Abstract:
An air bearing drive system has a base, a moving portion, an air bearing for forming an air film gap between the base and the moving portion, air nozzles and a suction inlet. The air nozzles and suction inlet are formed on the base. The air nozzles blow air toward the moving portion so as to exert a levitation force on the moving portion. The suction inlet applies suction to the air film gap so as to attract the moving portion toward the base and thereby exert an attraction force on the moving portion. An air supplying device supplies air to the air nozzles. A vacuum source applies suction to the suction inlets. Two adjusting devices are adapted to adjust the levitation force produced by the air issuing from the air nozzles and the attraction force of air drawn into the suction inlet so as to provide an accurate and low cost air bearing drive system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to air bearing drive systems, and more particularly, it relates to a mechanism of the drive system for controlling a gap of an air film (hereinafter, a gap of an air film is simply referred to as an air film gap) formed by an air bearing. 
     2. Description of the Related Art 
     Roundness testers are conventionally used for obtaining a variety of data about geometrical round precision of a cylindrical form such as roundness, concentricity, and coaxiality. By placing a workpiece on a turntable, rotating the workpiece by the turntable, and detecting a profile of the workpiece by a detection head, the roundness testers collect data about the profile of the workpiece, and then measure and calculate the geometrical round precision of the workpiece. 
     As disclosed, for example, in Japanese Unexamined Patent Application Publication Nos. 2000-120686 and 2000-348429, air bearings producing dramatically less frictional resistance, heat generation, and rotating vibration than ball bearings and the like are widely used for a variety of accurate drive systems, for example, for achieving a rotary motion of the turntable and a linear feed motion of the detection head of the roundness testers. 
     In general, a rotary drive system  10  using an air bearing shown in FIG. 1 has a stator (also referred to as base)  12 , a rotor  14 , an upper plate (also referred to as moving portion)  16 , and a lower plate  18 . The rotor  14  is formed integrally with the upper plate  16  and lower plate  18  and supported by the stator  12 . 
     By supplying air  26 , the lower surface of the upper plate  16  and the upper surface of the stator  12  have an upper air film gap  20  formed therebetween, the lower surface of the stator  12  and the upper surface of the lower plate  18  have a lower air film gap  22  formed therebetween, and the outer peripheral surface of the rotor  14  and the inner peripheral surface of the stator  12  have a tubular air film gap  24  formed therebetween. Thus, these gaps  20 ,  22 , and  24  allow the rotor  14  to rotate smoothly together with the upper and lower plates  16  and  18  relative to the stator  12 . 
     The drive system using the foregoing conventional air bearing has an air film gap generally formed between the base and each of the moving parts facing the base. For example, in the rotary drive system  10  shown in FIG. 1, the air flows in these three gaps  20 ,  22 , and  24 . In order to achieve an accurate rotation of the rotor  14  together with the upper and lower plates  16  and  18  while maintaining all the air film gaps, between the base and the moving parts facing the base, at respectively fixed amounts of spacing, all the foregoing upper and lower gaps  20  and  22  and tubular gap  24  are required to satisfy a large area of extremely strict precision, as described below, for example. 
     Within currently attainable levels of geometrical precision, for example, the stator  12  is required to satisfy flatness of its upper and lower surfaces, parallelism between the opposing surfaces (hereinafter, also referred to as surface-to-surface parallelism), squareness of the axis of its inner peripheral surface relative to the above surfaces, cylindricity of the inner peripheral surface, and so forth. Also, the rotor  14  is required to satisfy flatness of its upper and lower surfaces, parallelism between these surfaces, squareness of the axis of its outer peripheral surface relative to the above surfaces, cylindricity of the outer peripheral surface, and so forth. In addition, the upper plate  16  is required to satisfy flatness of its upper and lower surfaces and the like, and the lower plate  18  is required to satisfy flatness of its upper surface and the like. 
     However, since satisfying the foregoing variety of extremely strict geometrical precision requires a large amount of work and time for processing and finishing these components, and accordingly incorporates an increased cost of the drive system, a new drive system which can achieve a reduced cost has been strongly desired without deteriorating accurate and stable driving features required. 
     Furthermore, when the center of rotation and the center of gravity of a rotating workpiece placed on the upper plate  16  for measuring its roundness do not coincide with each other, an eccentric load is exerted on the rotating upper plate  16 , causing the air film between the lower surface of the upper plate  16  and the upper surface of the stator  12  to vary in accordance with the rotation of the rotating workpiece, giving rise to a problem in that it is difficult to accurately measure geometrical round precision of the workpiece because the axis of the rotation center of the workpiece is tilted. 
     SUMMARY OF THE INVENTION 
     In view of the above-described problems of the related art, it is an object of the present invention to provide an air bearing drive system that offers a reduced cost and achieves accurate and stable driving performances. 
     The present invention is made to achieve the above object. An air bearing drive system according to the present invention comprises (a) a moving portion performing a rotational motion or a linear motion, (b) a base for supporting the moving portion, (c) an air bearing having an air film gap, for supporting the moving portion on the base, between the base and the moving portion, (d) at least one air nozzle, (e) air supplying means, (f) at least one suction inlet, (g) attracting means, and (h) two adjusting means. 
     The air nozzle is formed on the base and faces the moving portion, for forming the air film gap by blowing air toward the moving portion and exerting a levitation force on the moving portion. The air supplying means supplies air to the air nozzle. 
     The suction inlet is formed on the base and faces the moving portion, for exerting an attraction force on the moving portion so as to attract the moving portion toward the base. The attracting means attracts the moving portion toward the base via the suction inlet. 
     The two adjusting means adjust the levitation force produced by the air blowing from air nozzle and the attraction force from the suction inlet. 
     According to the present invention, by adjusting the thickness of the air film between the moving portion and the base by using the air nozzle and the suction inlet, the thickness of the air film can be fine-tuned or adjusted without making increasing the size of the air bearing. 
     Further, in the air bearing drive system according to the present invention, it is preferable that the moving portion comprises a cylindrical rotor and a moving flat surface orthogonal to the axial center of the rotor, the base comprises at least one thrust flat surface facing the moving flat surface, and the thrust flat surface comprises the air nozzle and the suction inlet formed thereon so that the moving flat surface is levitated, at a prescribed height via the air film, relative to the thrust flat surface. 
     According to the present invention, the moving portion can be easily positioned in a non-contact manner since the moving portion has a levitation height controlled in the thrust direction thereof. 
     Further, in the air bearing drive system according to the present invention, the thrust flat surface is preferably a single surface disposed on one side of the stator. 
     Since the air bearing drive system according to the present invention has a smaller number of elements of components requiring precise processing, unskilled workers can process the components. Accordingly, a reduced processing cost of the components is achieved without a risk of deteriorating rotation accuracy of the drive system. 
     Further, in the air bearing drive system according to the present invention, the base may have a cylindrical opening therein for supporting the rotor in the radial direction of the rotor. Also, the base may comprise a plurality of the air nozzles disposed on the peripheral surface of the cylindrical opening so that the rotor is positioned in the center of the cylindrical opening. 
     According to the present invention, since the rotor is positioned in the center of the cylindrical opening in a non-contact manner, the air bearing drive system has an integrally formed radial and thrust bearing, leading to a reduced size of the drive system. 
     Further, in the air bearing drive system according to the present invention, either the air nozzle or the suction inlet may have a groove-shape. 
     According to the present invention, since the thickness of the air film between the base and the moving portion can be controlled easily and a variation in the thickness can be reduced accordingly, the drive system has improved driving accuracy. 
     Further, in the air bearing drive system according to the present invention, the moving portion may further comprise an anti-slip member for preventing the moving portion from slipping out from the base. 
     According to the present invention, a possibility of an accident during conveyance and the like can be eliminated. 
     Further, the drive system according to the present invention is preferably used for driving a turntable of a roundness tester. 
     Accordingly, the present invention provides a roundness tester comprising a precise rotary driving mechanism at a low cost. 
     Further, the air bearing drive system according to the present invention may further comprise controlling means for controlling the adjusting means to adjust a balance of the levitation force and the attraction force exerted on the moving portion so that the air film gap between the base and the moving portion maintains a required uniform thickness when the moving portion has the levitation force and the attraction force exerted thereon, wherein (i) at least one group of the plurality of the air nozzles and the plurality of the suction inlets is disposed on the base, (ii) the corresponding adjusting means independently adjusts at least one of a part of the levitation force by using each of the air nozzles and a part of the attraction force by using each of the suction inlets, and (iii) the control means controls the corresponding adjusting means to perform the above-stated independent adjustment so that the air film gap between the base and the moving portion maintains the required uniform thickness when the moving portion has the levitation force and the attraction force exerted thereon. 
     The required uniform thickness of the air film gap as described above is achieved by adjusting surface-to-surface parallelism and the like of the air film gap, for example, by adjusting a height and an angle of the moving portion relative to the base. 
     According to the present invention, even when an eccentric load is exerted on the rotating moving portion when the center of rotation and the center of gravity of a rotating workpiece placed on the moving portion for measuring its roundness do not coincide with each other, the air film between the lower surface of the moving portion and the upper surface of the base is controlled so as to have a constant thickness in accordance with a rotation of the rotating workpiece, thereby achieving an accurate rotation of the workpiece without tilting the axis of the rotation center of the workpiece. 
     Further, in the air bearing drive system according to the present invention, the control means may comprise at least one gap sensor, concentrically disposed with the peripheral circle of the rotor, for measuring a levitation height of the moving flat surface relative to the thrust flat surface. 
     According to the present invention, since the levitation height of the moving flat surface relative to the thrust flat surface can be measured accurately, a slanted angle of the moving flat surface can be calculated more accurately, allowing the control means to control the levitation height more accurately, thereby achieving a more accurate rotation of the workpiece. 
     Further, in the air bearing drive system according to the present invention, the control means may comprise at least one pressure sensor for measuring a negative pressure. 
     According to the present invention, since the pressure sensor for measuring a negative pressure is disposed in the air suction line, the levitation height of the moving flat surface relative to the thrust flat surface can be maintained constant regardless of the weight of the workpiece by adjusting the negative pressure so as to maintain the attraction force constant. 
     As described above, the drive system according to the present invention comprises (a) the base, (b) the moving portion, (c) at least one air nozzle formed on the upper surface of the base for blowing air toward the moving portion so as to exert a levitation force on the moving portion, (d) at least one suction inlet also formed on the upper surface of the base for attracting the moving portion toward the base so as to exert an attraction force on the moving portion, and (e) two adjusting means, one for adjusting the levitation force from the air nozzle and the other for adjusting the attraction force from the suction inlet. With this configuration, the adjusting means adjust a balance of the levitation force and the attraction force so as to form an air film gap having a required uniform thickness, thereby achieving a reduced cost as well as an accurate and stable drive of the drive system. 
     In the drive system according to the present invention, at least one group of a plurality of the air nozzles and a plurality of the suction inlets is disposed on the base. The corresponding adjusting means independently adjust at least one of a part of the levitation force by using each of the plurality of air nozzles and a part of the attraction force by using each of the plurality of suction inlets. Also, the drive system comprises control means for controlling the corresponding adjusting means to perform the above independent adjustment, thereby achieving further accurate and stable driving performances of the drive system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a known air bearing drive system; 
     FIGS. 2 and 3 are schematic views illustrating the configuration of an air bearing drive system, according to an embodiment of the present invention, applied to a roundness tester; 
     FIG. 4 illustrates a vertical section of an air bearing according to the present invention and the other schematic configuration of the drive system using the air bearing; 
     FIG. 5 is a top view of air nozzles and a suction inlet of the air bearing; and 
     FIGS. 6A to  7 B are illustrations of an operation of the air bearing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 2 and 3 are schematic views of the configuration of an air bearing drive system, according to an embodiment of the present invention, applied for rotating a turntable of a roundness tester. 
     A roundness tester  150  shown in FIG. 2 has a main unit  152 , an electrical unit  154 , and a computer  156 . 
     The main unit  152  has a platform  158  and a turntable  160  disposed on the platform  158  via an air bearing drive system  110  according to the embodiment, and the turntable  160  has a workpiece  162  placed thereon. The computer  156  inputs a drive command into the electrical unit  154  so as to rotate the turntable  160  at a given speed by means of the conventional motor (not shown). The rotating speed of the turntable  160  is sequentially detected by a rotary encoder (not shown) disposed below the turntable  160 , and the detected values are input into the computer  156  in a form of digital signals. 
     Also, the main unit  152  has a detection head  164 , for detecting irregularities of the profile of the workpiece  162 , and a stylus  166  attached on the detection head  164 . Displacement detection signals from the detection head  164  are converted into digital signals by the electrical unit  154  and are sequentially input into the computer  156 . The stylus  166  is constantly urged against the detection head  164 , for example, in the radial direction of the turntable  160 . By allowing the turntable  160  to rotate the workpiece  162  while the tip of the stylus  166  is kept in contact with the workpiece  162 , the detection head  164  detects an amount of displacement of the stylus  166  caused by the irregularities of the profile of the workpiece  162 . The displacement detection signals from the detection head  164  together with the detection signals of the rotating speed from the rotary encoder are stored in the computer  156 . The measured data is computed for obtaining roundness, coaxiality, and so forth by using calculation methods such as the least-squares method and the minimal domain method when needed. 
     For example, in order to achieve an accurate rotation of the turntable  160  of the roundness tester  150 , an air bearing drive system, which generates dramatically less frictional resistance, heat generation, rotating vibration, and so forth, is generally used. The air bearing drive system  110  is disposed, for example, below the turntable  160  of the main unit  152  as shown in FIG.  3 . Although the turntable  160  using an air bearing is required to have extremely high rotation accuracy, achieving such accuracy of rotation requires extremely strict geometrical precision of each element of the base and the moving parts facing the base, leading to a large amount of work and time for processing and finishing these components, and accordingly leading to an increased cost of the drive system  110 . 
     In view of the above problems, a self-attraction and levitation type air bearing shown in FIGS. 4 and 5 is employed in order to reduce the total number of elements requiring costly geometrical precision in the drive system  110  according to the embodiment. For example, surface-to-surface parallelism is achieved not by the conventional way of processing and finishing the components, but by adjusting an air film gap of a self-attraction and levitation type air bearing at a given amount of spacing. FIG. 4 illustrates a vertical section of an air bearing according to the embodiment and the schematic configuration of the drive system  110  using the air bearing. FIG. 5 is a top view of air nozzles and a suction inlet of the air bearing. 
     The drive system  110  using the air bearing according to the embodiment is provided with a gap control mechanism by using an air film in a gap between the upper surface (i.e., thrust flat surface) of a stator (i.e., base)  112  and the lower surface (i.e., moving flat surface), orthogonal to the axis of a rotor  114 , of an upper plate (i.e., moving portion)  116 . With this configuration, air blows toward the inner peripheral surface and the upper surface of the stator  112 . 
     The stator  112  has a plurality of air nozzles  170   a  to  170   h  on the upper surface thereof at a given pitch, concentrically disposed with the peripheral circle of the rotor  114 . Also, the drive system  110  has a plurality of air supplying lines  171 , air supplying means  172 , main air supply line  173 , air-supply adjusting means  174 , and the computer (i.e., control means)  156 . 
     The air nozzles  170   a  to  170   h  are connected, via the air supplying lines  171 , to the air-supply adjusting means  174 . The air-supply adjusting means  174  is connected, via the main air supply line  173 , to the air supplying means  172 . 
     With these connections, the air supplying means  172  supplies air  126  to each of the air nozzles  170   a  to  170   h  through the corresponding air supplying lines  171  via the air-supply adjusting means  174 . 
     The computer  156  is connected to the air-supply adjusting means  174  and controls an operation of the air-supply adjusting means  174  so that an amount of air blowing from each of the air nozzles  170   a  to  170   h  is independently adjustable. By adjusting the amount of air blowing from each of the air nozzles  170   a  to  170   h , a levitation force exerted on the lower surface of the upper plate  116  can be adjusted at each of the air nozzles  170   a  to  170   h . 
     An amount of air blowing into a tubular gap between the outer peripheral surface of the rotor  114  and the inner peripheral surface of the stator  112  is basically fixed. 
     In the drive system  110  according to the embodiment, since the gap control mechanism by using an air film is provided between the upper surface of the stator  112  and the lower surface of the upper plate  116  as describe above, an attraction force of the air is set in a direction perpendicular to the above upper surface, and the stator  112  has an approximately O-shaped ring-like vacuum suction groove (also referred to as suction inlet)  176  on the upper surface thereof and inside a circle formed by the air nozzles  170   a  to  170   h . Also, the drive system  110  has a plurality of air suction lines  177 , vacuuming means (i.e., attracting means)  178 , main air suction line  179 , air-suction adjusting means  180 , and the computer (i.e., control means)  156 . 
     The vacuum suction groove  176  is connected, via at least one air suction line  177 , to the air-suction adjusting means  180 . The air-suction adjusting means  180  is connected, via the main air suction line  179 , to the attracting means  178 . 
     With these connections, the vacuuming means  178  draws or vacuums air in the vacuum suction groove  176  through the air suction line  177  via the air-suction adjusting means  180 . 
     The computer  156  is connected to the air-suction adjusting means  180  and controls an operation of the air-suction adjusting means  180  so that an amount of air drawn or suctioned from the vacuum suction groove  176  is adjusted. By adjusting the amount of air drawn or suctioned from the vacuum suction groove  176 , an attraction force exerted on the lower surface of the upper plate  116  can be adjusted. 
     The stator  112  has three capacitance type gap sensors (not shown) on the upper surface thereof at a 120-degree pitch, concentrically disposed with the peripheral circle of the rotor  114 , for measuring distances at the three points between the upper surface of the stator  112  and the lower surface of the upper plate  116 . The computer  156  receives outputs of the gap sensors with respect to the three distances and computes a slant of the lower surface of the upper plate  116  relative to the upper surface of the stator  112 . On the basis of the computed result, the computer  156  independently adjusts an amount of air blowing from each of the air nozzles  170   a  to  170   h  so as to correct for the slant of the upper plate  116  by controlling the air-supply adjusting means  174 . A correcting operation for the above slant is always performed. Accordingly, even when a direction of the slant varies with rotation of the upper plate  116 , the slant is corrected as needed by adjusting the amount of air blowing from each of the air nozzles  170   a  to  170   h  so as to be an optimal amount. 
     As a result, the drive system  110  according to the embodiment does not require precise processing and finishing of the components in a conventional manner for achieving geometrical precision of the surface-to-surface parallelism of the components. Instead, as shown in FIGS. 6A to  7 B, when the upper plate  116  has a levitation force from the air nozzles  170   a  to  170   h  together with an attraction force from the vacuum suction groove  176  exerted on the lower surface thereof, the computer  156  controls an operation of each adjusting means so as to form a gap having a required uniform thickness of an air film between the upper surface of the stator  112  and the lower surface of the upper plate  116  by adjusting the balance of the levitation force and the attraction force, thereby allowing the upper plate  116  to be positioned on the stator  112  in a non-contact manner. 
     Accordingly, the parallelism between the upper surface of the stator  112  and the lower surface of the upper plate  116  can be improved by controlling the air film gap so as to have a required uniform thickness. 
     The foregoing configuration allows the turntable  160  to rotate accurately and stably and eliminates costly processing and finishing of the components for achieving the geometrical precision of the surface-to-surface parallelism of the components in a conventional manner. 
     Referring now to FIGS. 6A to  7 B, an exemplary operation of the air bearing will be described. When the upper plate  116  lies at a levitation height d+, which is greater than a prescribed height d, relative to the stator  112  as shown in FIG. 6A, the computer controls each adjusting means so as to reduce the amount of air blowing from the air nozzles  170   a  to  170   h , while the amount of air suctioned from the vacuum suction groove  176  is kept constant, when the upper plate  116  has an attraction force  182  from the vacuum suction groove  176  exerted on the lower surface thereof. Then, although the upper plate  116  has a levitation force  175  produced by air blowing from the air nozzles  170   a  to  170   h  together with the attraction force  182  from the vacuum suction groove  176  exerted on the lower surface thereof, the amount of air suctioned from the vacuum suction groove  176 , i.e., the attraction force, becomes relatively larger, and accordingly, as shown in FIG. 6B, the upper plate  116  moves down toward the stator  112 , resulting in the gap, having the required uniform thickness d of an air film, between the upper surface of the stator  112  and the lower surface of the upper plate  116 . 
     On the other hand, when the upper plate  116  lies at a levitation height d−, which is smaller than the required height d, relative to the stator  112  as shown in FIG. 7A, the computer controls each adjusting means so as to increase the blowing rates, while the attraction force  182  is kept constant, when the upper plate  116  has the attraction force  182  from the vacuum suction groove  176  exerted on the lower surface thereof. Then, although the upper plate  116  has the levitation force  175  produced by air blowing from the air nozzles  170   a  to  170   h  together with the attraction force  182  from the vacuum suction groove  176  exerted on the lower surface thereof, the amount of air blowing from the air nozzles  170   a  to  170   h , i.e., the levitation force, becomes relatively larger, and accordingly, as shown in FIG. 7B, the upper plate  116  moves up, resulting in the gap having the required uniform thickness d of an air film between the upper surface of the stator  112  and the lower surface of the upper plate  116 . 
     As shown in FIG. 4, the base or stator  112  has a cylindrical opening therein and a plurality of air nozzles  124  on the inner peripheral surface of the cylindrical opening so as to press the outer peripheral surface of the cylindrical rotor  114  by air in the radial direction of the rotor  114 , thereby allowing the rotor  114  to be positioned in a non-contact manner such that the axial center of the rotor  114  coincides with that of the cylindrical opening. 
     The rotor  114  has an anti-slip member  190 , as shown in FIG. 4, fixed on the lower surface thereof for eliminating the possibility of the rotor  114  from slipping out during the conveyance of the roundness tester  150  and the like. The lower surface of the rotor  114  and the anti-slip member  190  do not require highly precise processing. 
     As described above, the drive system  110  according to the embodiment does not require precise processing and finishing of the components in a conventional manner for achieving geometrical precision of the surface-to-surface parallelism of the components. Instead, when the upper plate  116  has a levitation force from the air nozzles  170   a  to  170   h  together with an attraction force from the vacuum suction groove  176  exerted thereon, the computer  156  controls an operation of each adjusting means so as to form an air film gap having a required uniform thickness between the upper surface of the stator  112  and the lower surface of the upper plate  116  by adjusting the balance of the levitation force and the attraction force. This arrangement improves the parallelism between the upper surface of the stator  112  and the lower surface of the upper plate  116 , thereby allowing the turntable  160  to rotate accurately and stably. 
     Also, the conventional requirement for geometrical precision such as parallelism between the upper surface of the stator and the lower surface of the upper plate can be eliminated. Furthermore, in place of the conventional lower plate, it is simply sufficient to provide the drive system  110  according to the embodiment with the anti-slip member  190 , which serves to prevent the rotor  114  from slipping out accidentally. Accordingly, the conventional requirement for geometrical precision such as parallelism between the lower surface of the stator and the upper surface of the lower plate can be eliminated. As a result, the total number of elements requiring geometrical precision decreases to about two thirds of that of the conventional drive system, allowing the drive system  110  according to the embodiment to have a simple structure and achieve an accurate rotation, and thereby leading to a reduced cost of the drive system  110  due to a reduced number of steps for processing and finishing the components. 
     As described in the embodiment, accurate controlling of the air film gap to be constant is practically realized for the first time by adjusting amounts of air introduced into, and removed from, the air film gap, wherein the upper plate  116  has a levitation force produced by air blowing from the air nozzles  170   a  to  170   h  together with an attraction force produced by air suctioned from the vacuuming groove  176  exerted thereon. 
     The foregoing accurate controlling of the air film gap to be constant cannot be achieved by simply blowing air and recovering it in a known manner, nor by either simply blowing air from the air nozzles  170   a  to  170   h  or suctioning air from the vacuum suction groove  176 . Simply adjusting an amount of blowing air is not enough for controlling the air film gap to be constant, since changing the amount of blowing air causes the upper plate  116  to vibrate or flutter. Even when the amount of blowing air is controlled in a prescribed manner, it is often the case that the actual air film gap does not vary in a prescribed manner, leading to a conclusion in that controlling the air film gap finely and quickly so as to provide the gap with a required thickness is difficult. 
     As opposed to the above way of controlling the air film gap, in the present invention adjusting amounts of blown air and suctioned air causes the upper plate  116  to move in a prescribed manner in accordance with the amounts of blown air and suctioned air, when these amounts are adjusted, and accordingly the thickness of the air film gap varies in a prescribed manner. As described above, the upper plate  116  can be controlled more reliably, leading to fine and quick controlling of the air film gap. Such controlling of the gap can be achieved only by adjusting the amounts of blowing air and suction air when the upper plate  116  has a levitation force from the air nozzles  170   a  to  170   h  together with an attraction force from the vacuum suction groove  176  exerted thereon as in the previously described and preferred embodiment. 
     Although an air bearing applied for achieving a rotary motion of the turntable  160  of the roundness tester  150  is described by way of example in this embodiment, the air bearing is also suitably used for achieving a linear motion necessary for an accurate and stable feed motion, for example, of the detection head  164  of the roundness tester  150 . However, the present invention is not limited to roundness testers, but is applicable to any air bearing drive systems. Moreover, the present invention is not limited to the foregoing configurations of the air nozzles and the suction inlet. 
     Furthermore, although the foregoing configuration, in which the air nozzles are plural, the suction inlet has a groove shape, and a levitation force from the air nozzles and an attraction force from the suction inlet are balanced by adjusting the levitation force while the attraction force is kept constant when the moving portion has the levitation force together with the attraction force exerted thereon, is described by way of example, other configurations can be employed as long as an air film gap is controlled by adjusting a levitation force from an air nozzle and an attraction force from a suction inlet when the moving portion has the levitation force together with the attraction force exerted thereon. 
     For example, it is preferable to balance a levitation force from an air nozzle and an attraction force from suction inlets by configuring the air nozzle in a groove-like shape and the suction inlets to be plural, and by adjusting the attraction force while the levitation force is kept constant when the moving portion has the levitation force together with the attraction force exerted thereon. 
     It is also preferable to balance a levitation force from the air nozzles and an attraction force from the suction inlet, when the moving portion has the levitation force together with the attraction force exerted thereon. 
     For example, if, before control operation starts, the upper plate  116  has its right half slanted upwardly relative to the stator  112 , with respect to the center line drawn from the air nozzles  170   a  to  170   e , then the smallest air film gap lies in the vicinity of the air nozzle  170   g  and the largest air film gap lies in the vicinity of the air nozzle  170   c . When the control operation starts, the computer  156  controls the air-supply adjusting means  174  for independently adjusting amounts of air blowing from the air nozzles  170   a  to  170   h , and while the upper plate  116  has an attraction force from the suction groove  176  exerted on the lower surface thereof, so that the surface-to-surface parallelism of the gap formed by the above air film is corrected. Accordingly, the stator  112  and the upper plate  116  have an air film gap of the required uniform thickness. 
     As a result, the upper plate  116  has a gap having a required uniform thickness relative to the stator  114 , thereby achieving required parallelism between the upper surface of the stator  112  and the lower surface of the upper plate  116  without requiring strict geometrical precision for processing and finishing the stator  112 , the upper plate  116 , etc. This configuration allows the drive system  110  to achieve a reduced cost for processing and finishing the components thereof and also allows the turntable  160  to rotate accurately and stably. 
     As opposed to the above described situation, if the upper plate  116  has its left half slanted upwardly relative to the stator  112 , with respect to the center line drawn from the air nozzles  170   a  to  170   e , then the largest air film gap lies in the vicinity of the air nozzle  170   g  and the smallest air film gap lies in the vicinity of the air nozzle  170   c . Therefore, when control stars, the computer  156  controls the air-supply adjusting means  174  for independently adjusting amounts of air blowing from the air nozzles  170   a  to  170   h , while the upper plate  116  has an attraction force from the suction groove  176  exerted on the lower surface thereof, so that surface-to surface parallelism of the above air film gap is corrected. Accordingly, the stator  112  and the upper plate  116  have an air film gap with a required uniform thickness. 
     As a result, the upper plate  116  has a gap having a required uniform thickness relative to the stator  114 , thereby achieving required parallelism between the upper surface of the stator  112  and the lower surface of the upper plate  116  without requiring strict geometrical precision for processing and finishing the stator  112 , the upper plate  116 , etc. This configuration allows the drive system  110  to achieve a reduced cost for processing and finishing the components thereof and also allows the turntable  160  to rotate accurately and stably. 
     As described above, by balancing the levitation force  175  from the air nozzles  170   a  to  170   h  and the attraction force  182  from the vacuum suction groove  176 , when the upper plate  116  has the levitation force  175  together with the attraction force  182  exerted on the lower surface thereof, the computer  156 , i.e., the control means, controls the air film gap so as to maintain a required uniform thickness, thereby allowing the turntable  160  to rotate accurately and stably. 
     Furthermore, it is preferable to provide the roundness tester  150  with additional functions such as an automatic detection of a thickness of an air film gap and an automatic control of surface-to-surface parallelism on the basis of detection results for achieving easy handling. 
     Although the drive system  110  is provided with three gap sensors by way of example in the above-described embodiment, a detection sensor for detecting a thickness of an air film gap between the stator  112  and the upper plate  116  may be disposed at each of the air nozzles  170   a  to  170   h  in place of the above gap sensors and may be connected to the computer  156  via the electrical unit  154 . The computer  156  has storing means for storing control information for achieving required surface-to-surface parallelism between the stator  112  and the upper plate  116 , and also for adjusting amounts of blowing air and suctioned air in response to a thickness of each air film gap. 
     Also, the computer  156  has signal input means and a CPU. When the CPU of the computer  156  receives a command signal for controlling the air film gap via the input means, the computer  156  reads the detection results of the sensors via the electrical unit  154 . 
     On the basis of the read detection results, the CPU computes each amount of blown air and suction air for achieving the required surface-to-surface parallelism from the control information stored in the storing means and inputs the computed values into the adjusting means  174  and  180 . Since the adjusting means  174  and  180  operate in response to the computed values, for example, the surface-to-surface parallelism can be adjusted automatically based on the detection data of the sensors, which detect each air film gap, and also based on the detection results of the computer  156  and the adjusting means  174  and  180 . 
     Since the sensors detect a gap of each air film in real time when the roundness tester  150  is in operation, the computer  156  and the adjusting means  174  and  180  perform an automatic control for adjusting the surface-to-surface parallelism in response to the detection results when the thickness of the gap varies, thereby providing easy handling and always maintaining high geometrical precision of the surface-to-surface parallelism and the like. 
     Although the foregoing gap sensors are of a capacitance type, non-contact sensors of an electromagnetic type, an optical type, and the like may be used. 
     Instead of the computer  156  used in the drive system  110  according to the foregoing embodiment, the air-supply adjusting means  174  may be controlled automatically by using pneumatic micrometers serving as gap sensors and also by using a pressure operator for computing an air pressure. 
     Furthermore, although the air-supply adjusting means  174  and the attracting means  178  adjust amounts of blowing air and suction air, respectively, in the above-described embodiment, instead of this configuration, the air suction lines  177  may be provided with a pressure sensor for measuring a negative pressure (i.e., a sensor for measuring an attraction force), and the air-suction adjusting means  180  may adjust an amount of suction air so as to keep a negative pressure (i.e., an attraction force) constant while keeping an amount of blowing air constant. With this configuration, when the workpiece  162  placed on the turntable  160  is heavy and the gap becomes smaller, the gap has less air blown therein and, accordingly, has an increased negative pressure (i.e., an increased degree of vacuum). By reducing the negative pressure, i.e., by reducing the attraction force, so as to restore the negative pressure to the prescribed value, the amount of air drawn or suctioned from the gap decreases, causing the gap to become wider. On the other hand, when the workpiece  162  is light and the gap becomes wider, reducing the attraction force so as to increase an amount of air blowing into the gap causes the gap to become narrower. In other words, by disposing a sensor for measuring a negative pressure in the air suction lines  177  for suctioning air from the suction inlet  176  and also by adjusting the negative pressure so as to maintain the attraction force constant, the levitation height of the moving flat surface can be maintained constant relative to the thrust flat surface, regardless of the weight of the workpiece  162 . The number of the pressure sensors for measuring a negative pressure may be at least one; however, a plurality of the pressure sensors may be disposed when the air suction lines  177  are complicated.