Abstract:
A magnetic bearing device has a rotor, electromagnets that control axial/radial positions of the rotor, and a power source that supplies power to the electromagnets. A switching circuit switches a voltage of a common node connected to each one end of the electromagnets. The switching circuit includes a first switch element that connects and disconnects between one end of the power source and the common node, and a first rectifier element connected between the other end of the power source and the common node. An excitation control circuit controls excitation of each of the electromagnets by a supply current that flows through the electromagnets in one direction or a regenerated current that flows through the electromagnets in one direction. The excitation control circuit includes a second switch element that connects and disconnects between the other end of one of the electromagnets and the other end of the power source, and a second rectifier element connected between the other end of one of the electromagnets and the one end of the power source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a U.S. national stage application of copending International Application No. PCT/JP2005/003347, filed Feb. 28, 2005, claiming a priority date of Mar. 4, 2004, and published in the non-English language. 
   TECHNICAL FIELD 
   The present invention relates to a magnetic bearing device and a turbo molecular pump with the magnetic bearing device mounted thereto. More specifically, the present invention relates to a magnetic bearing device capable of reducing the costs required for the manufacture, installation, or the like of a turbo molecular pump by reducing the number of elements of an amplifier circuit that drives electromagnets through excitation, and of reducing an error at the time of detecting a current flowing through the electromagnets, and to a turbo molecular pump with the magnetic bearing device mounted thereto. 
   BACKGROUND ART 
   With the development of electronics in recent years, demands for semiconductors for forming memories, integrated circuits, etc. are rapidly increasing. Those semiconductors are manufactured such that impurities are doped into a semiconductor substrate with an extremely high purity to impart electrical properties thereto, or semiconductor substrates with minute circuit patterns formed thereon are laminated. Those manufacturing steps must be performed in a chamber with a high vacuum state so as to avoid influences of dust etc. in the air. This chamber is generally evacuated by using a vacuum pump as a pumping system. In particular, a turbo molecular pump, which is one of the vacuum pumps, is widely used since the turbo molecular pump entails little residual gas, is easy of maintenance, and has other such characteristics. 
   The semiconductor manufacturing process includes a number of steps in which various process gases are caused to act onto a semiconductor substrate, and the turbo molecular pump is used not only to evacuate the chamber but also to discharge those process gases from the chamber. Further, in equipment for an electron microscope etc., the turbo molecular pump is used to create a high vacuum state in the chamber of the electron microscope etc. in order to prevent refraction etc. of an electron beam caused by the presence of dust or the like. 
   Such the turbo molecular pump is composed of a turbo molecular pump main body for sucking and discharging gas from the chamber of a semiconductor manufacturing apparatus, an electron microscope, or the like, and a control device for controlling the turbo molecular pump main body.  FIG. 10  is a vertical sectional view of the turbo molecular pump main body. 
   In  FIG. 10 , a turbo molecular pump main body  100  includes an outer cylinder  127  with an inlet port  101  formed on the top thereof. Provided inside the outer cylinder  127  is a rotor  103  having in its periphery a plurality of rotary vanes  102   a ,  102   b ,  102   c , . . . serving as turbine blades for sucking and discharging gas, formed radially in a number of stages. At the center of the rotor  103 , a rotor shaft  113  is mounted while being supported in a levitating state in the air and controlled in position, for example, by a 5-axis control magnetic bearing. 
   Upper radial electromagnets  104  are provided by arranging four electromagnets in pairs in X- and Y-axis and plus- and minus-side directions (not shown.; those electromagnets are denoted by  104 X+,  104 X−,  104 Y+, and  104 Y−, as necessary). Further, there is provided an upper radial sensor  107  constituted of four electromagnets arranged in close proximity to and in correspondence with the upper radial electromagnets  104 . The upper radial sensor  107  detects radial displacement of the rotor  103 , transmitting a signal to a control device (not shown). 
   In this control device, the upper radial electromagnets  104  are controlled through excitation by an amplifier circuit  150  (to be described later), through a compensation circuit having a PID adjusting function, on the basis of a displacement signal detected by the upper radial sensor  107 , which adjusts the radial position of the upper portion of the rotor shaft  113 . 
   The rotor shaft  113  is formed of a high-magnetic-permeability material (e.g., iron) and is adapted to be attracted by the magnetic force of the upper radial electromagnets  104 . Such adjustment is conducted independently in the X-axis direction and the Y-axis direction. 
   Further, lower radial electromagnets  105  and a lower radial sensor  108  are arranged in the same way as the upper radial electromagnets  104  and the upper radial sensor  107 . Like the radial position of the upper portion of the rotor shaft  113 , the radial position of the lower portion of the rotor shaft  113  is adjusted (the lower radial electromagnets  105  are similarly denoted by  105 X+,  105 X−,  105 Y+, and  105 Y−, as necessary). 
   Further, axial electromagnets  106 A and  106 B are arranged on the upper and lower sides of a circular metal disc  111  provided in the lower portion of the rotor shaft  113 . The metal disc  111  is formed of a high-magnetic-permeability material such as iron. To detect axial displacement of the rotor shaft  113 , an axial sensor  109  is provided, which transmits an axial displacement signal thereof to the control device. 
   The axial electromagnets  106 A and  106 B are controlled through excitation by the amplifier circuit  150 , through the compensation circuit having a PID adjusting function of the control device on the basis of the axial displacement signal. The axial electromagnet  106 A upwardly attracts the magnetic disc  111  by the magnetic force, and the axial electromagnet  106 B downwardly attracts the magnetic disc  111 . 
   In this way, the control device has a function to appropriately control the magnetic force exerted on the metal disc  111  by the axial electromagnets  106 A and  106 B, magnetically levitate the rotor shaft  113  in the axial direction, and retain the rotor shaft  113  in the space in a non-contact state. 
   Note that descriptions will be given later in more detail on the amplifier circuit  150  that drives, through excitation, the upper radial electromagnets  104 , the lower radial electromagnets  105 , and the axial electromagnets  106 A and  106 B. 
   Meanwhile, a motor  121  is equipped with a plurality of magnetic poles, which are arranged circumferentially to surround the rotor shaft  113 . The magnetic poles are respectively controlled by the control device to rotate the rotor shaft  113  through an electromagnetic force acting between the rotor shaft  113  and the magnetic poles. 
   In addition, the motor  121  also has an RPM sensor (not shown) incorporated to output a detection signal, which is used for detection of RPM of the rotor shaft  113 . A phase sensor (not shown) is attached, for example, in the vicinity of the lower radial sensor  108  to detect the phase of rotation of the rotor shaft  113 . From detection signals of the phase sensor and the RPM sensor both, the control device detects positions of the magnetic poles. 
   A plurality of stationary vanes  123   a ,  123   b ,  123   c , . . . are arranged so as to be spaced apart from the rotary vanes  102   a ,  102   b ,  102   c , . . . by small gaps. To downwardly transfer the molecules of exhaust gas through collision, the rotary vanes  102   a ,  102   b ,  102   c , . . . are inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft  113 . Similarly, the stationary vanes  123  are also inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft  113 , and extend toward the inner side of the outer cylinder  127  to be arranged alternately with the rotary vanes  102 . 
   The stationary vanes  123  are supported at one end by being inserted into gaps between a plurality of stationary vane spacers  125   a ,  125   b ,  125   c , . . . stacked together in stages. The stationary vane spacers  125  are ring-shaped members, which are formed of a metal, such as aluminum, iron, stainless steel, or copper, or a metal such as an alloy containing such the metal as a component. 
   In the outer periphery of the stationary vane spacers  125 , the outer cylinder  127  is secured in position with a small gap therebetween. At the bottom of the outer cylinder  127 , there is arranged a base portion  129 , and a threaded spacer  131  is arranged between the lower portion of the stationary vane spacers  125  and the base portion  129 . In the portion of the base portion  129  below the threaded spacer  131 , there is formed a exhaust port  133  which communicates with the outside. 
   The threaded spacer  131  is a cylindrical member formed of a metal, such as aluminum, copper, stainless steel, or iron, or a metal such as an alloy containing such the metal as a component, and has a plurality of spiral thread grooves  131   a  in its inner peripheral surface. The spiral direction of the thread grooves  131   a  is determined such that when the molecules of the exhaust gas move in the rotating direction of the rotor  103 , these molecules are transferred toward the exhaust port  133 . 
   Connected to the lowermost one of the rotary vanes  102   a ,  102   b ,  102   c , . . . of the rotor  103  is a rotary vane  102   d , which extends vertically downwards. The outer peripheral surface of the rotary vane  102   d  in a cylindrical shape sticks out toward the inner peripheral surface of the threaded spacer  131 , and is in close proximity to the inner peripheral surface of the threaded spacer  131  with a predetermined gap therebetween. 
   The base portion  129  is a disc-like member constituting the base of the turbo molecular pump main body  100 , and is generally formed of a metal, such as iron, aluminum, or stainless steel. The base portion  129  physically retains the turbo molecular pump main body  100 , and also functions as a heat conduction passage. Thus, the base portion  129  is desirably formed of a metal that is rigid and high in heat conductivity, such as iron, aluminum, or copper. 
   In the above-mentioned construction, when the rotary vanes  102  are driven by the motor  121  to be rotated together with the rotor shaft  113 , an exhaust gas from a chamber is sucked in through the inlet port  101  by the action of the rotary vanes  102  and the stationary vanes  123 . 
   The exhaust gas sucked in through the inlet port  101  passes between the rotary vanes  102  and the stationary vanes  123 , and is transferred to the base portion  129 . At this time, the temperature of the rotary vanes  102  is raised by frictional heat generated when the exhaust gas comes into contact with the rotary vanes  102  and by heat generated and conducted from the motor  121 . Such the heat is transferred to the stationary vanes  123  through radiation or through conduction of gas molecules of exhaust gas or the like. 
   The stationary vane spacers  125  are joined to one another on the outer periphery and transmit, to the outside, heat received by the stationary vanes  123  from the rotary vanes  102  as well as frictional heat generated upon contact between exhaust gas and the stationary vanes  123 . The exhaust gas transferred to the base portion  129  is sent to the exhaust port  133  while being guided by the thread grooves  131   a  of the threaded spacer  131 . 
   Herein, the turbo molecular pump requires control based on individually adjusted specific parameters (e.g., identification of the model and characteristics corresponding to the model). To store the control parameters, the turbo molecular pump main body  100  contains an electronic circuit portion  141  in its main body. The electronic circuit portion  141  is composed of a semiconductor memory, such as EEP-ROM, electronic parts, such as semiconductor devices for access to the semiconductor memory, a substrate  143  for mounting these components thereto, etc. This electronic circuit portion  141  is accommodated under an RPM sensor (not shown) near the center of the base portion  129  constituting the lower portion of the turbo molecular pump main body  100 , and is closed by a hermetic bottom cover  145 . 
   Given next is a detailed description of the amplifier circuit for driving, through excitation, the upper radial electromagnets  104 , the lower radial electromagnets  105 , and the axial electromagnets  106 A and  106 B of the turbo molecular pump main body  100  structured as above. Patent Document 1 is known as a conventional example of the amplifier circuit. 
     FIG. 11  shows a diagram of a conventional amplifier circuit. Electromagnet coils  151 ,  151 , . . . which respectively constitute the electromagnets  104 ,  105 ,  106 A, and  106 B are elements present on the turbo molecular pump main body  100  side. The electromagnet coils are shown for simplicity. 
   In  FIG. 11 , the electromagnet coil  151  is connected at one end  151   a  to a transistor  161  and a diode  165 , while the electromagnet coil  151  is connected at the other end  151   b  to a transistor  162  and a diode  166  through an electromagnetic current detecting circuit  155 . 
   Herein, the transistors  161  and  162  are both power MOSFETs. The transistor  161  has a drain terminal  161   a  connected to a positive electrode  153   a  of a power source  153  and has a source terminal  161   b  connected to the one end  151   a  of the electromagnet coil  151 . The transistor  162  has a drain terminal  162   a  connected to the other end  151   b  of the electromagnet coil  151  through the electromagnetic current detecting circuit  155  and has a source terminal  162   b  connected to a negative electrode  153   b  of the power source  153 . 
   In addition, the diodes  165  and  166  are both provided for current regeneration. The diode  165  has a cathode terminal  165   a  connected to the one end  151   a  of the electromagnet coil  151  and has an anode terminal  165   b  connected to the negative electrode  153   b . Similarly, the diode  166  has a cathode terminal  166   a  connected to the positive electrode  153   a  and has an anode terminal  166   b  connected to the other end  151   b  of the electromagnet coil  151  through the electromagnetic current detecting circuit  155 . 
   The electromagnetic current detecting circuit  155  connected to the other end  151   b  of the electromagnet coil  151  is, for example, a hole sensor type current sensor, and detects the amount of a current flowing in the electromagnet coil  151  (hereinafter referred to as “electromagnetic current iL”) to output an electromagnetic current detection signal  173  as the detection result to an amplifier control circuit  171  (to be described later) Also, provided between the positive electrode  153   a  and the negative electrode  153   b  of the power source  153  is a capacitor (not shown) for stabilizing the power source  153 . 
   The amplifier circuit  150  configured as described above is provided for each of the electromagnet coils  151 ,  151 , . . . which respectively constitute the electromagnets  104 ,  105 ,  106 A, and  106 B. 
   The amplifier control circuit  171  is a circuit within a digital signal processor portion (hereinafter referred to as “DSP portion”) (not shown) of the control device. The amplifier control circuit  171  compares the value of the electromagnetic current iL detected by the electromagnetic current detecting circuit  155  and a current command value. Based on the comparison result, the pulse width time of each of gate drive signals  174  and  175  to be outputted to the gate terminals of the transistors  161  and  162  is determined within a control cycle Ts, which is one cycle by PWM control. 
   In the above-mentioned structure, when the transistors  161  and  162  of the amplifier circuit  150  are both turned on, the electromagnetic current iL is increased due to a current supplied from the positive electrode  153   a  to the negative electrode  153   b  through the transistor  161 , the electromagnet coil  151 , and the transistor  162 . On the other hand, when the transistors  161  and  162  are both turned off, the electromagnetic current iL is decreased due to a current regenerated from the negative electrode  153   b  to the positive electrode  153   a  through the diode  165 , the electromagnet coil  151 , and the diode  166 . 
   In this case, when the value of the electromagnetic current iL detected by the electromagnetic current detecting circuit  155  is smaller than the current command value, control is performed such that the electromagnetic current iL is increased in the amplifier control circuit  171 . Therefore, as shown in  FIG. 12 , in one control cycle Ts, the pulse width time during which the transistors  161  and  162  are both kept turned on is set to be longer than the pulse width time during which the transistors  161  and  162  are both kept turned off. As a result, the electromagnetic current iL in one control cycle Ts is increased since an increasing time Tp 1  for the electromagnetic current iL is set to be longer than a decreasing time Tp 2  for the electromagnetic current iL. 
   On the other hand, when the value of the electromagnetic current iL detected by the electromagnetic current detecting circuit  155  is larger than the current command value, control is performed such that the electromagnetic current iL is decreased in the amplifier control circuit  171 . Therefore, as shown in  FIG. 12 , in one control cycle Ts, the pulse width time during which the transistors  161  and  162  are both kept turned off is set to be longer than the pulse width time during which the transistors  161  and  162  are both kept turned on. As a result, the electromagnetic current iL in one control cycle Ts is decreased since the decreasing time Tp 2  for the electromagnetic current iL is set to be longer than the increasing time Tp 1  for the electromagnetic current iL. 
   By the settings, the electromagnetic current iL in the control cycle Ts can be appropriately increased or decreased, so the value of the electromagnetic current iL and the current command value can be made to be the same. 
   Note that the detection of the electromagnetic current iL in the electromagnetic current detecting circuit  155  is performed once at the same detection timing Td in the control cycle Ts as shown in  FIG. 12 . 
   Patent Document 1: JP 3176584 B (FIG. 8 and FIG. 9) 
   As described above, the amplifier circuit  150  is provided for each of the electromagnet coils  151 ,  151 , . . . which respectively constitute the electromagnets  104 ,  105 ,  106 A, and  106 B, so, in the case of a magnetic bearing in a five axis control, ten amplifier circuits  150  are provided in the control device. The respective amplifier circuits  150  are each constituted of a bridged circuit which is composed of two transistors  161  and  162  and two diodes  165  and  166  as shown in  FIG. 11 , so twenty transistors and twenty diodes are required for driving, through excitation, all the electromagnet coils  151 ,  151 , . . . . 
   As a result, the amplifier circuit  150  is composed of a large number of elements, so it is difficult to reduce the size of the amplifier circuit  150 , and it is also difficult to reduce the size of the entirety of the turbo molecular pump. In view of this, a large space is required when the turbo molecular pump is installed in the clean room or the like, so there is a fear of increasing the costs of installation. Also, there is a fear of increasing the failure rate because the number of elements constituting the amplifier circuit  150  is increased. In addition, there is a fear of increasing power consumption and heat generation within the amplifier circuit  150 . Further, there is a fear of increasing the manufacturing costs or the like of the amplifier circuit  150  itself because of the increase in the number of the elements. 
   In addition, the electromagnet coil  151  is an element provided to the turbo molecular pump main body  100  side as shown in  FIG. 11 , so nodes at both ends  151   a  and  151   b  (these nodes are respectively referred to as “node R” and “node S”) of the electromagnet coil  151  are wirings constituting a cable between the control device and the turbo molecular pump main body  100 . Considering that ten amplifier circuits  150  are provided in the control device, twenty wirings serving as nodes R and S are assumed to be provided in the cable between the control device and the turbo molecular pump main body  100 . As a result, it is necessary to increase the number of cores for the cable between the control device and the turbo molecular pump main body  100 , or it is necessary to increase the size of a connector (not shown) serving as a port of the cable at the turbo molecular pump main body  100  side, so there is a fear that the respective costs of parts are increased. 
   Further, in control of a conventional amplifier circuit  150 , the electromagnetic current iL is always increased or decreased within the control cycle Ts (that is, not constant) as shown in  FIG. 12 . Thus, the electromagnetic current iL is in a transient state at a detection timing Td for detecting the electromagnetic current iL. In view of this, when only a small gap or the like is generated between the detection timing Td and a waveform of an actual electromagnetic current iL, there is a fear of generating a large error with respect to the value of the electromagnetic current iL that is intended to be detected. Further, when the increase and decrease of the electromagnetic current iL is switched in the vicinity of the detection timing Td, noise may be generated within the amplifier circuit  150 , or noise may be allowed to overlap the positive electrode  153   a  and the negative electrode  153   b  of the power source  153 , which leads to a fear of generating the detection error. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide a magnetic bearing device capable of reducing the costs required for the manufacture, installation, or the like of a turbo molecular pump by reducing the number of elements of an amplifier circuit that drives electromagnets through excitation, and capable of reducing an error at the time of detecting a current flowing through the electromagnets, and a turbo molecular pump with the magnetic bearing device mounted thereto. 
   Accordingly, the present invention relates to a magnetic bearing device including: a rotor; a plurality of electromagnets for controlling a radial position and/or an axial position of the rotor; a power source for supplying power to the electromagnets; a common node commonly connected to each one end of the electromagnets; switch means for switching a voltage of the common node; and excitation control means for controlling excitation of each of the electromagnets by a supply current supplied from the other end of one of the electromagnets to a negative electrode of the power source, or by a regenerated current regenerated from the other end of one of the electromagnets to a positive electrode of the power source, in which the switch means includes: a first switch element for connecting and disconnecting between the positive electrode and the common node; and a first rectifier element for causing a current to flow from the negative electrode to the common node, and the excitation control means includes: a second switch element for connecting and disconnecting between the other end of one of the electromagnets and the negative electrode; and a second rectifier element for causing a current to flow from the other end of one of the electromagnets to the positive electrode. 
   When the second switch element of the excitation control means is connected and the first switch element of the switch means is connected, the supply current is caused to flow, thereby increasing the electromagnetic current. On the other hand, when the second switch element is disconnected and the first switch element is disconnected, the regenerated current is caused to flow, thereby decreasing the electromagnetic current. In view of this, even when the excitation control means is merely composed of one switch element and one rectifier element, the excitation control means is controlled while performing control of the switch means, thereby making it possible to control excitation of the electromagnets by increasing or decreasing the electromagnetic current. 
   With the elements that constitute the excitation control means reduced in number, the failure rate of the magnetic bearing device having the excitation control means can be lowered. In addition, power consumption or heat generation in the magnetic bearing device can be also reduced. Further, since the excitation control means is not a so-called regulator circuit, there is no need to provide a capacitor for stabilization to the common node, and a choking coil or the like for protection, thereby making it also possible to reduce the respective costs of the parts or the like. 
   It should be noted that, when the second switch element is connected and the first switch element is disconnected, or when the second switch element is disconnected and the first switch element is connected, a flywheel current is caused to flow from the other end of the electromagnet to the positive electrode or to the negative electrode, thereby making it possible to maintain the electromagnetic current to be constant. 
   Further, the present invention relates to a magnetic bearing device including: a rotor; a plurality of electromagnets for controlling a radial position and/or an axial position of the rotor; a power source for supplying power to the electromagnets; a common node commonly connected to each one end of the electromagnets; switch means for switching a voltage of the common node; and excitation control means for controlling excitation of each of the electromagnets by a supply current supplied from a positive electrode of the power source to the other end of one of the electromagnets, or by a regenerated current regenerated from a negative electrode of the power source to the other end of one of the electromagnets, in which the switch means includes: a first switch element for connecting and disconnecting between the common node and the negative electrode; and a first rectifier element for causing a current to flow from the common node to the positive electrode, and the excitation control means includes: a second switch element for connecting and disconnecting between the positive electrode and the other end of one of the electromagnets; and a second rectifier element for causing a current to flow from the negative electrode to the other end of one of the electromagnets. 
   When the second switch element of the excitation control means is connected and the first switch element of the switch means is connected, the supply current is caused to flow, thereby increasing the electromagnetic current. On the other hand, when the second switch element is disconnected and the first switch element is disconnected, the regenerated current is caused to flow, thereby decreasing the electromagnetic current. 
   In view of this, even when the excitation control means is merely composed of one switch element and one rectifier element, it is possible to control excitation of the electromagnet by increasing or decreasing the electromagnetic current. Accordingly, it becomes possible to select the excitation control means and the switch means that are easy to be designed and controlled. 
   It should be noted that, also in this case, when the second switch element is connected and the first switch element is disconnected, or when the second switch element is disconnected and the first switch element is connected, a flywheel current is caused to flow from the other end of the electromagnet to the positive electrode or to the negative electrode, thereby making it possible to maintain the electromagnetic current to be constant. 
   Further, the present invention relates to the magnetic bearing device characterized in that the current caused to flow through each of the electromagnets is increased, decreased, or maintained to be constant by adjusting a switching phase of the switch means and a control phase of the excitation control means within a common control cycle. 
   The switching phase of the switch means and the control phase of the excitation control means are adjusted within the common control cycle, thereby making it possible to constitute the circuit with less elements and with ease. In addition, any one of the supply current, the regenerated current, and the flywheel current can be caused to flow through the electromagnet within the control cycle. As a result, the current flowing through the electromagnet can be increased, decreased, or maintained to be constant. 
   Further, the present invention relates to the magnetic bearing device characterized in that the first rectifier element includes a third switch element connected in parallel therewith. 
   The third switch element is connected when a current is caused to flow through the first rectifier element of the switch means, thereby making it possible to suppress the heat generated in the first rectifier element. 
   Further, the present invention relates to a magnetic bearing device including: a rotor; a plurality of electromagnets for controlling a radial position and/or an axial position of the rotor; a power source for supplying power to the electromagnets; a common node commonly connected to each one end of the electromagnets; switch means for switching a voltage of the common node; a first excitation control means for controlling excitation of at least one of the plurality of electromagnets by a supply current supplied from the other end of one of the electromagnets to a negative electrode of the power source, or by a regenerated current regenerated from the other end of one of the electromagnets to a positive electrode of the power source; and a second excitation control means for controlling excitation of electromagnets other than the at least one electromagnet controlled through excitation by the first excitation control means, by a supply current supplied from the positive electrode to the other end of another one of the electromagnets, or by a regenerated current regenerated from the negative electrode to the other end of the another one of the electromagnets, in which the switch means includes: a switch element for connecting and disconnecting between the common node and the negative electrode, and a switch element for connecting and disconnecting between the positive electrode and the common node; and a rectifier element for causing a current to flow from the common node to the positive electrode, and causing a current to flow from the negative electrode to the common node, respectively, the first excitation control means includes: a switch element for connecting and disconnecting between the other end of one of the electromagnets and the negative electrode; and a rectifier element for causing a current to flow from the other end of the one of the electromagnets to the positive electrode, and the second excitation control means includes: a switch element for connecting and disconnecting between the positive electrode and the other end of another one of the electromagnets; and a rectifier element for causing a current from the negative electrode to the other end of the another one of the electromagnets. 
   The switch means includes a switch element for connecting and disconnecting between the common node and the negative electrode and between the positive electrode and the common node, and a rectifier element for causing a current to flow from the common node to the positive electrode, and from the negative electrode to the common node. With such the structure, even when the plurality of electromagnets are divided into electromagnets controlled by the first excitation control means, and such magnets controlled by the second excitation control means, it is possible to control excitation of the electromagnets by increasing or decreasing the electromagnetic current. 
   It should be noted that, in the switch means, it is preferable to connect or disconnect the switch element so that the connection between the common node and the negative electrode does not overlap the connection between the positive electrode and the common node. As a result, a through current flowing from the positive electrode to the negative electrode can be prevented. 
   Further, the present invention relates to the magnetic bearing device characterized in that the current caused to flow through each of the electromagnets is increased, decreased, or maintained to be constant by adjusting a switching phase of the switch means and control phases of the first excitation control means and the second excitation control means within a common control cycle. 
   The switching phase of the switch means and the control phases of the first excitation control means and the second excitation control means are adjusted within the common control cycle, thereby making it possible to constitute the circuit with less elements and with ease. In addition, any one of the supply current, the regenerated current, and the flywheel current can be caused to flow through the electromagnet within the control cycle. 
   Further, the present invention relates to the magnetic bearing device characterized in that the plurality of electromagnets are constituted by being divided into two groups, one controlled by the first excitation control means and the other controlled by the second excitation control means so that the current caused to flow between the positive electrode and the common node and the current caused to flow between the common node and the negative electrode are made substantially equalized. 
   The plurality of electromagnets are divided into groups so that the electromagnetic currents flowing through the common node are substantially equalized, thereby making it possible to reduce the size of the switch element constituting the switch means or that of the rectifier element. In addition, currents flowing through those elements can also be reduced, so it is possible to prevent heat generation or the like. Further, the current to be supplied from the power source can also be reduced, so it is possible to reduce an input power supply capacity. 
   Further, the present invention relates to the magnetic bearing device, characterized by further including current detecting means for detecting a value of the current when a constant current is caused to flow through the electromagnets. 
   It is possible to maintain the electromagnetic current to be constant by causing a flywheel current to flow through the electromagnet, so the current detecting means detects the electromagnetic current at this time. 
   Accordingly, there is no need to detect the electromagnetic current in the transient state, so even when a deviation or the like is caused between the detection timing and the actual waveform of the electromagnetic current, a large error is not caused with respect to the value of the electromagnetic current that is intended to be detected. Further, it is possible to prevent switching between the increase and decrease of the electromagnetic current at about the detection timing, thereby making it possible to reduce noise generated in the excitation control means, the positive electrode, or the negative electrode, to reduce detection errors. 
   Further, the present invention relates to the magnetic bearing device characterized in that the current detecting means includes a resistance having one end connected to the negative electrode, and a detection portion for detecting a current flowing through the resistance. 
   As a result, a high voltage is not inputted to the current detecting means, so noise is not likely to be allowed to overlap when the electromagnetic current is detected, and the electromagnetic current can be detected with accuracy. 
   Further, the present invention relate to a turbo molecular pump including the magnetic bearing device mounted thereto characterized in that: the rotor has rotary vanes and a rotor shaft placed at the center of the rotary vanes; and each of the electromagnets levitates the rotor shaft by a magnetic force. 
   The above-mentioned magnetic bearing device is mounted on the turbo molecular pump, so the entirety of the turbo molecular pump can be reduced in size. Therefore, it is possible to reduce the costs for installing the turbo molecular pump in a clean room or the like. 
   Further, the present invention relates to the turbo molecular pump, characterized in that the turbo molecular pump includes: a turbo molecular pump main body having the rotor and the electromagnets; and a control device having the switch means and the excitation control means, or the switch means, the first excitation control means, and the second excitation control means, and in that: the turbo molecular pump main body and the control device are integrated into one. 
   The above-mentioned excitation control means, or the first excitation control means and the second excitation control means can be reduced in size, so it is also possible to reduce the size of the control device including the excitation control means or the like Accordingly, the control device and the turbo molecular pump main body can be integrated in to one, there by making it possible to further reduce the costs of the manufacture, installation, or the like of the turbo molecular pump. 
   EFFECTS OF THE INVENTION 
   According to the present invention, as described above, the magnetic bearing device is composed of the switch means for switching the voltage of the common node, and the excitation control means for controlling excitation of each of the plurality of electromagnets by the supply current or the regenerated current. Accordingly, even when the excitation control means is merely composed of one switch element and one rectifier element, the excitation control means is controlled while performing control of the switch means, thereby making it possible to control excitation of the electromagnets by increasing or decreasing the electromagnetic current. Therefore, the elements of the excitation control means can be reduced in number, thereby making it possible to reduce the costs of manufacture, installation, or the like of the turbo molecular pump. 
   Further, the magnetic bearing device includes the current detecting means for detecting the value of the current when the constant current flows through the electromagnet, so there is no need to detect the value of the electromagnetic current in the transient state, thereby making it possible to reduce the error in detecting the value. 

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, description will be made of a first embodiment of the present invention. 
     FIG. 1  shows a circuit diagram of an amplifier circuit according to the first embodiment of the present invention. Note that components identical with those of  FIG. 11  are denoted by the same reference symbols, and descriptions thereof are omitted. 
   In  FIG. 1 , a turbo molecular pump main body  200  has the electromagnet coils  151 ,  151 , . . . which respectively constitute electromagnets  104 ,  105 ,  106 A, and  106 B, and are provided with a common node (which will be referred to as “common node C”). One end  151   a  of each electromagnet coil  151  is connected to the common node C. The other end  151   b  of the electromagnet coil  151  is connected to a transistor  261  and a diode  265  that compose an amplifier circuit  250  (note that a node at the other end  151   b  will be referred to as “node E”) 
   Herein, the transistor  261  is a power MOSFET, and has a drain terminal  261   a  connected to the other end  151   b  of the electromagnet coil  151  and a source terminal  261   b  connected to the negative electrode  153   b  of the power source  153  through a current detecting circuit  255 . A diode  265  is one for current regeneration or a flywheel diode, and has a cathode terminal  265   a  connected to a positive electrode  153   a  of the power source  153  and an anode terminal  265   b  connected to the other terminal  151   b  of the electromagnet coil  151 . 
   The current detecting circuit  255  connected to the source terminal  261   b  of the transistor  261  has a detection resistor  256  which is connected at one end to the negative electrode  153   b  and at the other end to the source terminal  261   b  of the transistor  261 , and a detector portion  257  for detecting the electromagnetic current iL from voltage at the other end of the detection resistor  256 . The detector portion  257  is structured to detect the electromagnetic current iL flowing through the electromagnet coil  151  and output a current detection signal  273  as a detection result to an amplifier control circuit  271  to be described later. Note that by using the current detecting circuit  255  thus having the detection resistor  256  connected at the one end to the negative electrode  153   b , the current detecting circuit  255  does not receive an input of high voltage, so noise hardly develops upon detection of the electromagnetic current iL, allowing the electromagnetic current iL to be detected with accuracy. 
   The amplifier circuit  250  structured as described above is provided to each of the electromagnet coils  151 ,  151 , . . . which respectively constitute the electromagnets  104 ,  105 ,  106 A, and  106 B. 
   The amplifier control circuit  271  is provided within a DSP portion (not shown) similarly to the conventional art. The amplifier control circuit  271  compares a value of the electromagnetic current iL detected by the current detecting circuit  255  and a current command value, and determines a time (the above-mentioned increasing time Tp 1 ) for increasing the electromagnetic current iL and a time (the above-mentioned decreasing time Tp 2 ) for decreasing the electromagnetic current iL. Based on those times, the amplifier control circuit  271  determines a pulse width time of a gate drive signal  274  outputted to a gate terminal of the transistor  261  within the control cycle Ts corresponding to one cycle performed by PWM control. Note that in outputting the gate drive signal  274 , a signal outputted from the amplifier control circuit  271  may be passed through a Field Programable Gate Array (not shown; hereinafter, referred to as “FPGA”) before the gate drive signal  274  is outputted to the transistor  261 , thereby allowing a fast speed operation. 
   Further, in  FIG. 1 , a common node C of the amplifier circuit  250  is connected to a switch circuit  280 . In the switch circuit  280 , the common node C is connected to a transistor  281  and a diode  285 . 
   The diode  285  is one for current regeneration or a flywheel diode, and has a cathode terminal  285   a  connected to the common node C and an anode terminal  285   b  connected to the negative electrode  153   b  of the power source  153  shared by the amplifier circuit  250 . The transistor  281  is a power MOSFET, and has a drain terminal  281   a  connected to the positive electrode  153   a  of the power source  153  and a source terminal  281   b  connected to the common node C. The transistor  281  has a gate terminal to which a switch signal  276  is outputted from the amplifier control circuit  271 . The amplifier control circuit  271  is structured to determine a pulse width time of the switch signal  276  outputted to the gate terminal of the transistor  281  within the same control cycle Ts as performed by the control on the amplifier circuit  250 . 
   In the above-mentioned structure, when the transistor  261  of the amplifier circuit  250  and the transistor  281  of the switch circuit  280  are respectively turned on, a current is made to flow from the positive electrode  153   a  to the negative electrode  153   b  through the transistor  281 , the common node C, the electromagnet coil  151 , and the transistor  261  (and the current detecting circuit  255 ). Accordingly, the current is supplied to the electromagnet coil  151  from the positive electrode  153   a  of the power source  153 , which increases the electromagnetic current iL (this state will be referred to as “increasing mode A 1 ”). 
   On the other hand, when the transistor  261  of the amplifier circuit  250  and the transistor  281  of the switch circuit  280  are respectively turned off, a regenerated current is made to flow from the negative electrode  153   b  to the positive electrode  153   a  through the diode  285 , the common node C, the electromagnet coil  151 , and the diode  265  due to a counter electromotive force caused by the electromagnet coil  151 . Accordingly, electromagnetic energy generated from the electromagnet coil  151  is consumed, which decreases the electromagnetic current iL (this state will be referred to as “decreasing mode A 2 ”). 
   Further, when the transistor  261  of the amplifier circuit  250  is turned on and the transistor  281  of the switch circuit  280  is turned off, a flywheel current is made to flow from the negative electrode  153   b  to the negative electrode  153   b  through the diode  285 , the common node C, the electromagnet coil  151 , and the transistor  261  (and the current detecting circuit  255 ) due to the counter electromotive force caused by the electromagnet coil  151 . At this time, an electric potential difference does not occur between the both ends  151   a  and  151   b  of the electromagnet coil  151 , which maintains the substantially constant electromagnetic current iL (this state will be referred to as “constant mode A 3 ”). 
   Further, even in the case other than the constant mode A 3 , when the transistor  261  of the amplifier circuit  250  is turned off and the transistor  281  of the switch circuit  280  is turned on, a flywheel current is made to flow from the positive electrode  153   a  to the positive electrode  153   a  through the transistor  281 , the common node C, the electromagnet coil  151 , and the diode  265  due to the counter electromotive force caused by the electromagnet coil  151 . Accordingly, the substantially constant electromagnetic current iL is maintained also in this case (this state will be referred to as “constant mode A 4 ”). 
   Herein,  FIG. 2  is a time chart showing how adjustment is made between control phases of the amplifier circuit  250  with respect to the transistor  261  etc. and switch phases of the switch circuit  280  with respect to the transistor  281  etc. 
   In  FIG. 2 , control is performed with respect to the switch circuit  280  such that the time during which the transistor  281  is kept turned on and the time during which the transistor  281  is kept turned off within the control cycle Ts are the same. Herein, during the time from a time (time  0 ) at the beginning of the control cycle Ts to a time (time 0.5 Ts) at a midpoint of the control cycle Ts, the transistor  281  is kept turned off. Therefore, the voltage of the common node C becomes substantially the same voltage (hereinafter, referred to as “voltage VL”) as that of the negative electrode  153   b  due to the counter electromotive force or the like generated by the electromagnetic coil  151 . On the other hand, during the time from the time (time 0.5 Ts) at the midpoint of the control cycle Ts to an end (time Ts) of the control cycle Ts, the transistor  281  is kept turned on. Thus, the voltage of the common node C becomes substantially the same voltage (hereinafter referred to as “voltage VH”) as that of the positive electrode  153   a.    
   In the case where the value of the electromagnetic current iL detected by the current detecting circuit  255  is smaller than the current command value, control is performed such that the electromagnetic current iL is increased in the amplifier control circuit  271 . In this case, control is performed such that the state of the increasing mode A 1  is maintained only during the increasing time Tp 1  described above in one control cycle Ts. In other times, control is performed such that the state of the constant mode A 3  or A 4  is maintained. To be specific, during the time from the time 0.5 Ts to the time Ts, the transistor  281  of the switch circuit  280  is kept turned on, so with the time 0.5 Ts as a starting point, the transistor  261  is kept turned on only during the time Tp 1 , thereby setting the state of the increasing mode A 1  only for the increasing time Tp 1 . Further, after the time Tp 1  has elapsed, the transistor  261  is turned off to thereby set the state of the constant mode A 4 . On the other hand, during the time from the time  0  to the time 0.5 Ts, the transistor  281  of the switch circuit  280  is kept turned off (that is, the state of the increasing mode A 1  cannot be set), so the transistor  261  is turned on, thereby setting the state of the constant mode A 3 . As a result, the electromagnetic current iL is increased in one control cycle Ts only during the increasing time Tp 1 . 
   On the other hand, in the case where the value of the electromagnetic current iL detected in the current detecting circuit  255  is larger than the current command value, control is performed such that the electromagnetic current iL is decreased in the amplifier control circuit  271 . In this case, control is performed such that the state of the decreasing mode A 2  is maintained only during the above-mentioned decreasing time Tp 2  in one control cycle Ts. In other times, control is performed such that the state of the constant mode A 3  or A 4  is maintained. To be specific, during the time from the time  0  to the time 0.5 Ts, the transistor  281  of the switch circuit  280  is kept turned off, so until the time 0.5 Ts as an end point, the transistor  261  is kept turned off only during the time Tp 2 , thereby setting the state of the decreasing mode A 2  only for the decreasing time Tp 2 . Until the transistor  261  is turned off, the transistor  261  is kept turned on, thereby setting the state of the constant mode A 3 . On the other hand, during the time from the time 0.5 Ts to the time Ts, the transistor  281  of the switch circuit  280  is kept turned on (that is, the state of the decreasing mode A 2  cannot be set), so the transistor  261  is turned off, thereby setting the state of the constant mode A 4 . As a result, the electromagnetic current iL is decreased in one control cycle Ts only during the decreasing time Tp 2 . 
   Further, in the case where the value of the electromagnetic current iL detected in the current detecting circuit  255  coincides with the current command value, control is performed such that the electromagnetic current iL is kept constant in the amplifier control circuit  271 . In this case, control is performed such that the state of the constant mode A 3  or A 4  is always maintained in one control cycle Ts. To be specific, during the time from the time  0  to the time 0.5 Ts, the transistor  281  of the switch circuit  280  is kept turned off, so the transistor  261  is turned on, thereby setting the state of the constant mode A 3 . On the other hand, during the time from the time 0.5 Ts to the time Ts, the transistor  281  of the switch circuit  280  is kept turned on, so the transistor  261  is turned off, thereby setting the state of the constant mode A 4 . As a result, the electromagnetic current iL is kept constant. 
   According to the above structure, even when the amplifier circuit  250  is composed of only one transistor  261  and one diode  265 , an increase, decrease, or constant state of the electromagnetic current iL can thus be maintained by controlling the amplifier circuit  250  while controlling the switch circuit  280 . Thus, the value of the electromagnetic current iL can be made to coincide with the current command value. Due to such the structure of the amplifier circuit  250 , the elements of the amplifier circuit  250  are reduced in number, thereby making it possible to miniaturize the turbo molecular pump as a whole to reduce the costs for installing the turbo molecular pump in a clean room or the like. Further, the elements of the amplifier circuit  250  are reduced in number, so it is possible to decrease a failure rate of the amplifier circuit  250 , and power consumption and heat generation therein. Still further, the manufacture costs of the amplifier circuit  250  can also be reduced. 
   Unlike the control with respect to the conventional amplifier circuit  150 , in control of the amplifier circuit  250  of this embodiment, the electromagnetic current iL can be maintained constant. Therefore, ripples of the current flowing through the common node C can be reduced, thereby making it possible to reduce power consumption and heat generation in the amplifier circuit  250  and the switch circuit  280 . 
   Further, the switch circuit  280  of the present invention is not a so-called regulator circuit (that is, not such a circuit as to maintain the voltage of the common node C constant) Therefore, it is not required to provide a capacitor (not shown) for stabilization, a choking coil (not shown) for protection, or the like to the common node C. Thus, the costs of parts can be reduced. 
   Further, wirings between the amplifier circuit  250  and the electromagnet coils  151  include only one common node C and 10 nodes E of the other ends  151   b  of the electromagnet coils  151 . Therefore, there are provided only 11 wirings as the common node C and the nodes E (conventionally, 20 wirings are required) Accordingly, it is possible to reduce the costs of cables between the control device and the turbo molecular pump main body  200 , and the costs of a connector (not shown) of the turbo molecular pump main body  200 . Therefore, the costs of parts can be reduced. Further, miniaturization of the amplifier circuit  250  leads to miniaturization of the control device (not shown) itself, so a function of the control device can easily be incorporated into the turbo molecular pump main body  200  side. Accordingly, the control device and the turbo molecular pump main body  200  can be integrated with each other. 
   In addition, also in the amplifier circuit  250  of this embodiment, as shown in  FIG. 2 , detection of the electromagnetic current iL is performed at the same detection timing Td once in the control cycle Ts. However, in the amplifier circuit  250  of this embodiment, the electromagnetic current iL can be maintained constant. Thus, while the electromagnetic current iL is maintained constant (that is, while the state of the constant mode A 3  is maintained), detection of the electromagnetic current iL can be performed (note that in the constant mode A 4 , it is impossible to perform detection of the current because the electromagnetic currentic iL is not supplied to the current detecting circuit  255 ). 
   Accordingly, it is not required to perform detection of the electromagnetic current iL in a transient state. Therefore, even when a deviation is caused between the detection timing Td and a waveform of the electromagnetic current iL, a large error is not caused with respect to the value of the electromagnetic current iL that is intended to be detected. Switching between increase and decrease of the electromagnetic current iL at about the detection timing Td can be avoided, thereby making it possible to reduce noise generated in the amplifier circuit  250  or the power source  153  to reduce a detection error. 
   Note that in this embodiment, it is described that the switch circuit  280  is composed of the transistor  281  and the diode  285 , but this is not obligatory. For example, as shown in  FIG. 3 , in addition to the structure described above, there may be provided a transistor  282  having a drain terminal  282   a  and a source terminal  282   b  connected to the common node C and the negative electrode  153   b , respectively. With this structure, a switch signal  277  is outputted from the amplifier control circuit  271  to a gate terminal of the transistor  282  to perform control so that the transistor  282  is turned on when a current flows through a diode  285  in the state of the decreasing mode A 2  or the constant mode A 3  (that is, control is performed by a synchronous rectification method), thereby making it possible to suppress heat generation of the diode  285  in the above-mentioned mode. 
   Further, in this embodiment, the increasing time Tp 1  and decreasing time, Tp 2  are provided such that the time 0.5 Ts is set as the starting point or end point. Alternatively, the increasing time Tp 1  and decreasing time Tp 2  may be provided such that the time Ts is set as the endpoint or the time  0  is set as the starting point. 
   Further, this embodiment has described that in the case where the electromagnetic current iL is in a constant state (i.e., in the state of the constant mode A 3 ), the electromagnetic current iL is detected. In a more specific manner, the following may be performed. In other words, control may be performed such that the state of the constant mode A 3  is forcefully maintained with respect to the amplifier circuit  250  and switch circuit  280  in the control cycle Ts, thereby detecting the electromagnetic current iL in this period. In this case, the time during which the state of the constant mode A 3  is forcefully maintained may be the time during which the electromagnetic current iL can be detected in the current detecting circuit  255 . For example, as shown in  FIG. 4 , the time is set to be a time from the time  0  to the time 0.1 Ts in the control cycle Ts. Then, a detection timing Td is set within a range between the time  0  and the time 0.1 Ts, thereby detecting the electromagnetic current iL. After that, during the remaining time (from the time 0.1 Ts to the time Ts), like in the control described above, for example, during the time from the time 0.1 Ts to the time 0.55 Ts (during the former half of the remaining time), the transistor  281  may be kept turned on, and during the time from the time 0.55 Ts to the time Ts (the latter half of the remaining time), the transistor  281  may be kept turned off to set the time Tp 1  and the time Tp 2  with the time 0.55 Ts (half the time of the remaining time) being set as the starting point or the endpoint. As a result, the detection of the electromagnetic current iL can be reliably performed in the state of the constant mode A 3 . 
   Next, description will be made of a second embodiment of the present invention. The second embodiment is another example of the amplifier circuit  250  and switch circuit  280  of the first embodiment. 
     FIG. 5  shows a circuit diagram of an amplifier circuit according to the second embodiment of the present invention. Note that components identical with those of  FIG. 1  are denoted by the same reference symbols, and descriptions thereof are omitted. 
   In  FIG. 5 , in an amplifier circuit  350 , one end  151   a  of each electromagnet coil  151  is connected to the common node C. The other end  151   b  of the electromagnet coil  151  is connected to a transistor  361  and a diode  365  (note that a node at the other end  151   b  will be referred to as “node F”). 
   Herein, the transistor  361  is a power MOSFET, and has a drain terminal  361   a  connected to the positive electrode  153   a  of the power source  153  and a source terminal  361   b  connected to the other terminal  151   b  of the electromagnet coil  151 . The diode  365  is one for current regeneration or a flywheel diode, and has a cathode terminal  365   a  connected to the other terminal  151   b  of the electromagnet coil  151  and an anode terminal  365   b  connected to the negative electrode  153   b  of the power source  153  through the current detecting circuit  255 . 
   Further, the common node C of the amplifier circuit  350  is connected to a switch circuit  380 . In the switch circuit  380 , the common node C is connected to a transistor  381  and a diode  385 . 
   The transistor  381  is a power MOSFET, and has a drain terminal  381   a  connected to the common node C and a source terminal  381   b  connected to the negative electrode  153   b  of the power source  153  shared by the amplifier circuit  350 . The transistor  381  has a gate terminal to which a switch signal  376  is inputted from an amplifier control circuit  371 . The diode  385  is one for current regeneration or a flywheel diode, and has a cathode terminal  385   a  connected to the positive electrode  153   a  of the power source  153  and an anode terminal  385   b  connected to the common node C. 
   With such the structure, when the transistor  361  of the amplifier circuit  350  is turned on and the transistor  381  of the switch circuit  380  is turned on, a current is supplied from the positive electrode  153   a  to the negative electrode  153   b  through the transistor  361 , the electromagnet coil  151 , the common node C, and the transistor  381 . Therefore, the current is supplied to the electromagnet coil  151  from the positive electrode  153   a  of the power source  153 , which increases the electromagnetic current iL (this state will be referred to as “increasing mode B 1 ”) 
   On the other hand, when the transistor  361  of the amplifier circuit  350  is turned off and the transistor  381  of the switch circuit  380  is turned off, due to the counter electromotive force generated by the electromagnetic coil  151 , a regenerated current is made to flow from the negative electrode  153   b  to the positive electrode  153   a  through (the current detecting circuit  255  and) the diode  365 , the electromagnet coil  151 , the common node C, and the diode  385 . As a result, electromagnetic energy generated from the electromagnet coil  151  is consumed, which decreases the electromagnetic current iL (this state will be referred to as “decreasing mode B 2 ”). 
   Further, when the transistor  361  of the amplifier circuit  350  is turned off and the transistor  381  of the switch circuit  380  is turned on, due to the counter electromotive force generated by the electromagnetic coil  151 , a flywheel current is made to flow from the negative electrode  153   b  to the negative electrode  153   b  through (the current detecting circuit  255  and) the diode  365 , the electromagnet coil  151 , the common node C, and the transistor  381 . At this time, no electric potential difference is generated between the ends  151   a  and  151   b  of the electromagnet coil  151 , so the electromagnetic current iL is kept substantially constant (this state will be referred to as “constant mode B 3 ”). 
   Further, in the case other than the constant mode B 3 , when the transistor  361  of the amplifier circuit  350  is turned on and the transistor  381  of the switch circuit  380  is turned off, due to the counter electromotive force generated by the electromagnetic coil  151 , a flywheel current is made to flow from the positive electrode  153   a  to the, positive electrode  153   a  through the transistor  361 , the electromagnet coil  151 , the common node C, and the diode  385 . Therefore, also in this case, the electromagnetic current iL is kept substantially constant (this state will be referred to as “constant mode B 4 ”). 
   Herein,  FIG. 6  is a time chart showing how adjustment is made between control phases of the amplifier circuit  350  with respect to the transistor  361  etc. and switch phases of the switch circuit  380  with respect to the transistor  381  etc. 
   In  FIG. 6 , also in this embodiment, control is performed with respect to the switch circuit  380  such that the time during which the transistor  381  is kept turned on and the time during which the transistor  381  is kept turned off are made to be the same within the control cycle Ts. Herein, during the time from the time  0  to the time 0.5 Ts, the transistor  381  is kept turned on and the voltage of the common node C is changed to the voltage VL substantially the same as that of the negative electrode  153   b . During the time from the time 0.5 Ts to the time Ts, due to the counter electromotive force or the like generated by the electromagnetic coils  151 , the voltage of the common node C is changed to the voltage VH substantially the same as that of the positive electrode  153   a . The transition of the common node C is the same as that of the first embodiment ( FIG. 2 ). 
   In the case where the value of the electromagnetic current iL detected by the current detecting circuit  255  is smaller than the current command value, control is performed such that the electromagnetic current iL is increased in the amplifier control circuit  371 . In this case, control is performed such that the state of the increasing mode B 1  is maintained for the increasing time Tp 1  in one control cycle Ts. In other times, control is performed such that the state of the constant mode B 3  or B 4  is maintained. To be specific, during the time from the time  0  to the time 0.5 Ts, the transistor  381  of the switch circuit  380  is kept turned on, so until the time 0.5 Ts as an endpoint, the transistor  361  is turned on for the time Tp 1 , thereby maintaining the state of the increasing mode B 1  only for the increasing time Tp 1 . Until the transistor  361  is turned on, the transistor  361  is kept turned off to thereby maintain the state of the constant mode B 3 . On the other hand, during the time from the time 0.5 Ts to the time Ts, the transistor  381  of the switch circuit  380  is kept turned off, so the transistor  361  is turned on, thereby maintaining the state of the constant mode B 4 . As a result, in one control cycle Ts, the electromagnetic current iL is increased only during the increasing time Tp 1 . 
   On the other hand, in the case where the value of the electromagnetic current iL detected by the current detecting circuit  255  is larger than the current command value, control is performed such that the electromagnetic current iL is decreased in the amplifier control circuit  371 . In this case, control is performed such that the state of the decreasing mode B 2  is maintained for the decreasing time Tp 2  in one control cycle Ts. In other times, control is performed such that the state of the constant mode B 3  or B 4  is maintained. To be specific, during the time from the time 0.5 Ts to the time Ts, the transistor  381  of the switch circuit  380  is kept turned off, so with the time 0.5 Ts as a starting point, the transistor  361  is kept turned off for the time Tp 2 , thereby maintaining the state of the decreasing mode B 2  only for the decreasing time Tp 2 . After the time Tp 2  has elapsed, the transistor  361  is turned on, thereby maintaining the state of the constant mode B 4 . On the other hand, during the time from the time  0  to the time 0.5 Ts, the transistor  381  of the switch circuit  380  is kept turned on, so the transistor  361  is turned off, thereby setting the state of the constant mode B 3 . As a result, in one control cycle Ts, the electromagnetic current iL is decreased only during the decreasing time Tp 2 . 
   Further, in the case where the value of the electromagnetic current iL detected by the current detecting circuit  255  coincides with the current command value, control is performed such that the electromagnetic current iL is kept constant in the amplifier control circuit  371 . In this case, control is performed such that the state of the constant mode B 3  or B 4  is always maintained in one control cycle Ts. To be specific, during the time from the time  0  to the time 0.5 Ts, the transistor  381  of the switch circuit  380  is kept turned on, so the transistor  361  is turned off, thereby setting the state of the constant mode B 3 . On the other hand, during the time from the time 0.5 Ts to the time Ts, the transistor  381  of the switch circuit  380  is kept turned off, so the transistor  361  is turned on, thereby setting the state of the constant mode B 4 . As a result, the electromagnetic current iL is kept constant. 
   As described above, an increasing, decreasing, or constant state of the electromagnetic current iL can thus be maintained also in the amplifier circuit  350  and the switch circuit  380  which are different from those of the first embodiment ( FIG. 1 ). The amplifier circuit  350  is also composed of one transistor  361  and one diode  365 . Therefore, the elements of the amplifier circuit  350  can be reduced in number, thereby making it possible to reduce the costs of manufacture, installation, or the like of the turbo molecular pump. Accordingly, the amplifier circuits  250  and  350  that are easy to design can be chosen and a structure that is easy to control can be chosen in controlling the amplifier circuits  250  and  350 . 
   In addition, the amplifier circuit  350  of this embodiment can keep the electromagnetic current iL constant as in the case of the first embodiment. Therefore, when the electromagnetic current iL is kept constant (that is, in a state of the constant mode B 3 ), it is possible to perform detection of the electromagnetic current iL. Accordingly, it is not required to perform detection of the electromagnetic current iL in a transient state, so an error in detection of the electromagnetic current iL can be reduced. 
   Note that in this embodiment, it is described that the switch circuit  380  is composed of the transistor  381  and the diode  385 , but this is not obligatory. As shown in  FIG. 7 , there may be provided a transistor  382  having a drain terminal  382   a  and a source terminal  382   b  connected to the positive electrode  153   a  and the common node C, respectively. As a result, a switch signal  377  is outputted to a gate terminal of the transistor  382  to perform control by the synchronous rectification method, thereby making it possible to suppress heat generation of a diode  385  in the decreasing mode B 2  or the constant mode B 4 . 
   Further, also in this embodiment, it is described that when the electromagnetic current iL is constant, detection of the electromagnetic current iL is performed. However, as in the case described in the first embodiment ( FIG. 4 ), it is also possible to perform control for forcefully making a state of the constant mode B 3  in the control cycle Ts with respect to the amplifier circuit  350  and the switch circuit  380  to perform detection of the electromagnetic current iL within this period. As a result, the electromagnetic current iL can be reliably detected in the state of the constant mode B 3 . 
   Next, a third embodiment of the present invention will be described. In the first and second embodiments, the electromagnet coils  151 ,  151 , . . . which respectively constitute the electromagnets  104 ,  105 ,  106 A, and  106 B are controlled by one of the amplifier circuits  250  and  350 . In the third embodiment, the electromagnet coils  151 ,  151 , . . . are appropriately divided into two groups according to arrangements of the electromagnets  104 ,  105 ,  106 A, and  106 B. The groups are respectively controlled by the amplifier circuit  250  ( FIG. 1 ) having the same structure as that of the first embodiment and the amplifier circuit  350  ( FIG. 5 ) having the same structure as that of the second embodiment. 
   A circuit diagram of an amplifier circuit according to the third embodiment of the present invention is shown in  FIG. 8 . Note that the same elements as those of  FIGS. 1 and 5  are denoted by the same reference symbols and the descriptions of those are omitted. 
   In  FIG. 8 , connected to the plurality of electromagnet coils  151 ,  151 , . . . which respectively constitute the electromagnets  104 ,  105 ,  106 A, and  106 B is a combination of the amplifier circuit  250  ( FIG. 1 ) having the same structure as that of the first embodiment and the amplifier circuit  350  ( FIG. 5 ) having the same structure as that of the second embodiment. The plurality of electromagnet coils  151 ,  151 , . . . are divided into two groups (one including the electromagnet coils  151 ,  151 , . . . which are controlled by the amplifier circuit  250  is referred to as “group A” and one including the electromagnet coils  151 ,  151 , . . . which are controlled by the amplifier circuit  350  is referred to as “group B”). 
   Herein, how the grouping is performed is described while taking specific examples. As an example, description is made of the X-axis positive side electromagnet  104 X+ and the X-axis negative side electromagnet  104 X− of the upper radial electromagnets  104 , and the X-axis positive side electromagnet  105 X+ and the X-axis negative side electromagnet  105 X+ of the lower radial electromagnets  105 . 
   For example, when a position of the rotor  103  as a whole is controlled in a + direction of the X axis, the electromagnetic current iL flowing through the electromagnets  104 X+ and  105 X+ is increased and the electromagnetic current iL flowing through the electromagnets  104 X+ and  105 X+ is decreased. On the other hand, when the position of the rotor  103  as a whole is controlled in a—direction of the X axis, the electromagnetic current iL flowing through the electromagnets  104 X+ and  105 X+ is decreased and the electromagnetic current iL flowing through the electromagnets  104 X− and  105 X− is increased. As described above, in many cases, controls with respect to the upper radial electromagnets  104  and the lower radial electromagnets  105  in the X-axis directions are the same. 
   Accordingly, when, for example, the electromagnet  104 X+ is put into the group A, the electromagnet  105 X+ is put into the group B, whereby at the time of increase of the electromagnetic current iL, when a current flows from the common node C to the negative electrode  153   b  in the group A, a current flows from the positive electrode  153   a  to the common node C in the group B. Therefore, the electromagnetic currents iL flowing through the common node C are equalized. The same holds true for the case where the electromagnetic current iL is decreased or the electromagnetic current iL is constant. Further, the same holds true for the case where the electromagnet  104 X+ is put into the group B and the electromagnet  105 X+ is put into the group A. 
   Thus, among the upper radial electromagnet  104  and the lower radial electromagnet  105 , the electromagnet  104 X+ and the electromagnet  105 X+ are divided into different groups. Further, the same holds true for the other electromagnets  104 X− and the electromagnet  105 X−, the electromagnet  104 Y+ and the electromagnet  105 Y+, and the electromagnet  104 Y− and the electromagnet  105 Y− on the Y-axis side, so they are respectively divided into different groups. 
   On the other hand, with regard to a relationship between the X-axis positive side electromagnet  104 X+ and the X-axis negative side electromagnet  104 X−, when the position of the rotor  103  is controlled in the X-axis positive direction, there is a tendency that the electromagnetic current iL of the electromagnet  104 X+ is increased and the electromagnetic current iL of the electromagnet  104 X− is decreased. Therefore, by putting those electromagnets into the same group, the electromagnetic currents iL flowing through the common node C are easy to be equalized. Thus, among the upper radial electromagnets  104  and the lower radial electromagnets  105 , the electromagnet  104 X+ and the electromagnet  104 − are put into the same group. Further, the same holds true for the other electromagnet  104 Y+ and the electromagnet  104 Y−, the electromagnet  105 X+ and the electromagnet  105 X−, the electromagnet  105 Y+ and the electromagnet  105 Y− of the lower radial electromagnets  105 , and the axial electromagnet  106 A and the axial electromagnet  106 B, so they are respectively put into the same group. 
   The ends  151   a ,  151   a , . . . of the electromagnet coils  151 ,  151 , . . . respectively divided into the groups A and B as described above are all connected to the common node C. Further, a switch circuit  480  is connected to the common node C. 
   In the switch circuit  480 , connected to the common node C, is a combination of the transistor  281  and the diode  285  having the same structure as that of the switch circuit  280  of the first embodiment, and the transistor  381  and the diode  385  having the same structure as that of the switch circuit  380  of the second embodiment. Further, the switch signal  276  and the switch signal  376  are respectively outputted to the gate terminals of the transistors  281  and  381  from an amplifier control circuit  471 . The amplifier control circuit  471  has both functions of the amplifier control circuit  271  of the first embodiment and the amplifier control circuit  371  of the second embodiment. 
   According to the above-mentioned structure,  FIG. 9  is a time chart showing how adjustment is made on switch phases by the switch circuit  480  with respect to the transistors  281 ,  381 , etc. 
   In  FIG. 9 , with respect to the switch circuit  480 , control is performed such that a time during which the transistor  381  is kept turned on and a time during which the transistor  281  is kept turned on are made to be the same within the control cycle Ts. Herein, during the time from the time  0  to the time 0.5 Ts, the transistor  281  is kept turned off and the transistor  381  is kept turned on, and from the time 0.5 Ts to a time Ts, the transistor  281  is kept turned on and the transistor  381  is kept turned off. In this case, in order to prevent generation of noise or the like due to a flow of a through current between the positive electrode  153   a  and the negative electrode  153   b , in each of an interval after the transistor  381  is turned off and before the transistor  281  is turned on (around the time 0.5 Ts) and an interval after the transistor  281  is turned off and before the transistor  381  is turned on (around the time  0  and time Ts), it is desirable to provide a dead time in which the both transistors  281  and  381  are turned off (not shown). 
   Due to such the control with respect to the switch circuit  480 , the common node C makes a transition from the voltage VL in the time  0  to the time 0.5 Ts to the voltage VH in the time 0.5 Ts to the time Ts. Accordingly, the transition of the common node C is the same as that of the first embodiment ( FIG. 2 ) and that of the second embodiment ( FIG. 6 ). Thus, with respect to the amplifier circuit  250 , by performing the same control as that described in the first embodiment, the electromagnetic current iL can be increased, decreased, or kept constant. Further, also with respect to the amplifier circuit  350 , by performing the same control as that described in the second embodiment, the electromagnetic current iL can be increased, decreased, or kept constant. As a result, each amplifier circuit  250 ,  350  is composed of one transistor  261 ,  361  and one diode  265 ,  365 , respectively. Therefore, it is possible to reduce the elements of the amplifier circuit  250 ,  350  in number and reduce the costs required for the manufacture, installation, or the like of the turbo molecular pump. 
   Further, also during detection of the electromagnetic current iL, the electromagnetic current iL can be kept constant, so when each amplifier circuit  250 ,  350  is in the constant modes A 3  and B 3 , the detection of the electromagnetic current iL can be performed. Accordingly, in both the amplifier circuits  250  and  350 , it is not required to perform detection of the electromagnetic current iL in a transient state, so an error in detection of the electromagnetic current iL can be decreased. In particular, the amplifier circuits  250  and  350  become the constant modes A 3  and B 3 , respectively, immediately after the time  0  in the control cycle Ts. Therefore, the detection of the electromagnetic current iL can be performed at a common detection timing Td, thereby making it possible to easily perform control of the detection timing Td. 
   Further, by effecting the appropriate grouping to the electromagnet coils  151 ,  151 , . . . which respectively constitute electromagnets  104 ,  105 ,  106 A, and  106 B, the electromagnetic currents iL flowing through the common node C can be equalized. Therefore, the transistors  281  and  381  and the diodes  285  and  385  can be reduced in size, thereby making it possible to further miniaturize the turbo molecular pump. Further, it is also possible to decrease the current flowing through those elements, so heat generation or the like can be prevented. Still further, it is also possible to decrease the current required to be supplied from the power source  153 , so an input power supply capacity can be reduced. 
   Note that in this embodiment, it is described that when the electromagnetic current iL is in a constant state, the detection of the electromagnetic current iL is performed. However, as in the case described in the first embodiment ( FIG. 4 ), it is also possible to perform control for forcefully making a state of the constant mode A 3 , B 3  in the control cycle Ts with respect to the amplifier circuits  250 ,  350  and the switch circuit  480  to perform detection of the electromagnetic current iL within this period. 
   BRIEF DESCRIPTION OF THE DRAWINGS 
   [ FIG. 1 ] A circuit diagram of an amplifier circuit according to a first embodiment of the present invention. 
   [ FIG. 2 ] A time chart showing how adjustment is made between control phases of the amplifier circuit and switch phases of a switch circuit, according to the first embodiment of the present invention. 
   [ FIG. 3 ] Another example of  FIG. 1 . 
   [ FIG. 4 ] Another example of  FIG. 2 . 
   [ FIG. 5 ] A circuit diagram of an amplifier circuit according to a second embodiment of the present invention. 
   [ FIG. 6 ] A time chart showing how adjustment is made between control phases of the amplifier circuit and switch phases of a switch circuit, according to the second embodiment of the present invention. 
   [ FIG. 7 ] Another example of  FIG. 5 . 
   [ FIG. 8 ] A circuit diagram of an amplifier circuit according to a third embodiment of the present invention. 
   [ FIG. 9 ] A time chart showing how adjustment is made on switch phases of as witch circuit, according to the third embodiment of the present invention. 
   [ FIG. 10 ] A vertical sectional view of a turbo molecular pump main body. 
   [ FIG. 11 ] A circuit diagram of a conventional amplifier circuit. 
   [ FIG. 12 ] A time chart showing how control is made on the conventional amplifier circuit. 
   Description of Symbols 
   
       
         100 ,  200  turbo molecular pump main body 
         102  rotary vanes 
         103  rotor 
         104 ,  105 ,  106 A, and  106 B electromagnet 
         113  rotor shaft 
         150 ,  250 ,  350  amplifier circuit 
         151  electromagnet coil 
         153  power source 
         155 ,  255  current detecting circuit 
         161 ,  162 ,  261 ,  281 ,  282 ,  361 ,  381 ,  382  transistor 
         165 ,  166 ,  265 ,  285 ,  365 ,  385  diode 
         171 ,  271 ,  371 ,  471  amplifier control circuit 
         280 ,  380 ,  480  switch circuit