Title of the invention: Magnetic bearing for a rotating machine, associated power supply method and associated rotating machine

[1] The present invention relates to a magnetic bearing of a rotating machine, a rotating machine and a method for powering such a magnetic bearing.

[2] The rotating machines with magnetic bearings, notably the turbomolecular vacuum pumps of the state of the art, comprise a rotor rotating generally at very high speed in a stator by being supported and guided in levitation by magnetic bearings. These magnetic bearings comprise coils forming electromagnets to create magnetic fields that make it possible to frictionlessly guide the rotor of the rotating machine.

[3] The coils are generally arranged in pairs on a bearing, on either side of the rotor, one pair of coils making it possible to guide the rotor in a given direction corresponding to the direction linking the two coils of the pair. A bearing generally comprises at least two pairs of coils arranged at 90° to one another to make it possible to guide the rotor in a radial plane. An additional pair of coils is generally used to guide the rotor in the axial direction.

[4] In order to keep the rotor in position, the currents circulating in the coils of the pair and defining the intensity of the magnetic fields created are servocontrolled as a function of the position of the rotor. The position of the rotor is, for example, measured by dedicated sensors situated on the bearings, in proximity to the coils.

[5] Figure 1 represents a diagram of a pair of coils 5, 5’ of a magnetic bearing and of an exemplary embodiment of the power supply device of a coil 5, 5’.

[6] The power supply device comprises, for each coil 5, 5’, a half-bridge powered by a DC voltage source V and comprising two transistors T in series whose midpoint is linked to a first terminal of the coil 5, 5’, the transistors T of the halfbridge are driven in phase-opposition with a constant duty cycle of 50% . The second terminal of the coil 5, 5’ is linked on the one hand to the DC voltage source V via a diode D and on the other hand to the ground via a transistor TO controlled by a regulation loop BR configured to servocontrol the current in the coil 5, 5’ as a function of the position of the rotor 7. The current setpoints for the first 5 and the second 5’ coils of the pair are the superimposition of a so-called idle current 10 and of a current delta Ai calculated as a function of the position of the rotor. This current delta Ai is added in one of the coils 5, 5’ and subtracted in the other so as to servocontrol the position of the rotor.

[7] However, such a setup can present drawbacks. In fact, the pulsed voltages applied to the terminals of the coils tend to create current ripples which can induce losses and vibrations of the rotor which can be detrimental notably when the rotating machine is a vacuum pump used for pumping chambers in which nanometric etching methods requiring great accuracy and a very stable environment take place. Furthermore, the significant and rapid voltage variations on the coils and their power supply cables can provoke an electromagnetic radiation and induce disturbances, notably to the position sensors of the rotor situated in proximity to the coils if costly and expensive electromagnetic shielding measures are not taken.

[8] A solution therefore needs to be found that makes it possible to at least partially overcome the drawbacks of the state of the art and that makes it possible to reduce the vibrations of the rotor and to reduce the electromagnetic radiation of the coils and of their power supply cables.

[9] To this end, the subject of the invention is a magnetic bearing of a rotating machine comprising a rotor, the magnetic bearing comprising at least one pair of coils to ensure the positioning of the rotor in a given direction, wherein the first coil and the second coil of the pair are positioned respectively in a first branch and a second branch of an electrical power supply circuit of the magnetic bearing that are parallel, the two branches being linked to a common DC voltage source and wherein the first branch comprises a first voltage regulator associated with the first coil and the second branch comprises a second voltage regulator associated with the second coil, the electrical power supply circuit also comprising a control unit of the first and of the second voltage regulators, said control unit comprising two first feedback loops configured to servocontrol the current circulating respectively in the first and in the second coils as a function of a measured position of the rotor and two second feedback loops configured to servocontrol the voltage respectively at the terminals of the first and of the second coils as a function of the currents in the first and in the second coils. The first and the second loops are nested and make it possible to adjust the voltage at the terminals of the coils so that the current best follows the setpoint value.

[10] The use of first feedback loops configured to servocontrol the current circulating in the coils as a function of the position of the rotor and of second feedback loops configured to servocontrol the voltage at the terminals of the coils as a function of the current in the coils makes it possible to apply a DC voltage in the coils while ensuring that the rotor is kept in position which makes it possible to reduce the vibrations produced by the power supply of the magnetic bearing and to reduce the electromagnetic radiation of the power supply cables of the coils of the bearing.

[11] According to another aspect of the present invention, the first and second feedback loops of the control unit comprise:

- a first processing unit configured to determine a first current setpoint to be applied to the coils of a pair as a function of the measured position of the rotor and, for the first coil and the second coil of the pair:

- a second processing unit configured to compare the current setpoint determined by the first processing unit with the current circulating in the associated coil and to determine a voltage setpoint as a function of the result of the comparison of the currents,

- a third processing unit configured to compare the voltage setpoint supplied by the second processing unit and the voltage at the terminals of the coil and to determine a second current setpoint as a function of the result of the comparison of the voltages,

- a fourth processing unit configured to regulate the current in the associated voltage regulator as a function of the second current setpoint supplied by the third processing unit.

[12] According to another aspect of the present invention, the first voltage regulator and/or the second voltage regulator are voltage step-up circuits whose input is linked to the associated coil.

[13] According to another aspect of the present invention, the first voltage regulator and/or the second voltage regulator are voltage step-down circuits whose output is linked to the associated coil. [14] According to another aspect of the present invention, the first voltage regulator is a voltage step-up circuit whose input is linked to the first coil and the second voltage regulator is a voltage step-down circuit whose output is linked to the associated coil.

[15] According to another aspect of the present invention, the magnetic bearing also comprises at least one inductive position sensor configured to measure a position of the rotor in a given direction.

[16] According to another aspect of the present invention, the components of the electrical power supply circuit are arranged on an electronic circuit board remote from the coils and linked to the coils via electrical power supply cables.

[17] The present invention relates also to a rotating machine comprising a first and a second magnetic bearing as described previously, wherein the rotating machine comprises five pairs of coils, a first and a second pair of coils arranged on the first magnetic bearing in a first and a second right-angled radial direction, a third and a fourth pair of coils arranged on the second magnetic bearing in a first and a second right-angled radial direction and a fifth pair of coils in an axial direction of the rotor.

[18] According to another aspect of the present invention, the rotating machine is a turbomolecular vacuum pump.

[19] The present invention relates also to a method for powering a magnetic bearing of a rotating machine comprising a rotor, the magnetic bearing comprising at least one pair of coils, said coils of the pair being arranged on either side of the rotor and being powered by a common DC voltage source and wherein, on the one hand, the current in the first and in the second coils of the pair is servocontrolled as a function of a measured position of the rotor to keep the rotor centred with respect to the first and to the second coils and, on the other hand, the voltage respectively at the terminals of the first and of the second coils of the pair is servocontrolled as a function of the currents in the first and in the second coils.

[20] Other features and advantages of the invention will become more clearly apparent on reading the following description, given as an illustrative and nonlimiting example, and the attached drawings in which:

[21] [Fig 1] represents a schematic view of a power supply of a bearing according to the state of the art; [22] [Fig.2] represents a perspective schematic view of a magnetic bearing;

[23] [Fig.3] represents a schematic view of a power supply circuit of a pair of coils of a magnetic bearing according to a first variant;

[24] [Fig.4] represents a schematic view of a power supply circuit of a pair of coils of a magnetic bearing according to a second variant;

[25] [Fig.5] represents a schematic view of a rotating machine comprising two magnetic bearings.

[26] In these figures, the elements that are identical bear the same references.

[27] The following embodiments are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment, or that the features apply only to a single embodiment. Simple features of different embodiments can also be combined or interchanged to provide other embodiments.

[28] The present invention relates to a magnetic bearing of a rotating machine. The rotating machine is, for example, a vacuum pump and notably a turbomolecular vacuum pump which can notably be used in the process chambers in the fabrication of semiconductors. Figure 2 represents a diagram of an exemplary embodiment of a magnetic bearing 1 of such a rotating machine. The magnetic bearing 1 comprises an electromagnet 3 comprising, for example, two pairs of coils 5, 5’ distributed around a rotor 7 of the rotating machine. The electromagnet 3 comprises, for example, an air gap 4 on which the coils 5, 5’ are arranged. A first pair of coils 5, 5’ is arranged in a first direction represented by an axis XI and a second pair of coils 5, 5’ is arranged in a second direction represented by an axis X2, the first and the second directions being at right angles to one another and at right angles to the axis X3 of rotation of the rotor. Thus, by controlling the intensity of the current in the two pairs of coils 5, 5’, it is possible to keep the rotor 7 in a desired radial position. The magnetic bearing

1 also comprises a rotor 7 position sensor 9, for example an inductive sensor which makes it possible to determine the position of the rotor 7 (in practice, a plurality of position sensors (arranged for example on the two bearings) are used to determine the position of the rotor 7). The magnetic bearing 1 also comprises a control unit 11 configured to make it possible to servocontrol the current in the two pairs of coils 5, 5’ as a function of the measurement supplied by the position sensor 9 (in practice, the position sensors 9). [29] Figure 5 schematically represents a rotating machine 100 which comprises two magnetic bearings 1 and 1 ’ arranged respectively at a first and a second end of the rotor 7. The rotor 7 is surrounded by a stator 6 configured to drive the rotor 7 in rotation. Each magnetic bearing 1 comprises a first pair of coils 5, 5’ on an axis XI, a second pair of coils 5, 5’ on an axis X2 at right angles to the axis XI and an additional pair of coils 5, 5’ is used to control the position of the rotor 7 in its axial direction (that is to say on the axis X3). Position sensors 9, five of them, are arranged on the magnetic bearings 1 and 1’, for example two position sensors 9 following two directions at right angles to each magnetic bearing 1, 1’ to measure the radial displacements of the rotor 7 and an additional position sensor 9 to measure the axial position of the rotor 7. The position of the rotor 7 can thus be determined by processing means such as a microprocessor or a microcontroller of the processing unit 11 from the measurements of all the position sensors 9 and this measured position can then be used to determine the current setpoint to be applied to the coils 5, 5’. Alternatively, a different number of pairs of coils can be used, for example a number of pairs greater than two for each bearing 1, 1’. A different number of position sensors 9 can also be used.

[30] Figure 3 represents an exemplary embodiment of an electrical power supply circuit 13 of a magnetic bearing 1 associated with a pair of coils 5, 5’. A similar electrical circuit 13 can thus be associated with each pair of coils 5, 5’ of the rotating machine. The electrical circuit 13 can comprise a printed circuit or electronic circuit board notably comprising the control unit 11, the printed circuit being linked to the pairs of coils 5, 5’ and to the position sensor 9 via electrical cables. The electrical circuit 13 comprises a DC voltage source Vb _us /2 which can be a voltage source derived from a DC voltage source Vb _us of the rotating machine 100. The electrical circuit 13 also comprises two branches Bl and B2 that are parallel and linked at a first end to the common DC voltage source Vb _us /2 and at the second end to a DC voltage source Vb _us • The first branch Bl comprises the first coil 5 of the pair in which a current Ibl circulates and a first voltage regulator RV1 associated with the first coil 5. The second branch B2 comprises the second coil 5’ of the pair in which a current Ib2 circulates and a second voltage regulator RV2 associated with the second coil 5’.

[31] In the example of Figure 3, the voltage regulators are voltage step-up circuits (also called boost type convertors), the input of which is linked to the associated coil 5, 5’. The voltage step-up circuit corresponding to the first voltage regulator RV1 comprises a capacitor Cl arranged between an input point el of the first voltage regulator RV1 and the ground, an inductor LI arranged in series with a diode DI between the input point el and an output point si of the first voltage regulator RV1 and a transistor T1 arranged between the ground and a midpoint ml arranged between the inductor LI and the diode DI. The control of the switching off and of the switching on of the transistor T1 makes it possible to control the current Ibl circulating in the first coil 5 of the pair.

[32] Likewise, the voltage step-up circuit corresponding to the second voltage regulator RV2 comprises a capacitor C2 arranged between an input point e2 of the second voltage regulator RV2 and the ground, an inductor L2 arranged in series with a diode D2 between the input point e2 and an output point s2 of the second voltage regulator RV2 and a transistor T2 arranged between the ground and a midpoint m2 arranged between the inductor L2 and the diode D2. The control of the switching off and of the switching on of the transistor T2 makes it possible to control the current circulating in the second coil 5’ of the pair.

[33] The output points si and s2 are, for example, linked to the DC voltage source Vbus-

[34] Other types of voltage regulators can also be used such as, for example, voltage step-down circuits (also called Buck type convertors) or any other type of voltage regulators that make it possible to control the current circulating in an associated coil 5, 5’.

[35] The electrical power supply circuit 13 also comprises a control unit 11 of the first and of the second voltage regulators RV1 and RV2.

[36] The control unit 11 comprises two first feedback loops configured to servocontrol the current circulating respectively in the first coil 5 and in the second coil 5’ as a function of a measured position of the rotor 7 given by at least one position sensor 9.

[37] These first feedback loops comprise a first processing unit 15 configured to determine a first current setpoint Cil, Ci2 to be applied to the coils 5, 5’ of a pair as a function of the measured position of the rotor 7. The first processing unit 15 is configured to determine a current delta Ai corresponding to a correction to be added to a predefined current value 10 to provide a first current setpoint Cil associated with the first coil 5 and to be subtracted from the predefined current value 10 to provide a second current setpoint Ci2 associated with the second coil 5’ so as to obtain the desired position of the rotor (generally a position centred between the first 5 and the second 5’ coils of the pair). The sign of the current delta Ai therefore defines the direction in which the axis of the rotor 7 is pulled, towards the bearing in which most of the current passes, the coils 5, 5’ of the bearing being able only to "pull" towards them the rotor 7 which is not magnetized. The resulting force on the rotor 7 is therefore the composition of two directionally opposed forces, one proportional to (10 + Ai) ² and the other proportional to (10 - Ai) ² , which has the effect of linearizing the force applied to the current variation Ai and to 10. The predefined current value 10 is constant over time.

[38] The first feedback loops also comprise second processing units 17, 17’ configured to respectively compare the first current setpoint Ci 1 with the measured value of the current circulating in the first coil 5 and the second current setpoint Ci2 with the measured value of the current circulating in the second coil 5’. The second processing units 17, 17’ are also configured to determine a voltage setpoint Cvl, Cv2 as a function of the result of the comparison between the first current setpoint Cil, Ci2 and the measured value of the current circulating in the associated coil 5, 5’.

[39] The control unit 11 also comprises two second feedback loops nested in the first feedback loops and configured to servocontrol the voltage respectively at the terminals of the first 5 and of the second 5’ coils as a function of the currents Ibl, Ib2 in the first 5 and in the second 5’ coils.

[40] These second feedback loops comprise third processing units 19, 19’ configured to respectively compare the voltage setpoints Cvl, Cv2 supplied by the second processing units 17, 17’ and the voltage at the terminals of the coils 5, 5’ (in practice, the voltage measured at the input el, e2 of the voltage step-up circuit (or at the output s2’ in the case of a voltage step-down circuit) which reflects the voltage at the terminals of the associated coil 5, 5’) and to determine a second current setpoint Ciil, Cii2 as a function of the result of the comparison between the voltage setpoint Cvl, Cv2 and the voltages at the terminals of the coil 5, 5’.

[41] The control unit 11 also comprises fourth processing units 21, 21’ configured to respectively regulate the current in the first voltage regulator RV1 as a function of the second current setpoint Cii 1 supplied by the third processing unit 19 and the current in the second voltage regulator RV2 as a function of the second current setpoint Cii2 supplied by the third processing unit 19’. For that, the fourth processing units 21, 21’ compare the current circulating in the transistors Tl, T2 (and corresponding to the current circulating in the inductors LI, L2 when the transistors Tl, T2 are switched on) with the second current setpoint Ciil, Cii2.

[42] The first 15, the second 17, 17’, third 19, 19’ and fourth 21, 21’ processing units are, for example, produced by programming a microcontroller of the control unit 11.

[43] The second feedback loops are therefore nested in the first feedback loops to make it possible to servocontrol both the position of the rotor 7 with respect to the coils 5 and 5’ and the voltage at the terminals of the coils 5 and 5’.

[44] The voltage at the terminals of the coils 5, 5’ is therefore a DC voltage whose value can vary between -Vb _us /2 (when the voltage at the input el, e2 of the associated voltage regulators RV1, RV2 is Vb _us (considering the components ideal and disregarding the losses)) and Vb _us /2 (when the voltage at the input el, e2 of the associated voltage regulators RV1, RV2 is 0 (considering the components ideal and disregarding the losses)).

[45] Thus, the absence of switched-mode power supply (pulsed voltage) of the coils 5, 5’ as in the case of the prior art described in Figure 1 and the use of nested feedback loops to regulate both the value of the current in the first coil 5 and the second coil 5’ and the value of the voltage at the terminals of the first coil 5 and of the second coil 5’ make it possible to reduce, even eliminate, the current ripples at the coils 5, 5’ due notably to the switching of the power supply voltage. That induces a reduction, even an elimination, of the vibrations at the frequencies close to the switching frequency, such that the rotating machine 100 produces fewer vibrations and therefore disturbs its environment and any precision devices situated in proximity to the rotating machine less. Furthermore, the absence of current ripples in the coils 5, 5’ makes it possible not to be limited in the choice of the inductance values of these coils 5, 5’ in order to address these current ripples and therefore to be able to use lower inductance values for the inductances of the coils 5, 5’. [46] Moreover, the absence of abrupt voltage variations over time in the coils 5, 5’ makes it possible to greatly limit, even eliminate, the electromagnetic radiation of the coils 5, 5’ and of the electrical cables which link the coils 5, 5’ and the control unit 11. This absence of electromagnetic radiation makes it possible to notably avoid disturbing the inductive position sensors 9 situated in proximity to the coils 5, 5’.

[47] Figure 4 represents a variant embodiment which is differentiated from the preceding example by the fact that, here, the second branch B2’ comprises a second voltage regulator RV2’ corresponding to a voltage step-down circuit (also called Buck type convertor) whose input point e2’ is linked to the output point si of the first voltage regulator RV1 and whose output point s2’ is linked to the second coil 5’.

[48] The voltage step-down circuit corresponding to the second voltage regulator RV2’ comprises a transistor T2’ and an inductor L2’ arranged in series between the input e2’ and the output s2’. The voltage step-down circuit also comprises a diode D2’ arranged between the ground and a midpoint m2’ arranged between the transistor T2’ and the inductor L2’ and a capacitor C2’ arranged between the ground and the output point s2’. The control of the switching off and of the switching on of the transistor T2’ makes it possible to control the current circulating in the second coil 5’ of the pair.

[49] The other elements of the electrical power supply circuit 13 remain identical to the preceding variant embodiment, described on the basis of Figure 3. In this embodiment, the direction of the current in the second branch B2’ and notably in the second coil 5’ is opposite to the direction of the current in the second branch B2 of the variant embodiment of Figure 3. That makes it possible to greatly reduce the current Ivb supplied by the DC voltage source Vb _us /2- The current Ivb is in fact substantially equal to 2*Ai. That can be particularly advantageous if the same DC voltage source Vb _us /2 is used to power all the pairs of coils 5, 5’ of the rotating machine 100.

[50] Thus, the different variants of the power supply circuit 13 of a pair of coils 5, 5’ described previously can be duplicated to allow all the pairs of coils 5, 5’ of the rotating machine 100 to be powered and thus control the position of the rotor 7 accurately and by limiting the vibrations produced. [51] The present invention relates also to a rotating machine 100, notably a turbomolecular vacuum pump, with magnetic bearings 1, 1’ comprising at least one power supply circuit 13 as described previously. The different control units

11 of the different power supply circuits 13 associated with the different pairs of coils 5, 5’ can be arranged on a single electronic circuit board.

[52] The present invention relates also to a method for powering a magnetic bearing 1 as described previously in which, on the one hand, the current in the first and in the second coils of a pair is servocontrolled as a function of a measured position of the rotor 7 to keep the rotor 7 centred with respect to the first coil 5 and to the second coil 5’ and, on the other hand, the voltage respectively at the terminals of the first coil 5 and of the second coil 5’ of the pair is servocontrolled as a function of the currents Ibl, Ib2 in the first coil 5 and in the second coil 5’ so as to limit the vibrations generated by the powering of the magnetic bearing 1.