Vacuum pump and magnetic bearing device

Abstract

[Problem] To provide a vacuum pump and a magnetic bearing device in which an abnormal state can be detected even when a failure occurs simultaneously in an input circuit and an output circuit. [Solution] A magnetic bearing device 110 included in a vacuum pump 100 is characterized by comprising opposing electromagnets 104 positioned in relation to a rotating body 103, a current supply means 201 that supplies currents to the opposing electromagnets 104, a measuring means 202 that measures displacement of the rotating body 103 in relation to the opposing electromagnets 104, and a control means 203 that controls the currents on the basis of the displacement measured by the measuring means 202, the currents supplied to the opposing electromagnets 104 including a bias current Ib, and the control means 203 determining abnormality when the sum Is of the currents supplied to the opposing electromagnets 104 is less than a predetermined current value If.

Description

Vacuum pump and magnetic bearing device

[0001] The present invention relates to a vacuum pump and a magnetic bearing device.

[0002] Vacuum pumps such as turbomolecular pumps use magnetic bearing devices that magnetically levitate and hold a rotor without contact. The magnetic bearing device is equipped with a sensor (measuring means) that measures the displacement of the rotor and an electromagnet that adjusts the position of the rotor. The excitation of the electromagnet is controlled based on the displacement measured by the measuring means, thereby magnetically levitating the rotor at a predetermined position.

[0003] Such magnetic bearing devices have the function of detecting an abnormal condition caused by a fault (for example, a short circuit or an open circuit in the output circuit that supplies current to the electromagnet) and then alerting the user to the abnormal condition by sound or display (see, for example, Patent Document 1).

[0004] Generally, an abnormal state is detected in three cases: when the displacement of the rotating body measured by the measuring means is excessive (hereinafter referred to as an "excessive displacement state"), when the current value supplied to the electromagnet exceeds a predetermined value (hereinafter referred to as an "excessive command current state"), and when a short circuit or the like occurs in the output circuit described above, causing an excessive current to flow and activating a protective means such as a fuse (hereinafter referred to as an "overcurrent fault state").

[0005] Japanese Patent Application Laid-Open No. 2002-295398

[0006] However, failures in such vacuum pumps are rare, and simultaneous failures in multiple locations are extremely rare. However, if a malfunction such as damage to a fixed vane occurs while the vacuum pump is in operation, there is a risk of simultaneous disconnections in both the input circuit, through which the position signal from the measuring means flows, and the output circuit. In this case, it has been found that even if the pump has the function to detect the above three conditions, it is unable to detect the abnormal state. Therefore, even if the frequency of such malfunctions is low, they could lead to major problems.

[0007] In view of the above, an object of the present invention is to provide a vacuum pump and a magnetic bearing device that are capable of detecting an abnormal state even if failures occur simultaneously in the input circuit through which a position signal from a measuring means flows and the output circuit that supplies current to an electromagnet.

[0008] The present invention is a vacuum pump equipped with a magnetic bearing device that magnetically levitates a rotating body and holds it in a non-contact manner, the magnetic bearing device comprising: an opposing electromagnet arranged relative to the rotating body; a current supply means that supplies current to the opposing electromagnet; a measurement means that measures the displacement of the rotating body relative to the opposing electromagnet; and a control means that controls the current based on the displacement measured by the measurement means, wherein the current supplied to the opposing electromagnet includes a bias current, and the control means determines that an abnormality has occurred when the sum of the currents supplied to the opposing electromagnets is smaller than a predetermined current value.

[0009] In such a vacuum pump, the predetermined current value is preferably 50% of the current value of the bias current.

[0010] It is preferable that the control means judges that an abnormality has occurred if, starting from the point at which the sum of the currents becomes smaller than the predetermined current value, the cumulative time during which the sum of the currents is smaller than the predetermined current value becomes longer than 50% of the predetermined time.

[0011] Furthermore, it is preferable that the control means judges that an abnormality has occurred if the cumulative time during which the sum of the currents is smaller than the predetermined current value, starting from the time when the sum of the currents becomes smaller than the predetermined current value, is greater than or equal to a specified time relative to the cumulative time during which the sum of the currents is equal to or greater than the predetermined current value.

[0012] The present invention also provides a magnetic bearing device that magnetically levitates a rotating body and holds it in a non-contact manner, comprising: an opposing electromagnet arranged relative to the rotating body; current supply means that supplies current to the opposing electromagnet; measurement means that measures the displacement of the rotating body relative to the opposing electromagnet; and control means that controls the current based on the displacement measured by the measurement means, wherein the current supplied to the opposing electromagnet includes a bias current, and the control means determines that an abnormality has occurred when the sum of the currents supplied to the opposing electromagnets is smaller than a predetermined current value.

[0013] The present inventors have focused on electromagnets used to magnetically levitate rotating bodies in vacuum pumps and magnetic bearing devices, focusing on electromagnets arranged opposite a rotating body, and further on the bias current contained in the current supplied to these opposing electromagnets. Although the value of the current supplied to each opposing electromagnet changes in response to the displacement of the rotating body, when a bias current is included in each current, the sum of the currents supplied to the opposing electromagnets will be greater than a predetermined current value, regardless of the displacement of the rotating body measured by the measuring means, even when various factors are taken into consideration. In other words, by having the control means determine that an abnormal condition has occurred if the sum of the currents supplied to the opposing electromagnets is less than a predetermined current value, an abnormal condition can be detected even if simultaneous failures occur in the input circuit through which the position signal from the measuring means flows and the output circuit that supplies current to the electromagnets.

[0014] 1 is a longitudinal sectional view schematically showing an embodiment of a vacuum pump according to the present invention. FIG. 2 is a circuit diagram of an amplifier circuit of the vacuum pump shown in FIG. 1. FIG. 3 is a time chart showing control when a current command value is greater than a detection value. FIG. 4 is a time chart showing control when a current command value is smaller than a detection value. FIG. 5 is a schematic diagram of a magnetic bearing device of the vacuum pump shown in FIG. 1. FIG. 6 is a diagram summarizing abnormal states when a failure occurs in an input circuit to which a position signal from a measuring means flows and an output circuit that supplies current to electromagnets. FIG. 7 is a diagram showing the relationship between time and the sum of currents supplied to opposing electromagnets. FIG. 8 is a diagram showing results due to the difference between a conventional general method for determining an abnormal state and a determination method in an embodiment according to the present invention.

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A turbomolecular pump, which is an embodiment of a vacuum pump according to the present invention, will be described below with reference to the drawings.

[0016] A longitudinal cross-sectional view of this turbomolecular pump 100 is shown in FIG. 1 . In FIG. 1 , the X-axis direction is the left-right direction on the page, the Z-axis direction is the up-down direction on the page, and the Y-axis direction is the front-to-rear direction on the page. The turbomolecular pump 100 has an intake port 101 formed at the upper end of a cylindrical outer tube 127 extending along a central axis CA parallel to the Z-axis direction. Inside the outer tube 127, a rotor 103 is provided. The rotor 103 has multiple turbine blades 102 (102a, 102b, 102c, etc.) arranged radially and in multiple stages around its periphery for sucking in and exhausting gas (intake gas). A rotor shaft 113 is attached to the center of the rotor 103. In this embodiment, the rotor shaft 113 is levitated and supported in the air and its position is controlled by a five-axis controlled magnetic bearing device 110. The rotor 103 is generally made of a metal such as aluminum or an aluminum alloy. The magnetic bearing device 110 will be described in detail later.

[0017] The magnetic bearing device 110 has a total of four upper radial electromagnets 104, which are arranged radially outward of the rotor shaft 113, with two of the electromagnets facing each other in the X-axis direction and two of the electromagnets facing each other in the Y-axis direction. In Fig. 1, of the two upper radial electromagnets 104 facing each other in the X-axis direction, the electromagnet located on the + side in the X-axis direction (the right side with respect to the central axis CA in Fig. 1) is shown as upper radial electromagnet 104A, and the electromagnet located on the - side in the X-axis direction (the left side with respect to the central axis CA in Fig. 1) is shown as upper radial electromagnet 104B.

[0018] A total of four upper radial sensors 107 are provided adjacent to the upper radial electromagnets 104 to measure the displacement of the rotor shaft 113. The upper radial sensors 107 are, for example, inductance sensors or eddy current sensors having conductive windings. In this embodiment, the upper radial sensors 107 measure displacement in the X-axis direction and the Y-axis direction above the rotor shaft 113. Two upper radial sensors 107 are provided corresponding to two upper radial electromagnets 104 facing each other in the X-axis direction, and two upper radial sensors 107 are provided corresponding to two upper radial electromagnets 104 facing each other in the Y-axis direction. In FIG. 1 , with respect to the upper radial sensors 107 located in the X-axis direction, the sensor located on the positive side in the X-axis direction (the right side with respect to the central axis CA in FIG. 1 ) is shown as upper radial sensor 107A, and the sensor located on the negative side in the X-axis direction (the left side with respect to the central axis CA in FIG. 1 ) is shown as upper radial sensor 107B. Furthermore, the upper radial sensor 107 of this embodiment is a so-called differential sensor that measures the X-axis and Y-axis positions of the rotor shaft 113 from two directions, the positive and negative directions, respectively, and is configured to send a position signal corresponding to the radial position of the rotor shaft 113 to the controller 200. Although the controller 200 in FIG. 1 is shown as being provided outside the base unit 129, it may also be provided inside the base unit 129. The controller 200 will be described in detail later.

[0019] Furthermore, a total of four lower radial electromagnets 105 are provided radially outward below the rotor shaft 113, two of which face each other in the X-axis direction and two of which face each other in the Y-axis direction. In Fig. 1, of the two lower radial electromagnets 105 facing each other in the X-axis direction, the electromagnet located on the +X-axis direction side (the right side with respect to the central axis CA in Fig. 1) is shown as lower radial electromagnet 105A, and the electromagnet located on the -X-axis direction side (the left side with respect to the central axis CA in Fig. 1) is shown as lower radial electromagnet 105B.

[0020] A total of four lower radial sensors 108 similar to the upper radial electromagnets 104 are provided in close proximity to the lower radial electromagnets 105. The lower radial sensors 108 measure displacement in the X-axis direction and the Y-axis direction below the rotor shaft 113, with two lower radial sensors 108 provided corresponding to the two lower radial electromagnets 105 facing each other in the X-axis direction, and two upper radial sensors 107 provided corresponding to the two lower radial electromagnets 105 facing each other in the Y-axis direction. In FIG. 1 , with respect to the lower radial sensors 108 located in the X-axis direction, the sensor located on the positive side in the X-axis direction (the right side with respect to the central axis CA in FIG. 1 ) is shown as lower radial sensor 108A, and the sensor located on the negative side in the X-axis direction (the left side with respect to the central axis CA in FIG. 1 ) is shown as lower radial sensor 108B. Like the upper radial sensor 107, the lower radial sensor 108 is also a so-called differential sensor that measures the X-axis and Y-axis positions of the rotor shaft 113 from two directions, the positive and negative directions, respectively, and is configured to send a position signal corresponding to the radial position of the rotor shaft 113 to the controller 200.

[0021] In this controller 200, for example, a compensation circuit having a PID adjustment function generates an excitation control command signal for the upper radial electromagnet 104 based on a position signal measured by the upper radial sensor 107, and generates an excitation control command signal for the lower radial electromagnet 105 based on a position signal from the lower radial sensor 108. Then, an amplifier circuit 150 (described later) shown in Fig. 2 controls the excitation of the upper radial electromagnet 104 and the lower radial electromagnet 105 based on these excitation control command signals, thereby adjusting the radial positions of the upper and lower sides of the rotor shaft 113.

[0022] The rotor shaft 113 is formed from a high magnetic permeability material (iron, stainless steel, etc.) and is attracted by the magnetic force of the upper radial electromagnets 104 and also by the magnetic force of the lower radial electromagnets 105. The attraction by the upper radial electromagnets 104 is performed independently in the X-axis direction and the Y-axis direction, and similarly, the attraction by the lower radial electromagnets 105 is performed independently in the X-axis direction and the Y-axis direction.

[0023] Furthermore, axial electromagnets 106 are arranged above and below a circular metal disk 111 provided at the bottom of the rotor shaft 113. In Fig. 1, the upper electromagnet is shown as axial electromagnet 106A, and the lower electromagnet is shown as axial electromagnet 106B. The metal disk 111 is made of a highly magnetic permeable material such as iron.

[0024] An axial sensor 109 that measures the axial displacement of the rotor shaft 113 is provided below the rotor shaft 113. The axial sensor 109 is configured to detect the axial position of the rotor shaft 113 by detecting the position of a sensor target embedded in the lower end of the rotor shaft 113, and to send a position signal corresponding to this axial position to the controller 200.

[0025] In the controller 200, a compensation circuit having, for example, a PID adjustment function generates excitation control command signals for the axial electromagnet 106A and the axial electromagnet 106B based on the axial position signal detected by the axial sensor 109, and the amplifier circuit 150 controls the excitation of the axial electromagnet 106A and the axial electromagnet 106B based on these excitation control command signals, so that the axial electromagnet 106A attracts the metal disk 111 upward by magnetic force, and the axial electromagnet 106B attracts the metal disk 111 downward, thereby adjusting the axial position of the rotor shaft 113.

[0026] Meanwhile, motor 121 has a plurality of magnetic poles arranged circumferentially so as to surround rotor shaft 113. Each magnetic pole is controlled by controller 200 so as to rotate rotor shaft 113 via electromagnetic force acting between the magnetic pole and rotor shaft 113. Motor 121 also incorporates a rotational speed sensor (not shown), such as a Hall element, resolver, or encoder, and the rotational speed of rotor shaft 113 is detected by the detection signal of this rotational speed sensor.

[0027] Furthermore, a phase sensor (not shown) is attached, for example, near the lower radial sensor 108, to detect the phase of rotation of the rotor shaft 113. The controller 200 uses the detection signals of both this phase sensor and the rotational speed sensor to detect the position of the magnetic pole.

[0028] A plurality of fixed blades 123 (123a, 123b, 123c...) are arranged with a small gap between them and the rotating blades 102 (102a, 102b, 102c...). Each of the rotating blades 102 (102a, 102b, 102c...) is formed at an angle of a predetermined degree from a plane perpendicular to the axis of the rotor shaft 113 in order to transport intake gas molecules downward through collision. The fixed blades 123 (123a, 123b, 123c...) are made of metal such as aluminum, iron, stainless steel, copper, or an alloy containing any of these metals.

[0029] Similarly, the fixed blades 123 are formed at a predetermined angle from a plane perpendicular to the axis of the rotor shaft 113, and are arranged in a staggered manner with the stages of the rotor blades 102 toward the inside of the outer cylinder 127. The outer peripheral ends of the fixed blades 123 are supported by being inserted between a plurality of stacked fixed blade spacers 125 (125a, 125b, 125c, ...).

[0030] The fixed vane spacer 125 is a ring-shaped member made of a metal such as aluminum, iron, stainless steel, or copper, or an alloy containing any of these metals. An outer cylinder 127 is fixed to the outer periphery of the fixed vane spacer 125 with a small gap between them. A base 129 is disposed at the bottom of the outer cylinder 127. An exhaust port 133 is formed in the base 129 and communicates with the outside. Intake gas that enters the intake port 101 from the chamber (vacuum chamber) and is transferred to the base 129 is sent to the exhaust port 133.

[0031] Furthermore, a threaded spacer 131 is disposed between the lower portion of the fixed vane spacer 125 and the base portion 129. The threaded spacer 131 is a cylindrical member made of a metal such as aluminum, copper, stainless steel, iron, or an alloy containing any of these metals, and has multiple spiral thread grooves 131a engraved on its inner circumferential surface. The spiral direction of the threaded grooves 131a corresponds to the direction in which intake gas molecules are transported toward the exhaust port 133 when they move in the rotational direction of the rotor 103. A cylindrical portion 102d hangs down from the lowest portion of the rotor 103, adjacent to the rotor vanes 102 (102a, 102b, 102c, etc.). The outer circumferential surface of this cylindrical portion 102d is cylindrical and protrudes toward the inner circumferential surface of the threaded spacer 131, and is adjacent to the inner circumferential surface of the threaded spacer 131 with a predetermined gap therebetween. The intake gas transferred to the screw groove 131 a by the rotary blade 102 and the fixed blade 123 is sent to the base portion 129 while being guided by the screw groove 131 a.

[0032] The base portion 129 is a disk-shaped member that forms the base of the turbomolecular pump 100, and is generally made of metal such as iron, aluminum, stainless steel, etc. The base portion 129 not only physically holds the turbomolecular pump 100, but also functions as a heat conduction path, so it is desirable to use a metal that is rigid and has high thermal conductivity, such as iron, aluminum, or copper.

[0033] In this configuration, when the rotor 102 is rotated together with the rotor shaft 113 by the motor 121, the action of the rotor 102 and the stator 123 draws gas from the chamber through the intake port 101. The rotational speed of the rotor 102 is typically 20,000 rpm to 90,000 rpm, and the peripheral speed at the tip of the rotor 102 reaches 200 m / s to 400 m / s. The intake gas drawn through the intake port 101 passes between the rotor 102 and the stator 123 and is transported to the base 129. At this time, the temperature of the rotor 102 rises due to frictional heat generated when the intake gas comes into contact with the rotor 102 and conduction of heat generated by the motor 121, but this heat is transferred to the stator 123 by radiation or conduction by gas molecules of the intake gas.

[0034] The stator spacers 125 are joined together at their outer peripheries and transmit to the outside heat received by the stator 123 from the rotor 102 and frictional heat generated when intake gas comes into contact with the stator 123.

[0035] In the above description, the threaded spacer 131 is disposed on the outer periphery of the cylindrical portion 102d of the rotor 103, and the thread groove 131a is formed on the inner circumferential surface of the threaded spacer 131. However, conversely, there are also cases where a thread groove is formed on the outer circumferential surface of the cylindrical portion 102d, and a spacer having a cylindrical inner circumferential surface is disposed around it.

[0036] Depending on the application of the turbomolecular pump 100, the electrical equipment section may be surrounded by a stator column 122 to prevent the intake gas sucked in from the intake port 101 from entering the electrical equipment section, which is composed of the upper radial electromagnet 104, the upper radial sensor 107, the motor 121, the lower radial electromagnet 105, the lower radial sensor 108, the axial electromagnet 106, the axial sensor 109, etc., and the interior of this stator column 122 may be kept at a predetermined pressure by purge gas.

[0037] In this case, piping (not shown) is provided in the base portion 129, and purge gas is introduced through this piping. The introduced purge gas is sent to the exhaust port 133 through gaps between the protective bearing 120 and the rotor shaft 113, between the rotor and stator of the motor 121, and between the stator column 122 and the inner cylindrical portion of the rotor blades 102.

[0038] Here, the turbomolecular pump 100 requires control based on specific parameters (e.g., various characteristics corresponding to the model) that are individually adjusted and specific to the model. To store these control parameters, the turbomolecular pump 100 is provided with an electronic circuit unit 141 within its body. The electronic circuit unit 141 is composed of a semiconductor memory such as an EEPROM, electronic components such as semiconductor devices for accessing the memory, and a substrate 143 for mounting these components. The electronic circuit unit 141 is housed below a rotational speed sensor (not shown) near the center of a base unit 129 that constitutes the lower part of the turbomolecular pump 100, and is closed by an airtight bottom lid 145.

[0039] In the semiconductor manufacturing process, some process gases introduced into a chamber have the property of solidifying when their pressure exceeds a predetermined value or their temperature falls below a predetermined value. Inside the turbomolecular pump 100, the pressure of the intake gas is lowest at the intake port 101 and highest at the exhaust port 133. If the pressure of the intake gas exceeds a predetermined value or the temperature falls below a predetermined value while the intake gas is being transferred from the intake port 101 to the exhaust port 133, the intake gas solidifies and adheres to and accumulates inside the turbomolecular pump 100.

[0040] For example, when SiCl4 is used as the process gas in an Al etching apparatus, the vapor pressure curve shows that at low vacuum (760 torr to 10-2 torr) and low temperature (approximately 20°C), a solid product (e.g., AlCl3) precipitates and adheres to and accumulates inside the turbomolecular pump 100. As a result, when precipitates of the process gas accumulate inside the turbomolecular pump 100, these deposits narrow the pump flow path, causing a decrease in the performance of the turbomolecular pump 100. The aforementioned product is prone to solidification and adhesion in high-pressure areas near the exhaust port 133 and the threaded spacer 131.

[0041] Therefore, in order to solve this problem, conventionally, a heater (not shown) or a circular water-cooled pipe 149 is wound around the outer periphery of the base portion 129 or the like, and a temperature sensor (not shown, for example, a thermistor) is embedded in the base portion 129, and heating by the heater and cooling by the water-cooled pipe 149 are controlled based on a signal from this temperature sensor so as to maintain the temperature of the base portion 129 at a constant high temperature (set temperature) (hereinafter referred to as TMS; TMS; Temperature Management System).

[0042] Next, regarding the turbomolecular pump 100 configured as described above, a description will be given of the amplifier circuit 150 that controls excitation of the upper radial electromagnets 104, the lower radial electromagnets 105, and the axial electromagnets 106. A circuit diagram of this amplifier circuit 150 is shown in FIG.

[0043] 2, one end of the electromagnet winding 151 constituting the upper radial electromagnet 104 and the like is connected to the positive electrode 171a of the power supply 171 via a transistor 161, and the other end is connected to the negative electrode 171b of the power supply 171 via a current detection circuit 181 and a transistor 162. The transistors 161 and 162 are so-called power MOSFETs, and have a structure in which a diode is connected between the source and drain.

[0044] At this time, the transistor 161 has a diode cathode terminal 161a connected to the positive electrode 171a and an anode terminal 161b connected to one end of the electromagnet winding 151. The transistor 162 has a diode cathode terminal 162a connected to the current detection circuit 181 and an anode terminal 162b connected to the negative electrode 171b.

[0045] Meanwhile, the current regenerating diode 165 has its cathode terminal 165a connected to one end of the electromagnet winding 151 and its anode terminal 165b connected to the negative electrode 171b. Similarly, the current regenerating diode 166 has its cathode terminal 166a connected to the positive electrode 171a and its anode terminal 166b connected to the other end of the electromagnet winding 151 via a current detecting circuit 181. The current detecting circuit 181 is configured, for example, by a Hall sensor type current sensor or an electrical resistance element.

[0046] The amplifier circuit 150 configured as described above corresponds to one electromagnet. The magnetic bearing device 110 of this embodiment is a five-axis control device, and is provided with a total of ten electromagnets 104, 105, and 106. A similar amplifier circuit 150 is configured for each electromagnet, and the ten amplifier circuits 150 are connected in parallel to the power supply 171.

[0047] Furthermore, the amplifier control circuit 191 is configured, for example, by a digital signal processor section (hereinafter referred to as a DSP section) not shown in the figure of the controller 200, and this amplifier control circuit 191 is configured to switch the transistors 161 and 162 on / off.

[0048] The amplifier control circuit 191 compares the current value detected by the current detection circuit 181 (a signal reflecting this current value is called a current detection signal 191c) with a predetermined current command value. Based on the comparison result, the amplifier control circuit 191 determines the magnitude of the pulse width (pulse width times Tp1 and Tp2) to be generated within a control cycle Ts, which is one period of PWM control. As a result, gate drive signals 191a and 191b having these pulse widths are output from the amplifier control circuit 191 to the gate terminals of the transistors 161 and 162.

[0049] It is necessary to control the position of the rotor 103 at high speed and with strong force when, for example, the rotor 103 passes through a resonance point during acceleration of its rotational speed or when a disturbance occurs during constant-speed operation. For this reason, a high voltage of, for example, about 50 V is used as the power supply 171 so that the current flowing through the electromagnet winding 151 can be rapidly increased (or decreased). In addition, a capacitor (not shown) is usually connected between the positive electrode 171a and the negative electrode 171b of the power supply 171 to stabilize the power supply 171.

[0050] In this configuration, when both the transistors 161 and 162 are turned on, the current flowing through the electromagnet winding 151 (hereinafter referred to as electromagnet current iL) increases, and when both are turned off, the electromagnet current iL decreases.

[0051] Furthermore, when one of the transistors 161 and 162 is turned on and the other is turned off, a so-called flywheel current is maintained. By passing a flywheel current through the amplifier circuit 150 in this manner, hysteresis loss in the amplifier circuit 150 can be reduced, and the power consumption of the entire circuit can be kept low. Furthermore, by controlling the transistors 161 and 162 in this manner, high-frequency noise such as harmonics generated in the turbomolecular pump 100 can be reduced. Furthermore, by measuring this flywheel current with the current detection circuit 181, the electromagnet current iL flowing through the electromagnet winding 151 can be detected.

[0052] That is, when the detected current value is smaller than the current command value, both transistors 161 and 162 are turned on for a time period corresponding to a pulse width time Tp1 only once during a control cycle Ts (e.g., 100 μs), as shown in Fig. 3. Therefore, the electromagnet current iL during this period increases toward a current value iLmax (not shown) that can flow from the positive electrode 171a to the negative electrode 171b via the transistors 161 and 162.

[0053] On the other hand, if the detected current value is greater than the current command value, both transistors 161 and 162 are turned off for a time period corresponding to pulse width time Tp2 only once during the control cycle Ts, as shown in Fig. 4. Therefore, the electromagnet current iL during this period decreases toward a current value iLmin (not shown) that can be regenerated from the negative electrode 171b to the positive electrode 171a via diodes 165 and 166.

[0054] In either case, after the pulse width times Tp1 and Tp2 have elapsed, one of the transistors 161 and 162 is turned on. Therefore, a flywheel current is maintained in the amplifier circuit 150 during this period.

[0055] Next, the above-mentioned magnetic bearing device 110 and controller 200 will be described in detail with reference to Fig. 5. In the following explanation, only the magnetic bearing device 110 and controller 200 related to the upper radial electromagnets 104A and 104B that face each other in the X-axis direction among the above-mentioned electromagnets (104, 105, 106) will be explained, but the configurations of the other electromagnets are similar. For this reason, explanations of the other electromagnets will be omitted below, and the parts related to the other electromagnets will also be omitted in Fig. 5.

[0056] 5 , the controller 200 includes a current supply means 201, a measurement means 202, and a control means 203. The current supply means 201 includes the amplifier circuit 150 and an output circuit 204 that connects the upper radial electromagnets 104A and 104B to the amplifier circuit 150. The measurement means 202 includes a levitation position measurement unit 205 and an input circuit 206 that connects the upper radial sensors 107A and 107B to the levitation position measurement unit 205. The control means 203 includes a levitation control unit 207 that sends an excitation control command signal to the current supply means 201, a current measurement unit 208 that measures the current supplied to the upper radial electromagnets 104A and 104B, and a protection function processing unit 209. The output circuit 204 is provided with a protection means (e.g., a fuse) that cuts off the current when an excessive current flows due to, for example, a short circuit. As shown in FIG. 5, the magnetic bearing device 110 is made up of upper radial electromagnets 104A and 104B, upper radial sensors 107A and 107B, current supply means 201, measurement means 202, and control means 203.

[0057] In the magnetic bearing device 110 configured as described above, when the rotor shaft 113 is rotated by the motor 121, the levitation position measurement unit 205 of the measurement means 202 measures the displacement of the rotor shaft 113 using the upper radial sensors 107A and 107B. Specifically, when the distance between the upper radial sensor 107A and the rotor shaft 113 changes, the inductance of the coil provided in the upper radial sensor 107A changes according to the distance, and therefore the output voltage of the upper radial sensor 107A changes. Similarly, when the distance between the upper radial sensor 107B and the rotor shaft 113 changes, the inductance of the coil provided in the upper radial sensor 107B changes according to the distance, and therefore the output voltage of the upper radial sensor 107B changes. Then, to avoid the influence of noise, etc., the difference between the output voltages of the upper radial sensors 107A and 107B is taken as the displacement of the rotor shaft 113, and a position signal corresponding to this displacement is fed back to the control means 203.

[0058] The levitation control unit 207 of the control means 203 calculates the deviation between the actual position of the rotor shaft 113 and the target position based on the position signal fed back from the measurement means 202 and the signal related to the target position command, and sends a signal related to an excitation control command to the current supply means 201 based on this deviation. The control means 203 also sends a signal related to a bias current command to the current supply means 201. Then, based on this signal, the current supply means 201 causes each amplifier circuit 150 to supply a current to which the bias current Ib has been added to the upper radial electromagnets 104A and 104B. As a result, the magnetic attractive forces from the upper radial electromagnets 104A and 104B act on the rotor shaft 113, and the rotor shaft 113 can be moved to the target position.

[0059] Here, a specific description will be given of the operations of the control means 203 and the current supply means 201. For example, when the rotor shaft 113 is displaced so that its distance from the upper radial electromagnet 104A is greater than its distance from the upper radial electromagnet 104B, the control means 203 sends a signal to the current supply means 201 to cause the amplifier circuit 150 connected to the upper radial electromagnet 104A to output a current Ip (Ip = Ib + Ic) which is the sum of the above-mentioned bias current Ib and a current corresponding to this displacement (control current Ic), from the amplifier circuit 150 connected to the upper radial electromagnet 104A, and to output a current Im (Im = Ib - Ic) which is the difference between the bias current Ib and the control current Ic, from the amplifier circuit 150 connected to the upper radial electromagnet 104B.

[0060] The control means 203 of this embodiment also includes a protection function processing unit 209 that has the function of determining that the magnetic bearing device 110 is in an abnormal state when the displacement of the rotor shaft 113 measured by the measurement means 202 is excessive (excessive displacement state), when the value of the current supplied to the upper radial electromagnets 104A, 104B exceeds a predetermined value (excessive command current state), or when an excessive current flows in the output circuit 204 and the protection means is activated (overcurrent fault state). The protection function processing unit 209 also has the function of activating an alarm means (not shown) provided in the turbomolecular pump 100 when it determines that such an abnormal state has occurred (such as a buzzer or a warning lamp that alerts the user of the abnormal state by sound or display), and the function of stopping the rotating body 103 by cutting off the current to the motor 121.

[0061] Conventional magnetic bearing devices also have the configuration of measuring the displacement of rotor shaft 113, outputting a current based on that displacement from amplifier circuit 150, thereby applying the magnetic attractive force from upper radial electromagnets 104A, 104B to rotor shaft 113 to move rotor shaft 113 to a target position, as well as the function of detecting the three abnormal conditions described above: excessive displacement, excessive command current, and overcurrent fault. However, if a fault occurs simultaneously in input circuit 206, through which a position signal from measuring means 202 shown in Figure 5 flows, and output circuit 204, which connects upper radial electromagnets 104A, 104B to amplifier circuit 150, the function of detecting the three abnormal conditions described above may not be able to detect this fault as an abnormal condition. This point will be explained with reference to Figure 6.

[0062] In Figure 6, the "circuit of the opposing electromagnet" in the left column indicates the state of the output circuit 204, with "no fault" indicating a state in which no fault has occurred in the output circuit 204, "short-circuit fault" indicating a state in which an excessive current has flowed in the output circuit 204 and the protective means has been activated, "partial breakdown (disconnection)" indicating a state in which either the path connecting the upper radial electromagnet 104A and the amplifier circuit 150 in the output circuit 204 or the path connecting the upper radial electromagnet 104B and the amplifier circuit 150 has been disconnected, and "total breakdown (disconnection)" indicating a state in which both of the two paths have been disconnected.

[0063] In Figure 6, the "opposing sensor circuit" in the upper row indicates the state of the input circuit 206, with "no fault" meaning that no fault has occurred in the input circuit 206, "partial breakdown" meaning that either the output voltage from the upper radial sensor 107A or the output voltage from the upper radial sensor 107B cannot be obtained (for example, a state in which the upper radial sensor 107A has failed or the path related to the upper radial sensor 107A in the input circuit 206 has been broken), and "total breakdown" meaning that neither the output voltage from the upper radial sensor 107A nor the output voltage from the upper radial sensor 107B can be obtained.

[0064] As described above, the position signal fed back from the measuring means 202 to the control means 203 is based on the difference between the output voltage of the upper radial sensor 107A and the output voltage of the upper radial sensor 107B. Therefore, when the "opposing sensor circuit" is "partially broken," the output voltage from the upper radial sensor 107A or the output voltage from the upper radial sensor 107B becomes zero, and the position signal from the measuring means 202 becomes a signal when the rotor shaft 113 is displaced nearly to the maximum, even if the rotor shaft 113 is actually displaced very little relative to the upper radial sensor 107A or the upper radial sensor 107B. Also, when the "opposing sensor circuit" is "completely broken," the output voltages of the upper radial sensors 107A and 107B both become zero. In other words, the position signal from the measuring means 202 becomes the same as the signal when the distance from the upper radial sensor 107A to the rotor shaft 113 is equal to the distance from the upper radial sensor 107B to the rotor shaft 113.

[0065] In the magnetic bearing device 110 having such a function, when the "opposing electromagnet circuit" is "not faulty" and the "opposing sensor circuit" is "not faulty" as shown in Figure 6, the control means 203 is in a state where it can correctly detect abnormalities, and for example, when the displacement of the rotor shaft 113 measured by the measuring means 202 becomes excessive, it can detect that an excessive displacement state exists, and when the current value supplied to the upper radial electromagnets 104A, 104B exceeds a predetermined value, it can detect that a command current is excessive.

[0066] When the "opposing electromagnet circuit" has "no fault" or is "partially broken (disconnected)", if the current value supplied to the upper radial electromagnets 104A, 104B exceeds a predetermined value, the control means 203 can detect an excessive command current state. Furthermore, when the "opposing electromagnet circuit" has "no fault" or is "partially broken (disconnected)", if the "opposing sensor circuit" has "no fault" or is "partially broken", it is possible to detect an abnormality as an excessive displacement state. Note that when a "short circuit fault" occurs in the "opposing electromagnet circuit", the control means 203 can detect an overcurrent fault state regardless of the state of the "opposing sensor circuit".

[0067] If the "opposing electromagnet circuit" is "completely broken (disconnected)" and the "opposing sensor circuit" is "not faulty" or "partially broken," the control means 203 can detect an abnormality as an excessive displacement state. Furthermore, if the current value supplied to the upper radial electromagnets 104A and 104B exceeds a predetermined value, it is possible to detect an excessive command current state even if the "opposing electromagnet circuit" is "completely broken (disconnected)" and the "opposing sensor circuit" is "completely broken." However, if the "opposing sensor circuit" is "completely broken," even if the rotor shaft 113 is significantly displaced relative to the upper radial sensor 107A or the upper radial sensor 107B, the position signal from the measurement means 202 is the same as the signal when the distance from the upper radial sensor 107A to the rotor shaft 113 is equal to the distance from the upper radial sensor 107B. Therefore, excitation of the upper radial electromagnets 104A, 104B is not basically performed, and therefore the value of the current supplied to the upper radial electromagnets 104A, 104B basically does not exceed a predetermined value. Furthermore, if the "opposing electromagnet circuit" is also in a "total breakdown (disconnection)" state, an excessive current does not flow in the output circuit 204. For this reason, in conventional magnetic bearing devices, if the "opposing electromagnet circuit" is "total breakdown (disconnection)" and the "opposing sensor circuit" is also "total breakdown," it may not be possible to detect the three abnormal states described above (the state in which fault notification is not possible shown in FIG. 6), and there is a risk that the rotating body 103 may continue to rotate for a long time without being magnetically levitated.

[0068] In contrast, the control means 203 of this embodiment has a function of measuring the sum of the currents supplied to the opposing upper radial electromagnets 104A and 104B using a current measurement unit 208, and determining that an abnormality has occurred if this sum of currents is smaller than a predetermined current value If. That is, the sum Is of the currents of the upper radial electromagnets 104A and 104B measured by the current measurement unit 208 is usually the sum of the current Ip (Ip = Ib + Ic) output from the amplifier circuit 150 connected to the upper radial electromagnet 104A and the current Im (Im = Ib - Ic) output from the amplifier circuit 150 connected to the upper radial electromagnet 104B, and indicates a value approximately twice the bias current Ib (Is = Ip + Im = 2 × Ib). However, when a break or the like occurs in the output circuit 204 and current is not supplied to both the opposing upper radial electromagnets 104A and 104B, the sum Is of the currents measured by the current measurement unit 208 will be smaller than approximately twice the value of the bias current Ib, even when the effects of variations in the components constituting the turbomolecular pump 100, variations in assembly, and time delays in feedback control are taken into consideration. Therefore, by setting a value smaller than approximately twice the value of the bias current Ib as the predetermined current value If and determining that an abnormality has occurred when the sum Is of the currents of the opposing upper radial electromagnets 104A and 104B is smaller than the predetermined current value If by the protection function processing unit 209, it is possible to stop the rotating body 103 even when the "opposing electromagnet circuit" is "total breakdown (break)" and the "opposing sensor circuit" is "total breakdown."

[0069] Note that if the predetermined current value If is set to a value approximately twice the bias current Ib, there is a risk that the turbomolecular pump 100 may be determined to be in an abnormal state even when it is actually normal, depending on the operating conditions of the turbomolecular pump 100. For example, the turbomolecular pump 100 is not necessarily installed in the orientation shown in FIG. 1 (in which the rotor shaft 113 is oriented vertically). For example, if the rotor shaft 113 is installed in an orientation in which it is oriented horizontally, a magnetic attraction force that takes into account gravity acting on the rotor 103 must be generated. This may change the sum Is of the currents of the upper radial electromagnets 104A and 104B depending on the installation orientation. Taking these points into consideration, the inventors of the present application conducted extensive studies using various turbomolecular pumps 100 while assuming various operating conditions, and found that setting the predetermined current value If to 50% of the bias current Ib makes it possible to stably determine whether the turbomolecular pump 100 is in an abnormal state. For this reason, it is preferable to set the predetermined current value If to 50% of the bias current Ib.

[0070] Furthermore, the sum Is of the currents of the opposing upper radial electromagnets 104A and 104B may fluctuate, for example, as shown in Fig. 7 due to the influence of a fault in the output circuit 204, chattering of the observed current, noise from the surroundings, etc. Taking into account the influence of such fluctuations, extensive research was conducted into a method for more stably determining whether or not an abnormal state has occurred. As a result, it was found that it is effective to configure the control means 203 to determine an abnormality when the cumulative time (Td1+Td2+...Tdm in Fig. 7) during which the sum Is of the currents is smaller than a predetermined current value If (50% of the current value of the bias current Ib in Fig. 7) is longer than 50% of the predetermined time Tra, starting from the time when the sum Is of the currents becomes smaller than a predetermined current value If (i.e., when the conditional expression (Td1+Td2+...Tdm)>Tra×50% is satisfied).

[0071] Furthermore, in order to more stably determine whether or not an abnormal state exists, it is also effective to configure the control means 203 to determine that an abnormality exists when the cumulative time (Td1+Td2+...Tdm in Figure 7) during which the sum of the currents Is is smaller than the predetermined current value If, ​​starting from the time when the sum of the currents Is becomes smaller than the predetermined current value If, ​​becomes greater than a predetermined time Trb (as an example, the predetermined time Trb is 1 second) relative to the cumulative time (Tu1+Tu2+...Tun in Figure 7) during which the sum of the currents Is is equal to or greater than the predetermined current value If (i.e., when the conditional expression (Td1+Td2+...Tdm)-(Tu1+Tu2+...Tun)≧Trb is satisfied).

[0072] Here, the differences between the conventional and typical abnormality determination methods and the above-described determination method will be described with reference to FIG. 8 . The sum of currents Is shown in FIG. 8 has time on the horizontal axis and current value on the vertical axis. The current value gradually decreases over time, eventually ceasing to flow. The minute changes in the sum of currents Is shown in FIG. 8 are primarily due to noise and other factors. The comparative example shows the results of a conventional and typical abnormality determination method. In the conventional and typical abnormality determination method, an abnormality is determined when the sum of currents Is becomes smaller than a predetermined current value If for a specified time Trb. If the specified time Trb is not reached, a reset is performed, and time measurement is restarted from the point when the sum of currents Is becomes smaller than the specified current value If. In the comparative example shown in FIG. 8 , the horizontal axis represents time, and the vertical axis represents duration (the time during which the sum of currents Is remains smaller than the specified current value If). When the sum of currents Is shown in FIG. 8 is checked using the conventional determination method, resetting is performed several times if the specified time Trb is not reached, and an abnormality is detected at time T2. On the other hand, the example shown in Figure 8 shows the results of a method in which an abnormality is determined when the cumulative time during which the sum of the currents Is is less than the predetermined current value If is equal to or greater than the predetermined current value If and the cumulative time during which the sum of the currents Is is equal to or greater than the predetermined current value If is equal to or greater than the predetermined current value If. In this example, the horizontal axis represents time and the vertical axis represents duration (cumulative time during which the sum of the currents Is is less than the predetermined current value If - cumulative time during which the sum of the currents Is is equal to or greater than the predetermined current value If). When the sum of the currents Is shown in Figure 8 is checked using the determination method of this embodiment, a reset is not performed as in the conventional determination method, and therefore an abnormality can be detected at time T1, which is earlier than time T2.

[0073] Although one embodiment of the present invention has been described above, the present invention is not limited to the specific embodiment, and various modifications, changes, and combinations are possible within the scope of the spirit of the present invention as set forth in the claims unless otherwise specifically limited in the above description. Furthermore, the effects of the above embodiment are merely examples of the effects resulting from the present invention, and do not mean that the effects of the present invention are limited to the above effects.

[0074] For example, the functions described with reference to Figures 5 to 7 are not limited to the upper radial electromagnets 104A, 104B and the upper radial sensors 107A, 107B, but can also be applied to other electromagnets 104, 105, 106 and other sensors 107, 108, 109.

[0075] 100: Turbo molecular pump (vacuum pump) 103: Rotating body 104: Upper radial electromagnet 105: Lower radial electromagnet 106: Axial electromagnet 107: Upper radial sensor 108: Lower radial sensor 109: Axial sensor 110: Magnetic bearing device 201: Current supply means 202: Measuring means 203: Control means

Claims

1. A vacuum pump equipped with a magnetic bearing device that magnetically levitates and holds a rotating body in a non-contact manner, the magnetic bearing device comprising: an opposing electromagnet arranged relative to the rotating body; current supply means that supplies current to the opposing electromagnet; measurement means that measures the displacement of the rotating body relative to the opposing electromagnet; and control means that controls the current based on the displacement measured by the measurement means, wherein the current supplied to the opposing electromagnet includes a bias current, and the control means determines that an abnormality has occurred when the sum of the currents supplied to the opposing electromagnets is smaller than a predetermined current value.

2. A vacuum pump according to claim 1, wherein the predetermined current value is 50% of the current value of the bias current.

3. A vacuum pump according to claim 1 or 2, characterized in that, when the cumulative time during which the sum of the currents is smaller than the predetermined current value is longer than 50% of the predetermined time, the control means determines that an abnormality has occurred.

4. A vacuum pump according to claim 1 or 2, characterized in that, starting from the time when the sum of the currents becomes smaller than the predetermined current value, if the cumulative time during which the sum of the currents is smaller than the predetermined current value becomes equal to or longer than a specified time relative to the cumulative time during which the sum of the currents is equal to or greater than the predetermined current value, the control means determines that an abnormality has occurred.

5. A magnetic bearing device that magnetically levitates a rotating body and holds it in a non-contact manner, comprising: an opposing electromagnet arranged relative to the rotating body; current supply means that supplies current to the opposing electromagnet; measurement means that measures the displacement of the rotating body relative to the opposing electromagnet; and control means that controls the current based on the displacement measured by the measurement means, wherein the current supplied to the opposing electromagnet includes a bias current, and the control means determines that an abnormality has occurred when the sum of the currents supplied to the opposing electromagnets is smaller than a predetermined current value.

Images