/121—Guiding or setting position of armatures, e.g. retaining armatures in their end position
  • H01F7/122—Guiding or setting position of armatures, e.g. retaining armatures in their end position by permanent magnets
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F7/00—Magnets
  • H01F7/06—Electromagnets; Actuators including electromagnets
  • H01F7/08—Electromagnets; Actuators including electromagnets with armatures
  • H01F7/16—Rectilinearly-movable armatures
  • H01F7/1638—Armatures not entering the winding
  • H01F7/1646—Armatures or stationary parts of magnetic circuit having permanent magnet
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F7/00—Magnets
  • H01F7/06—Electromagnets; Actuators including electromagnets
  • H01F7/08—Electromagnets; Actuators including electromagnets with armatures
  • H01F7/18—Circuit arrangements for obtaining desired operating characteristics, e.g. for slow operation, for sequential energisation of windings, for high-speed energisation of windings
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F7/00—Magnets
  • H01F7/06—Electromagnets; Actuators including electromagnets
  • H01F7/08—Electromagnets; Actuators including electromagnets with armatures
  • H01F7/16—Rectilinearly-movable armatures
  • H01F2007/1692—Electromagnets or actuators with two coils
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F7/00—Magnets
  • H01F7/06—Electromagnets; Actuators including electromagnets
  • H01F7/08—Electromagnets; Actuators including electromagnets with armatures
  • H01F7/13—Electromagnets; Actuators including electromagnets with armatures characterised by pulling-force characteristics

Definitions

• the present invention relates to multistable electromagnetic actuators and more particularly actuators suitable for controlling fluid flow.
• Spring-loaded solenoid-based actuators are often employed to control locks or the flow of fluids, for example. However, they are typically monostable devices and require a continuous current to maintain the driving rod of the device in its actuated position. This leads to unwanted energy dissipation in the form of heat.
• EP-A-1119723 (filed by the present applicant under reference 554.02AV) describes a magnetic drive having a bistable characteristic, which can be configured to revert to (or remain in) one of its two states in the event of a power failure.
• US-3,772,540 relates to an electromechanical latching actuator for producing linear or rotary motion.
• Figures IA to ID depict an actuator which includes one or more sets of radially polarised permanent magnets and electric coils which annul and flux switch a magnetic field between adjacent magnetically isolated poles, thereby sequentially generating a force or torque that can be coupled to a suitable load.
• the present invention seeks to provide a robust and reliable electromagnetic actuator configuration, suitable for use in a broad range of applications.
• the present invention provides an electromagnetic actuator comprising an armature comprising a permanent magnet, wherein the armature is movable between first and second stable positions; two electric coils disposed on opposing sides of the armature along its direction of movement, with their axes substantially aligned with said direction; and a magnetic flux container substantially surrounding the armature and the coils to substantially contain magnetic flux generated thereby and to substantially shield its interior from external magnetic flux, wherein in each stable position magnetic flux generated by the permanent magnet extends around a magnetic circuit path including the container so as to retain the armature in its stable position, and wherein energising the coils causes the armature to move from one stable position to the other.
• the known actuator configurations acknowledged above have open flux arrangements wherein the permanent magnets create flux which extends outside the actuators themselves. Therefore their performance is susceptible to external influences. For example, it may be influenced by magnetic surrounding components such as another actuator or a ferromagnetic housing. In addition, an open magnetic field attracts ferromagnetic particles from the environment. A fluid or gas flowing close to the actuator may include small ferromagnetic particles, for example as the result of corrosion. Aggregation of such particles risks causing a blockage. This is undesirable in many applications, particularly critical roles in jet engine fuel flow control or the space industry for example.
• the magnetic flux container present in an actuator according to the invention extends around the armature and electric coils in such a way as to substantially contain within it the magnetic flux generated by these elements, thereby minimising any side effects resulting from flux leakage. Magnetic circuits formed during operation of the device are closed by the container.
• the container serves to shield the interior of the actuator from external magnetic fields.
• the actuator is substantially sealed against the ingress of magnetic flux from outside by the container.
• each coil is wound round a coil core which forms part of the magnetic circuit created when the armature is adjacent to the respective coil.
• the actuator may be configured such that, when the armature is in either of the stable positions, the shortest path from the armature to the container is less than the shortest path from the armature to the more distant of the two coil cores. This ensures that the armature is reliably latched against one of the coil cores in each stable rest position.
• the armature may include pole pieces on opposing sides of the permanent magnet along its direction of movement.
• the actuator is preferably configured such that, when the coils are energised, the path of magnetic flux through the pole piece closest to the corresponding coil core changes from a substantially axial orientation to a substantially radial orientation, and vice versa for the other pole piece.
• each pole piece defines a surface for engagement with a respective coil core, and each coil core defines a complementary engagement surface.
• each of said pole piece engagement surfaces may include a frustoconical portion. This serves to create a more uniform force of attraction characteristic between the two mating surfaces, relative to planar faces.
• the permanent magnet is orientated with its North and South poles aligned with the direction of movement of the armature. Relative to radial alignment of the poles, a significantly greater locking force is achieved as a greater area of high flux density faces the adjacent coil core.
• Actuators embodying the invention preferably include an energy storage arrangement for storing energy derived from movement of the armature into each stable position.
• This storage arrangement transfers energy to the armature as it moves away from each stable position. This provides internal energy recycling and so reduces the power required to switch the device. It also affords a "soft landing" effect, which will extend the lifetime of the actuator. Also, in applications where the actuator controls fluid flow by pinching a deformable tube, the deceleration caused by the energy storage arrangement as the actuator moves towards each stable position reduces the likelihood of damage to the tube. The extent of the energy storage may be readily adjusted as appropriate to alter the net latching force exerted on the armature to suit different applications.
• the energy storage arrangement may comprise a pair of resilient devices, such as coil springs for example, with one of the devices being compressed or extended as the armature moves into a respective stable position.
• the resilience of these devices may be selected to suit a particular requirement.
• Each resilient device may be disposed between a pole piece and a respective coil core, providing a compact and self-contained configuration.
• the resilient devices may be located outside the housing of the actuator to provide a greater area of engagement between the armature and the coil cores, thereby increasing the latching force.
• larger resilient devices may be more readily accommodated outside the actuator housing in this implementation.
• either resilient device is only compressed or extended as the armature moves through a final portion of its travel into a respective stable position.
• the actuator has a third stable position between the first and second stable positions. This third position is preferably defined by spring and passive magnetic forces acting on the armature.
• a pair of resilient devices may be arranged such that one of them is compressed (or extended) or compressed (or extended) further if the armature moves away from the third stable position, so as to urge the armature towards the third stable position.
• each resilient device is partially compressed (or extended) when the armature is in the third stable position.
• This pre-loading of each resilient device makes the third stable position more definite and more clearly defined and readily selectable.
• the extent to which each resilient device may be partially compressed (or extended) when the armature is in the third stable position may be adjustable so as to emphasise the third position to the degree needed to meet particular requirements.
• an actuator may be arranged such that when the armature moves from the third stable position to one of the first and second stable positions so as to compress (or extend) further one of the resilient devices, at least during a final portion of said movement (preferably substantially the whole of said movement), the degree of partial compression (or extension) of the other resilient device remains substantially unchanged. This has the effect that during movement of the armature from the third stable position to another stable position and back again, energy is not expended in deformation of the other resilient device and it does not therefore influence this action of the actuator.
• the magnetic flux container may form the housing of the actuator.
• the present invention provides a method of operating an actuator as described herein, comprising the step of moving the armature from one stable position to the other by energising the coils so as to generate axial magnetic flux through each coil in respective opposite directions.
• applying a current pulse momentarily to each coil in this manner serves to substantially nullify the flux created by the permanent magnet on one side whilst augmenting the flux density on the other side, causing the armature to switch positions.
• the armature is held in each stable rest position by spring and/or passive magnetic forces alone, with only a brief current pulse needed as and when the actuator is switched to a different stable rest position. Its power consumption is therefore very low.
• FIGS 1 and 2 are perspective and side cross- sectional views, respectively, of actuators embodying the invention
• FIGS 3 and 4 are side cross-sectional views of the actuator of Figure 1 which illustrate its switching action
• Figure 5 is a plot of force against armature-container spacing
• Figure 6 is a side cross-sectional view of an actuator embodying the invention in combination with a tube clamping device
• Figure 7 is a perspective cross-sectional view of an actuator according to a further embodiment of the invention.
• Figure 8 is a schematic graph plotting the forces exerted on the actuator armature against its displacement for an actuator having a configuration of the form shown in Figure 1;
• Figure 9 is a side cross- sectional view of a further actuator configuration embodying the invention.
• Figures 1OA to 1OC are side cross- sectional views of the actuator shown in Figure 9 in three different stable positions.
• Figure 11 is a schematic graph plotting the forces exerted on the actuator armature against its displacement for an actuator having a configuration of the form shown in Figure 9. Detailed description of the Drawings
• Figures 1 and 2 show cross-sectional views of an actuator embodying the invention. It is a fully magnetically sealed bistable push-pull actuator including an internal energy recycling mechanism. It is suitable for use as a directly linked mechanical driver, or to operate a valve or electric switch. It could be used as a direct replacement for traditional solenoid-based actuators, with a substantial reduction in power consumption.
• the actuator includes a magnetic flux container or cage 2 which also forms the actuator housing. Each end of the container is closed by end caps 4a and 4b.
• a driving element in the form of a push-pull rod 6 extends along the longitudinal axis of the actuator. In the embodiment of Figure 1, this rod extends through and beyond both end caps, whilst in the arrangement of Figure 2, it only protrudes from one end of the actuator.
• a permanent magnet 8 is mounted on a central portion of the rod 6.
• Pole pieces 16a and 16b, also mounted on the rod, are provided in contact with and on either side of the permanent magnet 8. The magnet and pole pieces together form an armature 10.
• each pole piece in the axial direction Facing each pole piece in the axial direction are coil cores 12a and 12b.
• a coil 14a, 14b is provided around each coil core in axial alignment with the rod 6. (The coils are not shown in the embodiment of Figure 2).
• Coil springs 18a and 18b are provided around rod 6 on either side of the armature 10.
• the springs may be configured such that they are in contact with the corresponding pole piece and coil core at all times, so that one of them begins to be compressed as soon as the armature moves away from one of its stable positions.
• compression of one of the springs may only begin part way through the travel of the armature into one of its stable positions to facilitate faster initial travel of the armature. This may be achieved by providing springs which are shorter in their uncompressed state than the maximum spacing between each pole piece and the corresponding coil core.
• a position sensor (not shown), such as a Hall sensor, may be located adjacent one of the stable positions of the actuator to provide a signal indicative of the armature location.
• the coil cores 12a, 12b are integrally formed with the end caps 4a,4b.
• the magnetic flux container or cage may be provided by a number of discrete elements coupled together.
• the container forms a continuous magnetic path which substantially externally surrounds the coils and permanent magnet.
• the container is formed of a material having a high magnetic permeability, such as steel for example.
• the pole pieces may be formed of laminated material for example. Soft ferrites may be used to form the pole pieces.
• the voids within the actuator may be filled with an inert liquid such as oil. It may be preferable to employ a gas instead as a relatively high viscosity fluid will tend to lead to a greater amount of energy being required to switch the actuator. It will be appreciated that the actuator may be constructed in a range of sizes. Merely by way of example, an embodiment suitable for small scale applications has a length of 28 mm and a diameter of 19mm.
• Figure 3 shows an actuator with the armature latched in one of its two stable positions.
• the path of flux lines emanating from the permanent magnet is shown by black arrows.
• the lines of flux travel from the North pole of permanent magnet 8 into right-hand pole piece 16b.
• the flux lines then extend radially outwards across the relatively small gap 20 between the pole piece and the container 2. They follow a path within the container 2 extending axially along the outer circumferential wall of the container and then radially inwards via end cap 4a.
• the path continues on axially inwards through coil core 12a, across the interface between the core and the adjacent pole piece 16a, before returning back to the permanent magnet 8.
• the left-hand coil core 12a engages a complementary mating face of the adjacent pole piece 16a, with the lines of magnetic flux therebetween parallel to the push-pull rod 6.
• the right-hand pole piece 16b is attracted to the adjacent magnetic container and the flux lines between them are perpendicular to the axis of the push-pull rod. This is because the spacing 20 between the pole piece 16b and the container 2 is significantly smaller than the distance 22 between the pole piece and the corresponding face of the opposing coil core 12b. Accordingly, the net magnetic locking force exerted on the armature 10 is axially directed towards the left-hand coil core 12a.
• Movement of the armature from one stable position to the other is initiated by applying a current pulse to each of the coils 14a, 14b so as to generate magnet flux through the centre of each coil in the respective opposite directions indicated by the arrows drawn in outline in Figure 4.
• This additional flux acts to substantially nullify the flux generated by the permanent magnet through the coil core 12a.
• the flux generated by the magnet is forced to change direction from a parallel flow between the coil core 12a and the pole piece 16a to a radial orientation along a path extending from the container 2 into the pole piece 16a. As a result, the magnetic locking force is substantially reduced.
• the other coil 14b generates flux in the same direction as the flux from the permanent magnet.
• the lines of flux previously running radially outwards from pole piece 16b to the magnetic container 2 are now attracted instead towards coil core 12b and re-orientated into an axial direction extending in-between pole piece 16b and coil core 12b.
• the net magnetic locking force exerted on the armature 10 is directed towards coil core 12b.
• the compressed spring 18a is no longer held by the locking force of the actuator and catapults the armature 10 away from coil core 12a, towards the other stable position.
• the coils 14a and 14b are arranged in a mirrored configuration such that a current pulse flowing outwardly along each coil from the inner ends thereof generates the opposite outward magnetic flux along the centre of each coil indicated by the outlined arrows in Figure 4.
• the actuator is therefore magnetically balanced both in its stable mode and during switching.
• (F) generated by an actuator embodying the invention is represented in the plot of Figure 5. If no gap was present, the path of flux generated by permanent magnet 8 would be closed locally by the wall of the container 2. In this case there would only be a weak locking force urging the armature against coil core 12. If gap 22 were smaller than gap 20, then the magnetic flux generated by the permanent magnet would follow a path via coil core 12b, the magnetic container 2 and coil core 12a, again resulting in a lower locking force.
• the size of the gap 20 is also a significant factor as it determines the ease with which the fluid can pass around the armature as it moves from one stable position to another.
• gap 20 means that the surface finish of the armature and the facing surface of the magnetic container is not as critical as it would be if there was a sliding fit between these two components.
• the actuator which Figure 5 relates has an armature travel distance of 3mm, and a gap of 0.5mm was found to be preferable.
• Figure 6 illustrates an actuator embodying the present invention in combination with a device for pinching a tube carrying a fluid.
• a head 30 is mounted on the end of push- pull rod 6.
• the fluid tube passes along a groove 32 defined by the valve.
• the valve is shown in its open position in Figure 6. Operation of the actuator moves armature 10 to its right-hand stable position, moving head 30 to the right and thereby pinching a tube mounted in the valve to cut-off fluid flow through the tube.
• the locking force generated by the permanent magnet of the actuator serves to hold the valve in the closed position without requiring any power input.
• Application of a further current pulse to the coils of the actuator switches the valve back to its open position.
• Figure 7 illustrates a further embodiment of the invention in which springs 38a,38b are provided outside the actuator housing.
• a flange 40 is mounted on a portion of push-pull rod 6 which protrudes from the housing 2.
• Springs 38a,38b are located axially on either side of the flange.
• the springs and flange are provided within an enclosure 42.
• One of the springs is provided between end cap 4b of the actuator and the flange 40, whilst the other spring is provided between flange 40 and end wall 44 of the enclosure 42.
• each spring 18a and 18b are located within the armature 10.
• the inner end of each spring bears against a collar 50 located axially on the rod 6 by a groove 52 defined by the rod.
• the outer end of each spring bears against a respective washer 54a, 54b which is slidably positioned around the rod 6.
• the outer surface of each washer in the axial direction is in engagement with the inner end of a respective sleeve 56a, 56b, or other suitable abutment arrangement fixed in position relative to the container.
• This outer surface of each washer is also preferably in contact with an inwardly facing annular shoulder 58a, 58b defined by the armature.
• Each spring is preferably in a partially compressed state. This serves to better define this central position as a third stable position, as discussed further below.
• Figures 1OA to 1OC illustrate the three stable positions exhibited by this actuator configuration, namely a central position and the left and right hand ends of its travel. It can be seen that when the armature has moved into a stable position at either end of its range of travel (as in Figure 1OB or 10C), one of the springs has been compressed as a result of the respective sleeve 56a, 56b maintaining the corresponding washer (54a in Figure 1OB and 54b in Figure 10C) in the same position relative to the actuator housing. In contrast, the other washer has been lifted away from its respective sleeve by the corresponding shoulder (58b in Figure 1OB and 58a in Figure 10C), with the extent of compression of the other spring consequently remaining unchanged.
• the resistance to movement of the armature out of its third, central rest position may be readily adjusted. For example this may be achieved by changing the spring constants of the springs, or by altering the extent to which the springs are compressed in this third stable position.
• Figure 11 is a graph showing plots of the axial forces exerted on the armature in an embodiment having a configuration of the form shown in Figures 9 and 10.
• the range of travel of the armature is divided into three zones A, B, and N in the absence of any forces other than the spring forces and passive magnetic forces acting on the armature.
• the armature With the armature within either of the end-of- travel zones A and B, it is urged by the resultant forces towards a respective end position.
• In the central zone N it is urged towards a central stable rest position.
• Plot 60 represents the magnetic forces
• plot 62 the spring forces
• plot 61 the combined effect of the magnetic and spring forces.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Reciprocating, Oscillating Or Vibrating Motors (AREA)
• Electromagnets (AREA)
• Magnetically Actuated Valves (AREA)

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• 2008
  • 2008-12-13 GB GBGB0822760.5A patent/GB0822760D0/en not_active Ceased
• 2009
  • 2009-10-23 GB GB0918632.1A patent/GB2466102B/en active Active
  • 2009-12-08 JP JP2011540208A patent/JP2012511823A/ja active Pending
  • 2009-12-08 WO PCT/GB2009/051668 patent/WO2010067110A1/en not_active Ceased
  • 2009-12-08 EP EP09771401.8A patent/EP2359376B1/en active Active
  • 2009-12-08 US US13/129,134 patent/US8710945B2/en active Active

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