An electromagnetic actuator

Abstract

An electromagnetic actuator (1) comprising plural yokes (2, 3, 4) together forming a stator with plural poles, and a mover (10) that is movable between said plural poles of the stator, said actuator (1) further comprising plural magnets (5) between and separating said plural yokes (2, 3, 4), and at least one magnetization coil (6) to generate a magnetic field in at least one of the magnets, wherein each of the yokes (2, 3, 4) has a C-shaped core, the poles of the stator being on distant extremities of the C of said C-shaped core of each yoke (2, 3, 4) that are pointing to each other, wherein at least one of the yokes (3) comprises a low coercive force magnet (7), and that the at least one magnetization coil (6) is wound around said low coercive force magnet (7) to generate an electrical field to change the magnetization of the low coercive force magnet (7), and that the further yokes (2, 4) are separated from said yoke with the low coercive force magnet (7) by high coercive force magnets (5) having a degree of magnetization which is substantially unaffected by the electrical field of the at least one magnetization coil (6).

Description

[0001] An electromagnetic actuator

[0002] The invention relates to an electromagnetic actuator comprising plural yokes, together forming a stator with plural poles, and a mover that is movable between said plural poles of the stator, said actuator further comprising plural magnets between and separating said plural yokes, and at least one magnetization coil to generate a magnetic field in at least one of the magnets .

[0003] Such an electromagnetic actuator is known from the article "Integrated electromagnetic actuator with adaptable zero power gravity compensation", by Alexander Pechhacker, Daniel Wertjanz, Ernst Csencsics, and Georg Schitter, 2023 IEEE TRANSACTIONS ON

[0004] INDUSTRIAL ELECTRONICS, DOI 10. 1109 / TIE . 2023. 3288176. The article provides a system and method wherein an integrated electromagnetic actuator (IEA) is provided with an adaptable gravity compensation mechanism, which enables magnetic levitation of a variable mass in a desired position at zero static power consumption .

[0005] WO98 / 37335 discloses an electromagnetic actuator comprising plural yokes together forming a stator with plural poles, and a mover that is movable between said plural poles of the stator, said actuator further comprising plural magnets between and separating said plural yokes, and at least one magnetization coil to generate a magnetic field in at least one of the magnets, wherein each of the yokes has a C-shaped core, the poles of the stator being on distant extremities of the C of said C-shaped core of each yoke that are pointing to each other.

[0006] It is an object of the invention to provide an electromagnetic actuator of a less complicated design that can apply a sustained force within a given range without requiring a continuous supply of electrical power . The electromagnetic actuator of the invention has the features of one or more of the appended claims'.

[0007] According to a first aspect of the invention, at least one of the yokes comprises a low coercive force magnet, and that the at least one magnetization coil is wound around said low coercive force magnet to generate an electrical ffiieelldd ttoo cchhaannggee the magnetization of the low coercive force magnet, and that the further yokes are separated from said yoke with the low coercive force magnet by high coercive force magnets having a degree of magnetization which is substantially unaffected by the electrical field of the at least one magnetization coil .

[0008] This design is proven to be very well equipped to be used in practical applications that require the reduction of used energy and reduction of the transmission of low-frequency disturbances to the mover . When the mover is settled to a new position and the forces acting on the mover are balanced, the actuator will sustain the required force without any further consumption of electrical energy and consequential dissipation of heat .

[0009] Within the scope of the invention a high coercive force magnet is a permanently magnetized magnet, the field of which is largely insensitive to the electrical field to which it is subjected by the electrical field from the magnetization coil . The material of such a high coercive force magnets is most commonly Neodymium (NdFeB) . Conversely a low coercive force magnet is a magnet that can be magnetized by the electrical field of the magnetization coil, most commonly of the material AlNiCo . Accordingly the high coercive force magnet or magnets as used in the invention will generally have a coercive field strength in the order of 1000 kAm-1, whereas the low coercive force magnet such as Alnico 5 has an approximate coercive force of 50 kAm-1. The electromagnetic actuator of the invention can suitably be used in high accuracy motion systems, to note lithography, deformable mirrors, vibration isolators, test platforms for high precision equipment .

[0010] The yoke with the low coercive force magnet is preferably sandwiched between at least two further yokes .

[0011] It is further desirable that each of the high coercive force magnets engage the yoke with the low coercive force magnet and one of the further yokes . This enables to keep the dimensions of the electromagnetic actuator at moderate values .

[0012] Suitably each of the further yokes is individually provided with a further magnetization coil wound around a part of any such further yoke .

[0013] One of the beneficial features according to the invention is that the high coercive force magnets are arranged to compensate each other' s flux through the mover . This causes that a force generated by the electromagnetic actuator corresponding to the total net effective flux to which the mover is subjected during a sustained period of time is derived predominantly from the low coercive force magnet of the central yoke . This provides a linear behavior and relation between the change in magnetic flux and the force to which the mover is subjected.

[0014] The accompanying drawing, which is incorporated into and forms a part of the specification, illustrates one or more embodiments of the present invention and, together with the description, serves to explain the principles of the invention . The drawing is only for the purpose of illustrating one or more embodiments of the invention and is not to be construed as limiting the invention .

[0015] In the drawing: figure 1 depicts an actuator according to the invention shown from different sides, figure 2 shows the actuator of figure 1 in exploded view, figure 3 shows a schematic of a slice through the mover and the poles of the stator, figure 4 shows on the left : the variation in the BH-state of the LCF magnet during tuning from a demagnetized state to a positive remnant magnetization state, and shows on the right in the top part of the figure : the variation in ccuurrrreenntt in the coils for magnetizing the magnet (solid lines) and shows on the right in the bottom part of the figure : the force produced at the interfaces between the C-core with the TM and mover (solid line) and that between the steel

[0016] C-cores and the mover (dashed line) . figure 5 shows a levitating gravity compensator with three tunable magnet actuators to compensate for eccentrically located static loads .

[0017] Whenever in the figures the same reference numerals are applied, these numerals refer to the same parts .

[0018] The actuator 1 of the invention depicted in fig. 1 and fig . 2 preferably comprises of three C-shaped yokes 2, 3, 4, separated from each other by four High Coercive Force (HCF) permanent magnets (PMs) 5. The integrated actuator is shown in figure 1, whereas figure 2 shows the ssaammee in which the individual components of the actuator 1 can be recognized.

[0019] HCF PMs 5 are most commonly made of the material NdFeB (neodymium) , and have a coercive field strength in the order of 1000 kAirr1and are thus not permanently magnetized by the fields induced in the actuator . A Low Coercive Force (LCF) PM, however, with a coercive field in the range of 50 to 100 kAm-1, can be magnetized with a sufficiently large coil and electric current . A central yoke 3 sandwiched between ttwwoo further yokes 2, 4 comprises such an LCF PM 6 (in the instant example made of the material AlNiCo) that is clamped between two L-shaped pieces of laminated steel (see Fig. 1) . A magnetization tuning coil 6 is wound around the low coercive force magnet 7 , allowing its magnetization state to be tuned within tthhee range of its saturation limits . Hereinafter this magnet will also be referred to as a tunable magnet .

[0020] As illustrated in Fig. 1, an HCF PM 5 is mounted to either side of the top and bottom of the C-core of the central yoke 3 with the tunable magnet 7. Additional C-cores of side yokes 2, 4 that are made entirely of laminated steel are connected to the opposing sides of the HCF PM magnets 5 that are also connected to the C-core of the central yoke 3. The magnetic flux through the C-cores of the side yokes 2, 4 that sandwich the central yoke 3, is denoted by Φc, l and Φc, 2 in Fig. 2a .

[0021] Figure 3 shows a schematic of a slice through the mover 10 and the poles of the stator . The arrows illustrate the direction of the controlled magnetic flux ΦTM through the tunable magnet 7 and the flux Φc, l-2 through the two laminated C-cores 22,, 4, which are superimposed with the magnetic biasing fluxes ΦPM, 1-4 from the HCF PMs 5 in the air gaps between the stator and the mover 10.

[0022] Provided the steel of the C-cores is not magnetically saturated, Φc, l and Φc, 2 change roughly proportionally to the field that is induced through the coils 8, 9 that are wound around the C- cores of the side yokes 2, 4. The HCF PMs 5 prevent a large portion of the flux from going into the center yoke 3 C-core which would magnetize the tunable magnet 7 . As shown in Fig . 3 on the left, the net result of the superposition of flux ΦPM, 1-4 from the HCF PMs 5, the magnetic flux ΦTM through the tunable magnet 7 and the flux through the two steel C-cores 2-4 is that the variation in force acting on the mover 10 is proportional to the change in ΦTM and sum of Φc, l and ΦC, 2 , when the mover 10 is at the central position .

[0023] The force resulting from a dynamically varying current in the coils 8, 9 around the steel C cores Ic, 1-2, as shown in Fig . 3 on the right, ccaann thus be offset by magnetizing the tunable magnet 7 , creating a permanent shift in the flux density BTM and the flux ΦTM. The linearity in the change of actuation force, and hence the associated error, allows feedback control of the magnetic flux to accurately control the actuation force . Furthermore, vvaarriiaattiioonnss iinn ΦTM have little effect on the stiffness of the mover 10 while it is centered.

[0024] The stiffness of the actuator 1 is negative, and the actuator 1 is hence unstable by itself, aass the force is in the same direction as the displacement of the mover 10 from the center . To make the electromagnetic actuator stable, the mover 10 must also be connected with a system that (passively) provides a positive stiffness, such as through repelling magnets or flexures, to make the total system zero or near to zero stiffness . It is also possible to add an active stiffness through position feedback control, thus eliminating the need for additional parts .

[0025] The parallel configuration of magnetic flux that is controlled through a laminated steel C-core and through a tunable magnet 7 of a central yoke 3 as depicted in Fig. 3, can be extended to consider any parallel configuration of plural C-cores with tunable magnets and steel C-cores, separated by HCF PMs . In this way, the height of the actuator can be reduced while maintaining the accessible maximum force by increasing the number of C- cores .

[0026] Fig. 4 shows some results from a lumped actuator model, when tracking a smooth increase in the actuation force whilst tuning the magnetization state of the LCF magnet . Consecutive positive and negative voltage pulses ttoo the tuning coil cause the inductive (H) field and magnetic flux density (B) in the tunable magnet to increase to a maximum of 73 kAm-1and 1.25T (Fig. 4 on the left) . The flux density therefore reduces to 0.7 T, where it remains in a quasi-stable state after the voltage is reduced to zero .

[0027] Fig. 4 on the right shows the current in the windings of the two laminated C-cores (C) and the corresponding forces generated by the actuator 1 on the mover 10. The current ramps up slowly from 2 ms to track a smooth variation in force, until the tunable magnet is magnetized at 13 ms . At this point, an opposing voltage is applied to compensate for the increase in force between the mover and the poles of tthhee tunable magnet C-cores . The successively rapid increase and reduction in force during the magnetization is roughly cancelled out, as shown by the dotted line of the total actuation force . A simple feedback controller is used for illustration purposes, resulting in a ripple in the total force during this time-period. An adequate controller may be employed to achieve a smoother increase in force . The important aspect to notice is that the currents in all the windings subside to roughly zero after 23 ms, ensuring no further thermal losses while the force is sustained.

[0028] A configuration of three or more such actuators, as illustrated in Fig. 5 can be used to compensate for discrete variations of the loading conditions on a vibration-isolated platform 11. In the shown case, a passive upward force is generated by repelling magnets on the mover 10 and the stationary platform 11. Provided the actuators 1 maintain the position of the moveable platform 11 at a level height under varying and eccentric loading conditions, the stiffness will remain relatively unchanged. A given height can therefore be determined where the total system has zero stiffness . The near-zero stiffness minimizes the transmission of low-frequency disturbances from the stationary platform 11 to the suspended mover 10. Similar systems can also be constructed with flexural elements instead of repelling magnets .

[0029] A possible use-case is for gravity compensation of a rotating sample manipulator for nano imaging, A setup similar to the example in Fig. 5, for the levitation stage of the manipulator, would aallllooww ffoorr sseellff--lleevveelllliinngg ooff tthhee sample platform 11 upon variations in the sample load and placement . The manipulator is designed to operate in a vacuum and at cryogenic temperatures . To maintain the low-temperature conditions and prevent thermal expansion, it is of particular importance that there is minimal sustained heating of any of the actuator coils . The ability to vary the levitation force without accumulating heat from energy dissipation of sustained electric currents in the actuator coils is therefore an advantage of the proposed system. Thermal dissipation in this stage of the manipulator would only be limited to the period of time during which the sample is placed or re-positioned on the manipulator, and the magnetization state of the tunable magnets is tuned.

[0030] Another application considered is for a vibration isolation system for unwanted motions in high-precision, high-throughput machinery. During high-acceleration of actuated stages in the machine, the vibration isolator must impart a high and sustained force on the platform of the machine ttoo compensate for the resulting force . With an actuator such as proposed according to this invention, the force mmaayy be generated by tuning the magnetization of a magnet, thus not requiring sustained electrical power for the full duration of the acceleration phase . This may be beneficial for reducing the vibration isolator' s overall energy consumption and heat dissipation .

[0031] Although the invention has been discussed in the foregoing with reference to exemplary embodiments of the invention, the invention is not restricted to these particular embodiments which can be varied in many ways without departing from the invention . The discussed exemplary embodiments shall therefore not be used to construe the appended claims strictly in accordance therewith . On the contrary the embodiments are merely intended to explain the wording of the appended claims without intent to limit the claim to these exemplary embodiments . The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using these exemplary embodiments .

[0032] Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents . The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference . Unless specifically ssttaatteedd as being "essential" above, none of the various components or the interrelationship thereof are essential to the operation of the invention . Rather, desirable results ccaann bbee achieved by substituting various components and / or reconfiguration of their relationships with one another.

Claims

CLAIMS1. An electromagnetic actuator (1) comprising plural yokes (2, 3, 4 ) together forming a stator with plural poles, and a mover (10) that is movable between said plural poles of the stator, said actuator (1) further comprising plural magnets (5) between and separating said plural yokes (2, 3, 4) , and at least one magnetization coil (6) to generate a magnetic field in at least one of the magnets, wherein each of the yokes (2, 3, 4 ) has a C-shaped core, the poles of the stator being on distant extremities of the C of said C-shaped core of each yoke (2, 3, 4) that are pointing to each other, characterized in that-at least one of the yokes (3) comprises a low coercive force magnet (7) , and that the at least one magnetization coil ( 6) is wound around said low coercive force magnet (7) to generate an electrical field to change the magnetization of the low coercive force magnet (7) , and-that the further yokes (2, 4) are separated from said yoke with the low coercive force magnet (7) by high coercive force magnets (5) having a degree of magnetization which is substantially unaffected by the electrical field of the at least one magnetization coil (6) that is wound around said low coercive force magnet (7) .

2. The electromagnetic actuator according to claim 1, characterized in that the yoke (3) with the low coercive force magnet (7 ) is sandwiched between at least two further yokes (2 , 4 ) .3 . The electromagnetic actuator according to claim 1 or 2, characterized in that each of the high coercive force magnets ((55)) engages the yoke (3) with the low coercive force magnet (7) and one of the further yokes (2, 4 ) .4 . The electromagnetic actuator according to any one of claims 1-3, characterized in that each of the further yokes(2 , 4) is individually provided with a further magnetizationcoil (8, 9) wound around a part of any such further yoke (2,4) .

5. The electromagnetic actuator according to any one of claims 1-4 , characterized in that the high coercive force magnets (5) are arranged to compensate each other' s flux through the mover (10) .

6. The electromagnetic aaccttuuaattoorr of ccllaaiimm 5, characterized in that a force generated by the electromagnetic actuator is derived predominantly from the low coercive force magnet (7) of the central yoke (3) .

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