Abstract:
A magnetic bearing enables to rotatably support a levitated body non-contactingly disposed in a hermetically sealed container filled with a gaseous process substance of a corrosive nature, while without contaminating the gaseous environment and suffering from corrosion. The magnetic bearing has an electromagnet for supporting a levitated body, a displacement sensor for detecting a levitated position of the levitated body, and a controller for supplying signals and excitation currents to the displacement sensor and the electromagnet through cables. An electromagnet target of the magnetic bearing that generates variations in magnetic fields due to rotation of the levitated body, is constructed of a single piece of ferromagnetic material, and is provided with an electrical insulation structure oriented parallel to magnetic fluxes generated by the electromagnet.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a radial magnetic bearing for freely rotatably levitating a rotation shaft having a circulation fan disposed inside a container having a corrosive gas sealed-in, such as excimer laser apparatus, and a circulation fan apparatus provided with the radial magnetic bearing. 
     2. Description of the Related Art 
     Magnetic bearing, being different from contact type bearing such as sliding bearing or ball bearing, supports a rotor non-contactingly, thereby offering advantages such as: 1) mechanical loss is low; 2) friction and wear are non-existent; 3) lubricating oil is not required; 4) low vibration and noise; and 5) maintenance free. Some examples of application of magnetic bearing having such features include turbomolecular pumps used for generating a vacuum environment that contains little impurities and spindles for highspeed machining tools. 
     If the magnetic bearing is to be used in an environment that is extremely averse to impurities or corrosive environment, problems arise from emission of gaseous substances from materials of the magnetic bearing such as magnetic strips, copper coils and organic materials, for example, and from corrosion of these materials. For this reason, a protective coating is applied to the magnetic bearing so as to protect the materials of the magnetic bearing from the corrosive environment. An example of using magnetic bearings for freely rotatably levitating the rotation shaft of a circulation fan is an excimer laser apparatus. 
     FIG. 6 shows a cross sectional view of a conventional excimer laser apparatus, and FIG. 7 is an enlarged view of a key section of FIG.  6 . In the conventional excimer laser apparatus, as shown in FIG. 6, a laser vessel  10  that seals in a laser gas such as a halogen group gas, is provided with: a pre-ionizing electrode (not shown) for pre-ionizing the laser gas; and at least a pair of main discharge electrodes  12 ,  12  to obtain an electric discharge to enable oscillation of laser light. Further, inside the laser vessel  10  is provided a circulation fan  14  for producing a highspeed flow of the laser gas between the main discharge electrodes  12 ,  12 . 
     The circulation fan  14  has a rotation shaft  16  passing through the laser vessel  10  and extending between both end sections of the laser vessel  10 . The rotation shaft  16  is freely rotatably supported by magnetic bearings  20 ,  22  and an axial magnetic bearing  24  non-contactingly, which are placed at each end portions of the laser vessel  10 . Also, a motor  26  is provided on the axial-end side of the radial magnetic bearing  20  at one-end. 
     A displacement sensor  20   a  and an electromagnet  20   b  of one-end radial magnetic bearing  20  and the stator  26   a  of the motor  26  are housed in the motor housing  28 , and their inside surfaces are protected by a thin walled cylindrical isolation wall  30  made of a material that is resistant to corrosion against halogen group gases contained in the laser gas, for example, austenite type stainless steels such as SUS316L and the like. Accordingly, the displacement sensor  20   a , electromagnet  20   b  and the stator  26   a  of the motor  26  are prevented from coming into contact with the laser gas. A displacement sensor  22   a  of the radial magnetic bearing  22  and the electromagnet  22   b  at the opposing-end are similarly constructed, and are housed inside the bearing housing  32 , and their inner surfaces are protected by an isolation wall  34 . 
     Displacement sensor targets  20   c ,  22   c  and electromagnet targets  20   d ,  22   d  of the radial magnetic bearings  20 ,  22 , and the rotor  26   b  of the motor  26  are affixed to the rotation shaft  16 , and are disposed so as to oppose the respective displacement sensors  20   a ,  22   a  and electromagnets  20   b ,  22   b  of the radial magnetic bearings  20 ,  22 , and the stator  26   a  of the motor  26 . The displacement sensor targets  20   c ,  22   c , and electromagnet targets  20   d ,  22   d  for the radial magnetic bearings  20 ,  22 , and the rotor  26   b  of the motor  26  affixed to the rotation shaft  16  are installed inside the sealed container communicating with the laser vessel  10 . Therefore, they are required to be resistant to corrosion by the laser gas and not contaminate the laser gas. 
     Therefore, the displacement sensor targets  20   c ,  22   c  and electromagnet targets  20   d ,  22   d  are generally made by applying a Ni plating on the surface of a laminated steel plate or cladding the surface with stainless steel, or using a single piece ferromagnetic material resistant to corrosion by the laser gas, for example, Permalloy (Fe—Ni alloy containing 35-80% Ni). Also, because the rotor  26   b  of the motor  26  is made of a composite of laminated steel plate and aluminum alloys or a permanent magnet, Ni-plating does not adhere tightly and uniformly to the surface, and for this reason, contact with the laser gas is prevented by creating a sealed space on its surface produced by installing the isolation wall  36  made of a stainless steel. 
     However, in the conventional radial magnetic bearings, if the electromagnet target is made of a structure produced by surface treatment such as Ni plating on laminated steel sheets, Ni plating does not adhere tightly to the laminated steel sheets, so that there is a possibility that the plating can peel off to expose the laminated steel to corrosion. Furthermore, because of the lamination structure, the surface area is large and gases can be trapped on the surface to cause potential contamination of the laser gas. 
     Also, when a structure made of stainless steel cladding is used, because the distance between the electromagnet and the electromagnet target of the radial magnetic bearing must be increased by an amount equal to the sheet thickness of the isolation wall, the size of the electromagnets tends to increase. 
     Further, when a structure made of a single piece ferromagnetic material resistant to corrosion is used for the radial magnetic bearing  20 , as shown in FIG. 7, eddy current E is generated in the interior of the electromagnet target  20   d  due to variations in the magnetic fields introduced by the rotation of the rotation shaft  16 , and the magnetic flux M generated by the electromagnet  20   b  is reduced by the eddy current E in the electromagnet target  20   d  so that the magnetic strength is lowered. Especially, the eddy current E increases in proportion to the square of the speed of magnetic field change so that as the rotational speed of the rotation shaft  16  increases, drop in the magnetic strength becomes noticeable. The same phenomenon occurs at the opposing-end radial magnetic bearing  22 . 
     SUMMARY OF THE INVENTION 
     The present invention is performed in view of the background presented above, and it is an object of the present invention to provide a magnetic bearing that does not generate gas contamination and has good corrosion resistance, and enables to rotatably support a levitated body without contact while generating a magnetic force of appropriate strength, and a circulation fan apparatus equipped with the magnetic bearing. 
     According to an aspect of the present invention, there is provided a magnetic bearing having an electromagnet for supporting a levitated body, a displacement sensor for detecting a levitated position of the levitated body, and a controller for supplying signals and excitation currents to the displacement sensor and the electromagnet through cables; wherein an electromagnet target of the magnetic bearing that generates variations in magnetic field due to rotation of the levitated body, is comprised of a single piece of ferromagnetic material, and is provided with an electrical insulation structure oriented parallel to magnetic fluxes generated by the electromagnet. 
     According to the above magnetic bearing, because the electromagnet target is comprised of a single piece ferromagnetic material, the surface area of the electromagnet target is less compared with a similar electromagnet target made by laminated steel sheets, and gas trapping sites are reduced so that contamination from the electromagnet target can also be lessened significantly. Further, because the electromagnet target has an electrical insulation structure oriented parallel to the magnetic flux generated by the electromagnet, even when variations in the magnetic field is created due to rotation of the levitated body and so on, eddy current generated in the interior of the electromagnet target is reduced. That is, the specific resistance in the longitudinal direction of the magnetic circuit formed among the electromagnet, the electromagnet target and the electrical insulation structure is increased so that the electromagnet target is able to reduce the eddy current generated by magnetic field change so that a stable magnetic force can be generated. The result is that the levitated body can be levitated in a stable manner at all times. 
     According to the present invention, the electrical insulation structure preferably comprises of slit groove. 
     According to this structure, because the electrical insulation structure on the electromagnet target is provided in a form of slit groove, gas trapping sites are reduced and the surface area of the electromagnet target can be limited to a minimum. Accordingly, electrical insulation structure that does not act as a gaseous contamination source can be provided for the electromagnet target at low cost. 
     According to another aspect of the present invention, there is provided a circulation fan apparatus having a rotation shaft of a circulation fan, disposed in a hermetically sealed container filled with a gaseous process substance of a corrosive nature, said rotation shaft being supported with not less than two radial magnetic bearings, wherein an electromagnet target of the radial magnetic bearing is comprised of a single piece ferromagnetic material and is provided with an electrical insulation structure at a given spacing along an axial direction of the rotation shaft. 
     According to the above circulation fan apparatus, the electromagnet target of the radial magnetic bearing supporting the rotation shaft of the circulation fan is comprised of a single piece ferromagnetic material, and has an electrical insulation structure formed at a given spacing along the axial direction of the rotation shaft. For this reason, the electromagnet target does not contaminate the gaseous environment in the container, and it can easily made resistant to corrosive gases. Further, even when variations in the magnetic field is created due to rotation of the rotation shaft and so on, eddy current generated in the interior of the electromagnet target is lessened. That is, the specific resistance of the magnetic circuit in the direction of the rotation shaft, formed among the electromagnet, the electromagnet target and electrical insulation structure, is increased so that the electromagnet target is able to reduce the eddy current generated by magnetic field change and a stable magnetic force can be generated. The result is that the levitated body can be levitated in a stable manner at all times. 
     According to the present invention, the circulation fan apparatus has an electromagnet target, which has the electrical insulation structure that preferably comprises of slit groove. 
     According to the above structure, because the electrical insulation structure on the electromagnet target is provided in a form of slit groove, gas trapping sites are reduced and the surface area of the electromagnet target is limited to a minimum, and accordingly, an electrical insulation structure can be provided at low cost for the electromagnet target that does not act as a gaseous contamination source. 
     As explained above, according to the present invention, even if variations in the magnetic field occur in the electromagnet target due to rotation of the rotation shaft as a levitated body, eddy current losses can be suppressed, thereby enabling to produce a stable levitation force. Also, when the rotation shaft as a levitated body is rotated at high speed, by selecting the number of slit grooves appropriately, it is possible to provide a radial magnetic bearing that produces less eddy current losses and prevents gaseous contamination of the working environment. 
     According to the present invention, an excimer laser apparatus comprising a circulation fan apparatus according to claim  3  or claim  4 . 
     The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of an excimer laser apparatus as an example of the circulation fan apparatus equipped with the magnetic bearing according to the first embodiment of the present invention; 
     FIG. 2 is an enlarged view of a key part in FIG. 1; 
     FIG. 3 is a cross sectional view through a plane along line A—A in FIG. 2; 
     FIG. 4 is an enlarged cross sectional view of a key part of an excimer laser apparatus equipped with the magnetic bearing according to the second embodiment of the present invention (drawing corresponding to FIG.  2 ); 
     FIG. 5 is a cross sectional view through a plane along line B—B in FIG. 4; 
     FIG. 6 is a cross sectional view of a conventional excimer laser apparatus; and 
     FIG. 7 is an enlarged view of a key part in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments will be explained in the following, with reference to FIGS. 1 to  5 . 
     FIG. 1 is a cross sectional view of an excimer laser apparatus as an example of a circulation fan apparatus equipped with the magnetic bearing of the first embodiment of the present invention, FIG. 2 is an enlarged view of a key part of FIG. 1, and FIG. 3 is a cross sectional view through a plane along line A—A in FIG.  2 . Here, those parts of the present apparatus that are the same as those in the conventional example shown in FIGS. 6,  7  are referred to by the same reference numerals, and their explanations are partly omitted. 
     This excimer laser apparatus, similar to the conventional example shown in FIGS. 6 and 7, has a laser vessel  10  having a halogen group gas such as fluorine sealed in, and provided inside the vessel  10  are a pre-ionization electrode (not shown) for pre-ionizing the laser gas and at least a pair of main discharge electrodes  12  to obtain an electric discharge to enable oscillation of laser light. Further, a circulation fan  14  for generating a flow of highspeed laser gas between the pair of main electrodes  12  is disposed in the vessel  10 . 
     The circulation fan  14  has a rotation shaft  16  passing through the laser vessel  10  and extending between both end sections of the laser vessel  10 . The rotation shaft  16  is freely rotatably supported by magnetic bearings  40 ,  42  and an axial magnetic bearing  24  non-contactingly placed at both end portions of the laser vessel  10 . Also, a motor  26  is provided on the axial end side of the radial magnetic bearing  40  at one end. 
     The radial magnetic bearing  40  at one end comprises a displacement sensor  40   a , an electromagnet  40   b , a displacement sensor target  40   c  and an electromagnet target  40   d . A positional signal detected from the displacement sensor  40   a  is input into a controller (not shown) through a cable (not shown), and the rotation shaft  16  is levitated at the target position by applying the excitation current to electromagnet  40   b  based on the input positional signal. 
     The displacement sensor  40   a  and the electromagnet  40   b  are housed inside the motor housing  28 , and their inner surfaces are covered by a thin walled cylindrical isolation wall  30  made of a material that is resistant to corrosion by a halogen group gas contained in the laser gas, an austenite type stainless steel such as SUS316L, for example. By this manner, the displacement sensor  40   a  and the electromagnet  40   b  are prevented from coming into contact with the laser gas. 
     On the other hand, the displacement sensor target  40   c  and the electromagnet target  40   d  are affixed to the rotation shaft  16 , and are disposed in a hermetic space that communicates with the laser vessel  10 . The displacement sensor target  40   c  and the electromagnet target  40   d  are both made of a single piece ferromagnetic material resistant to corrosion by a halogen gas group gas contained in the laser gas, for example, Permalloy (an Fe—Ni alloy containing 35-80% Ni). And, the electromagnet target  40   d  is provided with slit grooves  44  extending from the outer periphery to inward of the electromagnet target  40   d  at a given spacing along the axial direction of the rotation shaft  16 . 
     According to this structure, even when variations in the magnetic field are produced due to rotation and other effects of the rotation shaft  16 , eddy current E generated in the interior of the electromagnet target  40   d  can be reduced. That is, as shown in FIG. 2, the specific resistance of the magnetic circuit in the electromagnet target  40   d  along the axial direction of the rotation shaft  16  formed among the electromagnet  40   b , the electromagnet target  40   d  and the slit grooves  44  is increased, so that a magnetic flux M due to electromagnet  40   b  is generated in each region of the divided slit grooves  44  of the electromagnet target  40   d , causing the eddy current E to flow around each magnetic flux M so that eddy current generated by the magnetic field change is minimized, thereby generating a stable magnetic force. The result is that the rotation shaft  16  can be levitated stably at all times. 
     It is preferable that the width of the slit groove  44  be as narrow as possible to secure the magnetic pole area, and, as shown in FIG. 3, the depth should preferably be at least equal to or more than the size of the magnetic path  46  of the magnetic flux M generated by the electromagnet  40   b . Here, higher the number of slit grooves  44  more effective they are in reducing the eddy current, but the magnetic pole area is decreased proportionately so that it is preferable to determine this number according to the speed that induces magnetic field change, that is, based on a parameter determined by the outer radius of the electromagnet target  40   d  and the rotational speed. 
     The radial magnetic bearing  42  at the opposing-end similarly comprises a displacement sensor  42   a , an electromagnet  42   b , a displace sensor target  42   c  and an electromagnet target  42   d . A positional signal detected from the displacement sensor  42   a  is input into the controller (not shown) through the cable (not shown), and the rotation shaft  16  is levitated at the target position by applying the excitation current to the electromagnet  42   b  based on the input positional signal. The displacement sensor  42   a  and the electromagnet  42   b  are housed inside the bearing housing  32 , and their inner surfaces are covered by an isolation wall  34  of a thin cylindrical shape. 
     Also, the displacement sensor target  42   c  and the electromagnet target  42   d  made of a single piece ferromagnetic material such as Permalloy are affixed to the rotation shaft  16 . And, the electromagnet target  42   d  is provided with slit grooves  50  at a given spacing along the axial direction of the rotation shaft  16  so that even if variations in the magnetic fields occur due to rotation and so on of the rotation shaft  16 , eddy current generated in the interior of the electromagnet target  42   d  can be reduced. 
     FIG.  4  and FIG. 5 show an excimer laser apparatus equipped with the magnetic bearing of the second embodiment of the present invention. FIG. 4 is an enlarged cross sectional view of a key part of the excimer laser apparatus (drawing corresponding to FIG.  2 ), FIG. 5 is a cross sectional view through a plane along line B—B in FIG.  4 . 
     In this second embodiment, a magnetic bearing  40 , differ from the first embodiment, has an electromagnet  40   b  whose projections of the cores  40   e  pierce an isolation wall  30  and expose their inner surface. Herein, the cores  40   e  of the electromagnet  40   b  are required to have resistant to corrosion because they come into contact with the laser gas. Therefore, the cores  40   e  of the electromagnet  40   b  are made of Permalloy that is resistant to corrosion by the laser gas. The cores  40   e  of the electromagnet  40   b  are affixed to the isolation wall  30  by welding and the like so as to prevent a coil winding  40   f  of the electromagnet  40   b  that has poor resistant to corrosion by the laser gas from coming into contact with the laser gas. 
     According to the second embodiment, a magnetic gap between the cores  40   e  of the electromagnet  40   b  and an electromagnet target  40   d  is prevented from being enlarged by the existence of the isolation wall  30 . Therefore, it can realize improved efficiency of a magnetic bearing, reduced electric power consumption, and compactness of the magnetic bearing. 
     In these embodiments, each application of the magnetic bearings was exemplified by an excimer laser apparatus, it is not limited to such excimer laser apparatus only. Also, it is obvious that the magnetic bearing is applicable to any application that requires improvement in corrosion resistance in the electromagnet target of the magnetic bearing and eliminate contamination of laser gas. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.