This application is a U.S. National Stage of International Application No. PCT/EP2020/053523, filed Feb. 12, 2020, which claims the benefit of European Patent Application No. 19159991.9, filed Feb. 28, 2019, both of which are incorporated herein by reference in their entireties.
The present invention relates in one aspect to a motor for an electrodynamic loudspeaker and in another aspect to an electrodynamic loudspeaker comprising the motor. The present invention relates in a first aspect to a motor for an electrodynamic loudspeaker, comprising a magnetic circuit assembly arranged about a motor axis. The magnetic circuit assembly comprises: an outer magnet, a magnetically permeable top plate, a magnetically permeable bottom plate, a center pole piece and an air gap for receipt of a voice coil. The magnetic circuit assembly additionally comprises an outwardly projecting magnetically permeable member arranged above the top of the air gap. The center pole piece comprises a magnetic member extending axially from at least the bottom of the air gap to a magnetically permeable bottom member or to the magnetically permeable bottom plate. The magnetic member exhibits a relative AC magnetic permeability smaller than 10, such as smaller than 5 or smaller than 2, such as about 1 which corresponds to the relative AC magnetic permeability of free air.
An electrodynamic loudspeaker has a motor that converts electrical energy into mechanical motion. The most common operating principle is a moving-coil, wherein an electrical input or drive current flows in a voice coil of the electrodynamic loudspeaker. The voice coil is suspended in a permanent magnetic field with a strong radial component. The drive current through the voice coil and the radial magnetic field produce a so-called Lorentz force along an axis of the voice coil. The voice coil is typically rigidly attached to a diaphragm or membrane of the electrodynamic loudspeaker. The Lorentz force thereby displaces the diaphragm in an outwardly and inwardly based motion to create sound pressure.
The Lorentz force or drive force on the diaphragm is the product of the drive current I, flux density B in the air gap and a length of the wire I that is inside the radial magnetic field. More accurately, it is I times the integral of the radial component of B over the length of the wire of the voice coil.
This integral is often designated as the BI product or force factor of the motor. The motor accordingly transduces (converts) energy in both directions between electrical and mechanical domains. Consequently, the motor also acts as a dynamo so that mechanical motion produces electrical energy. The magnetic field induces a voltage (EMF) in the voice coil being proportional to a velocity of the voice coil and the diaphragm assembly. The proportionality factor is again the force factor. Practical motors of electrodynamic loudspeakers possess several pronounced non-linear mechanisms which produces undesired linear and non-linear distortions in the generated sound pressure.
One non-linear distortion mechanism is caused by a position/displacement-dependent variation of the BI-product such that the B*I product varies with the position of the voice coil in the magnetic gap. The force factor falls off gradually from a maximum that is typically found at the rest position of the voice coil at zero drive current in the voice coil. This first non-linearity distortion mechanism is static, i.e. only depends on the position of the voice coil.
Another dynamic non-linear distortion mechanism also exists. The drive current in the voice coil creates its own magnetic field in response to the flow of current. Part of the generated magnetic field by the voice coil circulates through the magnetic circuit, i.e. the voice coil behaves as a cored inductor with the magnetic circuit acting as the core. The magnetic flux generated by the voice coil current is superimposed on the permanent magnetic flux in the magnetic gap such that the magnetic flux in the magnetic gap varies with coil current in an undesirable manner.
The force on the voice coil and diagram is no longer strictly proportional to the voice coil current, i.e. drive current, since the force factor itself has become dependent on the voice coil current. This effect depends on the position of the voice coil but non-linearity exists because of the superposition of the two magnetic fields, not because of the movability of the voice coil. Depending on how the problem is described, force factor modulation is also known as position-dependent inductance, flux modulation and reluctance force. It is described in detail in AES Paper “Force Factor Modulation in Electro Dynamic Loudspeakers” presented at 141st Convection 2016 Sep. 29-Oct. 3.
The force-factor modulation causes a 2nd order non-linear distortion in the form of a force component proportional to the voice coil current squared:
where L is the position dependent generalized inductance of the coil as defined in the subject AES paper, x is the coil position and i is the coil current.
In other words, the 2nd order non-linear distortion is proportional to the voice coil current squared and the spatial derivative of the coil inductance. The variable voice coil inductance also produces distortion in yet another way. The voice coil inductance is part of the electrical impedance of the voice coil such that when it is driven by a voltage source (as it is in the vast majority of cases), the voice coil current becomes dependent on the applied drive voltage in a position dependent manner. It is shown in the above-mentioned 2016 AES paper that the equation for the non-linear component of the force can be generalized to include frequency dependency of the voice coil inductance. As mentioned before, the magnetic circuit acts as a core for the voice coil which means that the voice coil inductance becomes frequency dependent when the permeability of parts of the magnetic circuit is frequency dependent.
The cause of frequency dependent permeability is the introduction of eddy currents which flow in all parts or member of the magnetic circuit or system that are electrically conductive, such as iron parts, when the voice coil magnetic flux changes, either because current changes or because the coil moves. The eddy currents will flow in such a manner as to counteract changes in magnetic flux (Lenz's law)—or stated alternatively, the eddy currents act as shorted coil turns which reduce the inductance of the voice coil.
Because the conductivity of the materials in which those eddy current flows is finite, the current will die down when the coil flux remains static for some time, i.e. there are no eddy currents to counteract the inductance at DC, or 0 Hz, and at very low frequencies. Consequently, the voice coil inductance at DC is solely determined by the geometry and permeability of the materials of the magnetic circuit. At higher frequencies the eddy currents become more pronounced so as to reduce inductance below that found at DC.
Certain prior art electrodynamic loudspeakers have included so-called shorting rings around the pole piece and the voice coil. These rings are made of an electrically conductive, but non-magnetic material, such as copper or aluminum. The aim is to reduce the voice coil inductance, at least at higher frequencies. Thanks to the lower resistivity of copper or aluminum compared to iron, most of the eddy currents flow in the shorting rings instead of in the iron. For the same reason the eddy currents are also larger, and therefore more strongly counteract the magnetic field variation that the voice coil tries to induce or create in the magnetic circuit. This reduces force factor modulation, at least at higher frequencies. Further side benefits include reduced inductance, meaning higher sensitivity for a given voltage applied across the voice coil, and a reduction in the non-linear inductance caused by magnetic hysteresis in the iron. That does not mean that a shorting ring, however placed, will unconditionally improve linearity. Since force factor modulation is equivalent to a position dependency of the generalized voice coil inductance, it is quite possible, at elevated frequencies, to reduce the inductance whilst at the same time driving up the spatial gradient of that inductance (absolute change per millimeter of motion). At low frequencies, the prior art shorting rings have no effect. The lower the frequency at which an effect is desired, the greater the section of the shorting ring has to be at lower frequencies that section becomes too large for the amount of space available inside the magnetic circuit of a practical loudspeaker.
The present inventors have realized that if the motor and magnetic circuit of an electrodynamic loudspeaker is designed, or configured, such that the voice coil inductance is displacement/position-independent, then non-linear distortion due to the force factor modulation and non-linear distortion due to voice coil current modulation are both eliminated. Therefore, an ideal motor for an electrodynamic loudspeaker exhibits a voice coil inductance that does not change with displacement of the voice coil, i.e. it is position independent.
Consequently, one aim or objective of the present invention is to provide an electrodynamic loudspeaker motor which substantially eliminates the harmful displacement dependency of the voice coil inductance or at least markedly reduces displacement/position dependency of the voice coil inductance compared to prior art loudspeaker motors. This reduction will improve linearity of the motor and thereby reduce several types of non-linear distortion of the electrodynamic loudspeaker for the reasons described above. Thus, improving the objective and subjective sound quality of the loudspeaker.
A first aspect of the invention relates to a motor for an electrodynamic loudspeaker, comprising: a magnetic circuit assembly arranged about a motor axis. The magnetic circuit assembly may comprise: an outer magnet, a magnetically permeable top plate, a magnetically permeable bottom plate, a center pole piece and an air gap for receipt of a voice coil. The air gap is formed by an inner axially extending wall of the magnetically permeable top plate facing an axially extending peripheral wall section of the center pole piece to define a width, a bottom, a top and height of the air gap. The magnetic circuit assembly additionally comprises outwardly projecting magnetically permeable member arranged above the top of the air gap. The center pole piece comprises a magnetic member extending axially from at least the bottom of the air gap to a magnetically permeable bottom member or to the magnetically permeable bottom plate. The magnetic member exhibits a relative AC magnetic permeability smaller than 10, such as smaller than 5 or smaller than 2, such as about 1 which corresponds to the relative AC magnetic permeability of free air.
In the present specification, the term “AC magnetic permeability” of the magnetic member refers to a slope of a tangent of a curve/plot of flux density, B, versus magnetic field strength, H, at zero voice coil current. The term “relative AC magnetic permeability”, μr, refers to the “AC magnetic permeability” expressed as a multiple of the magnetic vacuum permeability μ0. The tangent can be viewed as a linearized small signal, or AC, model around a DC operating point of the magnetic member. The slope of the tangent is the permeability of the small-signal model of the magnetic member, i.e. the “AC magnetic permeability” of the magnetic member. At large magnetic field strengths, for example above 1.5 Tesla, this B-H curve becomes flatter, meaning that the AC magnetic permeability decreases as the material of the magnetic member saturates. A permanent magnet is by nature highly magnetically saturated and therefore typically possesses an AC magnetic permeability that is not much larger than that of air. Neodymium magnets may exhibit a relative AC magnetic permeability below 1.5 or below 1.1.
Hence, the small AC magnetic permeability of the magnetic member in combination with the outwardly projecting magnetically permeable member provides a synergistic effect by markedly reducing the increase of the voice coil inductance at inwards displacements of the voice coil, and additionally compensating a small residual voice coil inductance increase by the arrangement of the outwardly projecting magnetically permeable member above the top of the air gap. This geometry ensures that the voice coil inductance also increases at outwards displacement of the voice coil in nearly the same proportion as the inductance increases at inwards displacement of the voice coil, hence making the displacement dependent variation of inductance of the voice coil extremely small as discussed in additional detail below with reference to the appended drawings.
The magnetic member of the center pole piece may comprise a permanent magnet such as a Neodymium magnet or a Ferrite magnet which by nature are highly magnetically saturated as discussed above. Alternatively, the magnetic member of the center pole piece may comprise magnetically permeable material, such as an isotropic, high resistive Soft Magnetic Composite (SMC) material, driven into DC magnetic saturation by at least one of: a permanent magnet and a field coil.
The outwardly projecting magnetically permeable member may generally be arranged inside, or outside, an outwardly projecting plane or surface defined by the axially extending peripheral wall section of the center pole piece as discussed in additional detail below with reference to the appended drawings—for example in connection with the motor embodiments of
In one embodiment of the motor, the center pole piece comprises a magnetically permeable top member which is extending axially from the bottom of the air gap to the top of the air gap and thereby forms or defines the axially extending peripheral wall section of the center pole piece. The outwardly projecting magnetically permeable member may be arranged on top of the magnetically permeable top member and either integrally formed therewith or provided as a separate element bonded or abutted to a top surface of the magnetically permeable top member as discussed in additional detail below with reference to the appended drawings. The magnetically permeable top member and/or the outwardly projecting magnetically permeable member may formed by or comprise, a highly permeable material, e.g. a ferromagnetic material such as AISI CR1010 steel or an isotropic, high resistive Soft Magnetic Composite (SMC) material discussed in additional detail below with reference to the appended drawings.
According to one embodiment of the motor, the outer magnet comprises an annular permanent magnet co-axially arranged around a cylindrical center pole piece centered about the motor axis.
According to another embodiment of the motor, a height of the outwardly projecting magnetically permeable member exceeds a height of the magnetically permeable top plate for example 1.5 times the height of the magnetically permeable top plate.
According to another embodiment of the motor, a height of the magnetic member of the center pole piece is larger than a difference between a height of the voice coil and the height of the air gap.
Additional embodiments of the invention are set out in the below-appended dependent patent claims.
A second aspect of the invention relates to an electrodynamic loudspeaker comprising:
The magnetic circuit assembly of the electrodynamic loudspeaker is preferably configured such that a variation of inductance of the voice coil over a predetermined displacement range of the voice coil defined by an outward displacement limit and an inward displacement limit is less than 10%, such as less than 7.5%, or even less than 5%, measured at 31 Hz; wherein said displacement range corresponds to 0.5 times a difference between a height of the voice coil and a height of the air gap. The skilled person will appreciate that the outward and inward displacement limits may be symmetrical about a rest or neutral position of the voice coil. The magnetic circuit assembly of the electrodynamic loudspeaker is preferably configured such that also the variation of inductance of the voice coil over the predetermined displacement range falls within the same percentage limits at one or more additional test frequencies selected from a group of: 1 Hz, 100 Hz, 316 Hz, 1 kHz, and 3.16 kHz.
Preferred embodiments of the invention are described below in additional detail in connection with the appended drawings, in which:
In the following, various exemplary embodiments of the present motor for an electrodynamic loudspeaker are described with reference to the appended drawings. The skilled person will understand that the accompanying drawings are schematic and simplified for clarity and therefore merely show details which are essential to the understanding of the invention, while other details have been left out. Similar reference numerals refer to like elements or components throughout the application. Similar elements or components will therefore not necessarily be described in-detail with respect to each figure. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to the described sequence is not actually required.
The motor 200 may be rotationally symmetrical about a central motor axis 205 of the motor 200. The motor 200 comprises a magnetic circuit assembly which is configured to generate a radially oriented essentially static magnetic field in a ring-shaped air gap 220. The magnetic circuit assembly comprises an outer annular permanent magnet 240, a magnetically permeable top plate 235, e.g. formed as an annular disc, a magnetically permeable bottom plate or yoke 230 and a center pole piece 245. The air gap 220 is configured for receipt of a ring-shaped or annular voice coil 225, which may form part of a diaphragm assembly of the electrodynamic loudspeaker. The annular or ring-shaped voice coil 225 is suspended freely in the ring-shaped air gap 220 and therefore displaceable along the central motor axis 205 outwardly away from the magnetic circuit assembly and inwardly into the magnetic circuit assembly about a rest position 0 of the voice coil. The rest position corresponds to DC zero current in the ring-shaped voice coil 225, and preferably corresponds to a centered position of the ring-shaped voice coil 225 in the air gap 220. The rest position of the ring-shaped voice coil 225 is schematically indicated by “0” on the “X” arrow of the drawing, while the outward displacement of the voice coil 225 away from the magnetic circuit assembly corresponds to positive/+ direction of X, and inward displacement of the voice coil 225 into the magnetic circuit assembly corresponds to negative/− direction of X.
The magnetically permeable top plate 235 may be formed from a highly permeable material, e.g. a ferromagnetic material such as CR1010 steel and a have height between one-sixth and two-thirds of the height of the ring shaped voice coil 225. The magnetically permeable bottom plate or yoke 230 may be formed from a highly permeable material, e.g. a ferromagnetic material such as AISI CR1010 steel, and a have height or thickness between 4 mm and 16 mm depending on the outer dimensions of the motor 200.
The center pole piece or center pole assembly comprises a magnetic member 250 which extends from a bottom 220b of the air gap 220 to a magnetically permeable bottom member which may be formed as an upwardly projecting cylindrical protrusion integrally formed with the magnetically permeable bottom plate or yoke 230. The magnetically permeable bottom member is physically and magnetically coupled to the lower surface of the magnetic member 250. Hence, the magnetic member 250 in the present embodiment of the motor 200 is arranged in-between a magnetically permeable pole top 210, which may be a flat disc, and the magnetically permeable bottom member. In other embodiments of the magnetic circuit assembly, the magnetic member 250 may extend axially all the way from the bottom 220b of the air gap 220 to the magnetically permeable bottom plate or yoke 230. The height of the magnetic member 250 is preferably at least 0.5 times the height of the annular permanent magnet 240, for example more than 0.7 times, or 0.9 times the height of the annular permanent magnet 240. Alternatively, or additionally, the height of the magnetic member 250 is larger than a difference between a height of the voice coil and the height of the air gap 220. Each of these limitations will typically ensure that the height of the magnetic member 250 is sufficiently large to markedly reduce the inductance of the voice coil at inward displacements because of the reduction of the amount of magnetically permeable material inside the voice coil.
The magnetically permeable pole top 210 is extending axially from a bottom 220b of the air gap 220 (refer to
The magnetically permeable pole top 210 comprises an outwardly projecting portion or protrusion 215 or “hat” 215 arranged above, i.e. outwardly of, the top 220a of the air gap 220. Hence, in the present embodiment, the outwardly projecting portion or protrusion 215 is also arranged above an upper flat surface 237 of the magnetically permeable top plate 235. The outwardly projecting “hat” 215 is arranged inside, i.e. towards the central motor axis 205, an outwardly projecting plane or surface (not shown) defined by the axially extending peripheral wall section 217 of the center pole piece 245. Hence, allowing unrestricted axial displacement of the voice coil 225.
The magnetically permeable pole top 210 may therefore comprise a first cylindrical portion or section 212 that defines the above-discussed inner wall (axially extending peripheral wall section) 236 of the air gap 220. The magnetically permeable pole top 210 of the center pole piece 245 additionally comprises the above-mentioned outwardly projecting protrusion 215, which in the present embodiment is formed by a second cylindrical portion of the magnetically permeable pole top 210, arranged on top of the first cylindrical portion 212 and either integrally formed therewith or provided as a separate element bonded or abutted to a top surface of the first cylindrical portion 212. The skilled person will appreciate that the outwardly projecting protrusion 215 need not be cylindrical. The first and second cylindrical portions 212, 215, respectively, of the magnetically permeable pole top 210 may be integrally formed—for example by milling or machining a suitably shaped cylindrical Ferrite member or other highly magnetically permeable material such as AISI CR1010 steel or an isotropic, high resistive Soft Magnetic Composite (SMC) material like Somaloy® material such as Somaloy 1P, Somaloy 3P or Somaloy 5P manufactured and sold by Höganas AB. A cross-sectional area of the second cylindrical portion 215 may be smaller than a cross-sectional area of the first cylindrical portion or section 212 to define a recessed upper outer circular wall 215a relative to the inner wall 236 of the magnetically permeable pole top 210, which defines the inner surface or inner wall 236 of the magnetic gap 220. In other words, the outwardly projecting protrusion 215 extends outwards above the magnetic gap 220 in the axial direction 205 of the motor 200.
In certain alternative embodiments, the first and the second cylindrical portions 212, 215, respectively, may have identical diameters to eliminate the recessed properties of the upper outer circular wall 215a.
The magnetic member 250 may exhibit a relative AC magnetic permeability smaller than 10, such as smaller than 5, or smaller than 2. In certain embodiments, the magnetic member 250 comprises, or is formed by, a permanent magnet such as a Neodymium magnet or a Ferrite magnet. In other embodiments of the motor 200 as discussed in additional detail below, the magnetic member 250 comprises a magnetically permeable material, for example an isotropic, high resistive Soft Magnetic Composite (SMC) material, which material is driven into DC magnetic saturation by at least one of: a permanent magnet and a field coil. The SMC material may comprise the above-discussed Somaloy® material.
Each of the outer annular permanent magnets 240 and the magnetic member 250 are axially magnetized as schematically illustrated by the magnetic field lines, which are used to drive magnetic flux through the magnetic circuit assembly and across the air gap which therefore carries a radially oriented magnetic field. The outer annular permanent magnets 240 may comprise a Ferrite magnet or Neodymium magnet.
The arrangement of the magnetically permeable outwardly projecting protrusion or hat 215 increases the inductance of the voice coil 225 at outwards displacement, i.e. positive “X” values, of the voice coil 225, such that the increase of inductance is effectively counteracting, or compensating for, the increased inductance of the voice coil 225 at inwards displacements thereof.
The reduced cross-sectional area of the magnetically permeable hat 215 directs the DC magnetic flux, i.e. static DC magnetic flux, of the magnetic circuit assembly to flow in the air gap 220. This feature ensures that the DC magnetic flux is focused in the air gap 220 and that the magnetic field strength is low in the magnetically permeable hat 215. This feature in turn ensures that the magnetically permeable hat 215 is kept out of magnetic saturation leading to a high permeability and a more effective compensation of the displacement dependent inductance L(x) of the voice coil 225.
In contrast, the magnetic member 250 which is arranged below the bottom 220b of the air gap 220, e.g. having an upper end surface substantially aligned with the bottom 220b of the air gap 220, preferably exhibits or possesses a small relative AC magnetic permeability as specified above in order to reduce the displacement dependency of the voice coil inductance. The small AC relative magnetic permeability can be achieved in several ways, for example by means of high DC or static magnetic saturation e.g. by the use of a permanent magnet or using a soft magnetic material such as ferromagnetic material driven into DC saturation by a permanent magnet or field coil as explained below. In both cases the AC relative magnetic permeability may be very small, e.g. below 10 or below 5.
The above-mentioned increase of the voice coil inductance at inwards displacements of the voice coil 225 is caused at one hand by the reduced distance from the voice coil 225 to the magnetically permeable bottom plate or yoke 230 including the upwardly projecting cylindrical projection. Another significant contribution to the increase of voice coil inductance in prior art motor designs at inwards displacements of the voice coil 225 is the high magnetic permeability of ferromagnetic material of the center pole piece.
The skilled person will appreciate that the combined properties of the magnetic member 250 and the magnetically permeable hat 215 largely eliminate, or at least markedly reduce, this undesired increase of the voice coil inductance at inwards displacements of the voice coil 225 of the present motor 200. The small AC relative magnetic permeability of the magnetic member 250, which in some embodiments may be comparable to free air, i.e. μr=1.0, at least reduces the presence of magnetically permeable material inside the voice coil 225 at inwards displacements. The voice coil inductance may still be at its maximum when the voice coil 225 is fully drawn inwards, because the magnetically permeable top member 210 and yoke 230 still help to shorten the magnetic field lines compared to free air. Crucially though, that voice coil inductance is markedly reduced compared to the design with the magnetically permeable center pole piece near to the coil.
Hence, the magnetic member 250 and the magnetically permeable hat 215 provide a synergistic effect by firstly markedly reduce the voice coil inductance at inward displacements of the voice coil 225 by the magnetic member 250, and in addition compensate the small residual voice coil inductance increase at inward displacements by the arrangement of the magnetically permeable hat 215 above the top of the air gap 220, such that the voice coil inductance also increases at outwards displacement of the voice coil 225. In other words, to combine the magnetically permeable hat 215 with the magnetic member 250 in the center pole piece 245 which thanks to its low AC magnetic permeability, makes it amenable for precisely this purpose.
The center pole piece of the present magnetic circuit additionally comprises a magnetically permeable pole top 710 which conducts and directs magnetic flux radially through the air gap 725. The magnetically permeable pole top 710 is preferably integrally formed with an outwardly projecting, and recessed, portion or protrusion 715 or “hat” arranged above, i.e. outwardly of, the top of the air gap 720 in a similar manner as the first embodiment of the invention discussed above.
A first plot 1010 represents the simulated inductance of the motor design 200 which includes the magnetically permeable outwardly projecting protrusion or “hat” 215 arranged above, i.e. outwardly of, the top 220a of the air gap 220. A second plot 1020 represents the simulated inductance of the same motor design 200, but without the magnetically permeable “hat” 215.
As evident from first plot 1010 for a peak-peak displacement range of 10 mm of the voice coil about the rest position (x=0), the inductance variation of the voice coil 225 is merely about 0.06 mH/2.45 mH=2.5%. Furthermore, this level of performance is also achieved at somewhat higher frequencies such as 31 Hz. The 10 mm displacement range corresponds to about 0.5 times the difference between the height of the voice coil 225 and the height of the air gap 220 for the present motor design. As evident from second plot 1020 without the “hat” for the same peak-peak displacement range of 10 mm about the rest position (X=0), the inductance variation of the voice coil is much larger and about 0.25 mH/2.2 mH=11%.
The series of plots of the voice coil inductance includes a first plot 1110 simulated at 31 Hz, a second plot 1120 simulated at 100 Hz, a third plot 1130 simulated at 316 Hz, a fourth plot 1140 simulated at 1 kHz and a fifth plot 1150 simulated at 3.16 kHz. As evident from each of these voice coil inductance plots, the variation of the inductance of the voice coil is very small for all test frequencies. For example, at 31 Hz the inductance variation is about 2-3% for a peak-peak displacement range of 10 mm of the voice coil 225 about the rest position (x=0). Furthermore, a substantially similar level of performance is also achieved at the higher frequencies such as at 316 Hz, 1 kHz and 3.16 kHz.
Number | Date | Country | Kind |
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19159991 | Feb 2019 | EP | regional |
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PCT/EP2020/053523 | 2/12/2020 | WO |
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WO2020/173699 | 9/3/2020 | WO | A |
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