Abstract:
An electrostatic actuator includes a stator having a circular surface carrying a plurality of drive electrodes, a rotor arranged opposite to the surface of the stator and a drive circuit for applying a drive voltage to the drive electrodes. Each of the drive electrodes has an arcuate profile extending from the center toward the outer periphery of the circular surface. The rotor can be driven to rotate in either direction of rotation by changing the mode of applying voltages to the drive electrodes. The rotor is floated above the stator by dynamic pressure generated by an oscillator, typically a piezoelectric element, provided under the stator. The distance between the rotor and the stator can be adjusted by controlling drive voltages applied to the oscillator.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an electrostatic actuator and a method of driving the same. More specifically, the present invention relates to an electrostatic actuator that does not comprise any structural bearing nor guide and is compact and adapted to support the rotor in a non-contact way and drive it in a desired direction of rotation with a low voltage. 
     2. Related Background Art 
     Various electrostatic actuators have been proposed in recent years. The electrostatic actuator provides the following advantages relative to the electromagnetic actuator. 
     (1) It is highly efficient and does not give rises to any loss in a stationary state. 
     (2) Since it comprises only electrodes, it is structurally simple and adapted to down-sizing. 
     FIG. 1 of the accompanying drawings is a schematic perspective view of a floating rotary drive system using an electrostatic actuator that was proposed at page 537 of the released papers for the lectures in the 1998 Spring Meeting of the Japan Society for Precision Engineering. The floating rotary drive system uses an electrostatic actuator comprising a rotor  401  and a stator  402  and is designed to eliminate the frictional force between the rotary  401  and the stator  402  by causing the rotor  401  to acoustically float relative to the stator  402 . FIG. 2 schematically illustrates the pattern of arrangement of the electrodes on the stator  402  of the electrostatic actuator of FIG.  1 . 
     Referring to FIGS. 1 and 2, the oscillator  404  is driven to operate according to a signal from oscillator driver  405 . With this electrostatic actuator, an acoustic floating force is generated on the surface of the stator  402  as the oscillator  404  is driven to operate and the oscillation of the oscillator  404  is amplified by means of a horn  403  so that the rotor  401  is supported above the surface of the stator  402  in a non-contact way. Thus, the rotor  401  is driven to rotate as a drive voltage is applied to drive electrodes  406  according to the signal from electrostatic drive circuit  407 . The drive voltage output from the electrostatic drive circuit  407  is a 3-phase AC voltage. The rotor is made of glass. 
     As shown in FIG. 2, a large number of circular electrodes  406  are arranged on the stator  402  so that both a torque and a centripetal force arise simultaneously and are applied to the rotor  401 . The drive voltage is 1 kV p-p and the rotor  401  rotates only clockwise. 
     The above described known electrostatic actuator is, however, accompanied by the following problems. Firstly, the rotor of the electrostatic actuator rotates only clockwise. Secondary, the electrodes of the stator have a circular profile and are separated from adjacent ones by a large distance so that the inductive electric charge generated on the rotor is located remote from the electrodes on the stator to consequently reduce the rotary force that is actually applied to the rotor. In other words, a large drive voltage is required to make the electrostatic actuator operate properly. Then, the overall arrangement will be rather bulky. 
     SUMMARY OF THE INVENTION 
     In view of the above identified problems of the prior art, it is therefore the object of the present invention to provide an electrostatic actuator that is compact and adapted to drive the rotor in a desired direction of rotation with a low drive voltage and a method of driving such an electrostatic actuator. 
     According to the invention, the above object is achieved by providing an electrostatic actuator comprising: 
     a stator formed by a circular substrate having a surface carrying thereon a plurality of drive electrodes, each of the drive electrodes having an arcuate profile extending from the center of the substrate toward the outer periphery thereof; 
     a rotor arranged opposite to the surface of the stator carrying thereon the drive electrodes; and 
     a drive circuit for applying a drive voltage to the drive electrodes of the stator. 
     According to the invention, there is also provided a rotary stage comprising: 
     an electrostatic actuator as defined above; and 
     a sample holding jig arranged on the rotor of the electrostatic actuator. 
     According to the invention, there is also provided an optical scanning device comprising: 
     a light source adapted to emit a laser beam; 
     an electrostatic actuator as defined above; and 
     a polygon mirror arranged on the rotor of the electrostatic actuator and adapted to deflect the laser beam emitted from the light source. 
     According to another aspect of the invention, there is also provided a method of driving an electrostatic actuator comprising a stator formed by a circular substrate having a surface carrying thereon a plurality of drive electrodes, each of the drive electrodes having an arcuate profile extending from the center of the substrate toward the outer periphery thereof, a rotor arranged opposite to the surface of the stator carrying thereon the drive electrodes, a drive circuit for applying a drive voltage to the drive electrodes of the stator and an oscillator for generating dynamic pressure between the stator and the rotor, the method comprising the steps of: 
     generating dynamic pressure between the stator and the rotor by means of the oscillator to float the rotor above the stator; and 
     applying a multiphase drive voltage to the drive circuit to drive the rotor to rotate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view of a floating rotary drive system, using a conventional electrostatic actuator. 
     FIG. 2 is a schematic illustration of the pattern of arrangement of the electrodes on the stator of the electrostatic actuator of FIG.  1 . 
     FIG. 3 is a schematic perspective view of a preferred embodiment of electrostatic actuator according to the invention. 
     FIG. 4 is a schematic illustration of the pattern of arrangement of the electrodes on the stator of the electrostatic actuator of FIG.  3 . 
     FIG.  5 A and FIG. 5B are schematic cross sectional views of the embodiment of electrostatic actuator of FIG. 4 taken between A and B in FIG.  4 . 
     FIG.  6 A and FIG. 6B are schematic illustrations of waveforms of two different signals that can be applied to the drive electrodes of the embodiment of electrostatic actuator of FIG.  4 . 
     FIG.  7 A and FIG. 7B are schematic illustrations of possible potential distributions on part of the stator of the embodiment of electrostatic actuator of FIG.  4 . 
     FIG.  8 A and FIG. 8B are schematic illustrations of possible potential distributions on the rotor located above the stator of FIGS. 7A and 7B. 
     FIG. 9 is a schematic illustration of the pattern of arrangement of the electrodes of the stator of the electrostatic actuator of Example 1. 
     FIG. 10 is a schematic illustration of the pattern of arrangement of the electrodes of the stator of the electrostatic actuator of Example 2. 
     FIG. 11 is a schematic illustration of a rotary stage using an electrostatic actuator according to the invention. 
     FIG. 12 is a schematic illustration of an optical scanning device using an electrostatic actuator according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be described in greater detail by referring to the accompanying drawings that illustrate a preferred embodiment of the invention. 
     FIG. 3 is a schematic perspective view of an embodiment of electrostatic actuator according to the invention. Referring to FIG. 3, an oscillator  107  and a rotor  101  are arranged respectively under and above a stator  102 . As an AC voltage is applied by a power source  112  between the drive electrodes  110   a  and  110   b  of the oscillator  107 , a squeeze film is produced between the rotor  101  and the stator  102  to support the rotor  101  above the stator  102  in a non-contact way. A control circuit  113  is arranged between the power source  112  and the oscillator drive electrodes  110   a  and  110   b.  The rotor  101  can be moved vertically to regulate the clearance between the stator  102  and the rotor  101  by changing the amplitude of the AC voltage by means of the control circuit  113 . 
     A piezoelectric element can be used for the oscillator  107  so as to down-size the electrostatic oscillator. 
     In this embodiment, the surface of the stator  102  that is fitted to and covers the oscillator  107  has a surface area greater than the surface area of the oscillator  107  that is held in contact with the stator  102 . With this arrangement, the rotor  101  can reliably be held on the stator  102 . 
     The rotor  101  is driven to rotate by means of the electrostatic force generated relative to the inductive electric charge generated on the rotor  101 . As shown in FIG. 4, a plurality of drive electrodes having an arcuate profile are arranged radially on the stator  102 . In this embodiment, the drive electrodes are divided into three drive electrode groups  109   a  through  109   c  and the drive electrodes of each of the groups are electrically connected to each other. A3-phase drive voltage is applied to the drive electrode groups  109   a  through  109   c  from a drive circuit  111 . 
     Now, the principle underlying the operation of driving an electrostatic actuator according to the invention will be described below. FIG.  5 A and FIG. 5B are schematic cross sectional views of the embodiment of the electrostatic actuator of FIG. 4 taken peripherally between A and B in FIG.  4 . The stator  102  includes a substrate  104  and drive electrode groups  109   a  through  109   c  and an insulating thin film  103  is formed between adjacently located electrodes and also on the surfaces of the electrodes. The oscillator  107  is located under the lower surface of the stator  102 . Assume that the rotor  101  is driven to rotate in the direction of rotation represented by arrow θ in FIG.  4 . 
     Firstly, the electrical potentials of the drive electrode groups  109   a ,  109   b , and  109   c  are set respectively to 0, +, and − as shown in FIG.  5 A. At that time, electric charges + and − are locally induced on the rotor  101  that is floated. Then, if the electrical potentials of the drive electrode groups  109   a ,  109   b , and  109   c  are changed respectively to −, 0, and + as shown in FIG. 5B, both the electric charges induced on the rotor  101  and the electrical potentials of the stator  102  are subjected to an electrostatic force to consequently move the rotor  101  in the direction of +θ. If the change of the potentials is repeated, the rotor  101  is moved further in the direction of +θ. The voltage applied to the drive electrode groups  109   a  through  109   c  are made to have a waveform that changes with time, such as a rectangular waveform as shown in (a) through (c) of FIG. 6A or a 3-phase AC wave shown in (a) through (c) of FIG.  6 B. 
     According to the invention, as the drive electrodes are formed to show an arcuate profile, the rotor is subjected to a rotary force and a centripetal force simultaneously so that it is driven to rotate on a stable basis without using any mechanical bearing. Additionally, according to the invention, because the distance separating adjacently located drive electrodes can be reduced, it is possible to bring the inductive electric charges generated on the rotor and the drive electrodes very close to each other to consequently raise the level of the force that is generated and applied to the rotor to drive the latter. Then, an electrostatic actuator according to the invention can be driven to operate with a low voltage if compared with conventional electrostatic actuators. Still additionally, since 3-phase drive electrodes can be arranged densely on the stator of an electrostatic actuator according to the invention so as to generate a rotary force at any position on the rotor, it is possible to drive the rotor to rotate bidirectionally. 
     Now, the principle underlying the generation of the centripetal force of an electrostatic actuator according to the invention will be described below by referring to FIGS. 7A through 8B. FIG.  7 A and FIG. 7B are schematic illustrations of possible potential distributions on part of the stator of the embodiment of the electrostatic actuator of FIG.  4 . Each of the sectors is connected arcuately from the lower-left direction toward the upper-right direction and corresponds to one of the drive electrode groups  109   a  through  109   c  shown in FIG. 4, to which a 3-phase voltage can be applied. FIG.  8 A and FIG. 8B are schematic illustrations of possible potential distributions on part of the rotor located above the stator illustrated in FIG.  7 A and FIG.  7 B. Note that in FIGS. 7A through 8B, the lower end corresponds to the center of the rotor, while the upper end corresponds to an outer peripheral area of the rotor. 
     Firstly assume that the electric potentials of the drive electrode groups  109   a ,  109   b  and  109   c  are −, 0, and + respectively as shown in FIG.  7 A. Then, electric charges are locally induced on the rotor  101  as shown in FIG.  8 A. If the electric potentials of the drive electrode groups  109   a,    109   b  and  109   c  are changed respectively to +, −, and 0 as shown in FIG. 7B, the rotor  101  is driven to rotate clockwise by the rightward rotary force and the downward centripetal force that are generated there as shown in FIG.  8 B. 
     If the change of the potentials is repeated, the rotor  101  is moved to turn clockwise further in the direction of +θ. Since the rotor  101  is constantly subjected to a centripetal force toward the rotation axis, it can be driven to rotate on a stable basis without using any structural bearing. 
     Now, the present invention will be described further by way of examples. 
     EXAMPLE 1 
     Now the electrostatic actuator of Example 1 will be described below. The electrostatic actuator of this example has a configuration the same as that of the above-described embodiment shown in FIG.  3 . The stator  102  is made of glass and has a diameter of 50 mm and a height of 0.5 mm. FIG. 9 is a schematic illustration of the pattern of arrangement of the electrodes of the stator  102  of the electrostatic actuator of Example 1. Note that FIG. 9 shows only part of the electrode pattern as enlarged in the rotational direction for the purpose of simplicity and easy understanding. 
     The drive voltage is applied to the drive electrode groups  609   a  through  609   c  of FIG. 9 at respective phases of the voltage. Each of the drive electrodes of the drive electrode groups  609   a  through  609   c  has an arcuate profile extending from the center toward the outer periphery of the stator with its width proportional to the distance from the rotation center of the stator. With this arrangement, the induced electric charges on the rotor show a uniform distribution to make the rotor rotate on a stable basis. 
     In this example, any two adjacently located drive electrodes form a central angle of 1 degree and have a width of 17 to 87 μm. On the other hand, the rotor  101  is made of glass and has a diameter of 16 mm and a height of 0.5 mm. The oscillator  107  is a Langevin type oscillator (NBL-15602S: tradename available from Tokin) adapted to be tightened by means of a bolt and having a resonance frequency of 60 kHz. 
     The electrostatic actuator of this example was operated with an electrostatic drive voltage of 75V rms, a drive frequency of 10 to 50 Hz, an oscillator drive voltage of 35V rms and a gap of 10 μm separating the stator and the rotor to find that the electrostatic actuator could be driven to rotate bidirectionally on a stable basis. 
     EXAMPLE 2 
     Now, the electrostatic actuator of Example 2 will be described below. The electrostatic actuator of this example has a configuration the same as that of the above-described embodiment shown in FIG.  3 . The stator  102  is made of glass and has a diameter of 50 mm and a height of 0.5 mm. FIG. 10 is a schematic illustration of the pattern of arrangement of the electrodes of the stator  102  of the electrostatic actuator of Example 2. Note that FIG. 10 shows only part of the electrode pattern as enlarged in a rotational direction for the purpose of simplicity and easy understanding. 
     The drive voltage is applied to the drive electrode groups  709   a  through  709   c  of FIG. 10 at respective phases of the voltage. Each of the drive electrodes of the drive electrode groups  709   a  through  709   c  has sector-like sections that are connected to show a quasi-arcuate profile extending from the center toward the outer periphery of the stator. With this arrangement where each of the drive electrodes has sector-like sections, the drive electrodes can be densely distributed to reduce the distance separating adjacently located drive electrodes so that it is possible to bring the inductive electric charges generated on the rotor and the drive electrodes very close to each other to consequently raise the level of the force that is generated and applied to the rotor to drive the latter. Then, an electrostatic actuator of this example can be driven to operate with a low voltage when compared with conventional electrostatic actuators. Still additionally, since 3-phase drive electrodes can be arranged densely on the stator of an electrostatic actuator of this example so as to generate a rotary force at any position on the rotor, it is possible to drive the rotor to rotate bidirectionally. 
     In this example, any two adjacently located drive electrodes form a central angle of 1 degree and have a width of 28 to 140 μm so that they are located extremely close to each other. On the other hand, the rotor  101  is made of glass and has a diameter of 16 mm and a height of 0.5 mm. The oscillator  107  is a Langevin type oscillator (NBL-15602S: tradename available from Tokin) adapted to be tightened by means of a bolt and having a resonance frequency of 60 kHz. 
     The electrostatic actuator of this example was operated with an electrostatic drive voltage of 50V rms, a drive frequency of 10 to 50 Hz, an oscillator drive voltage of 35V rms and a gap of 10 μm separating the stator and the rotor to find that the electrostatic actuator could be driven to rotate bidirectionally on a stable basis. 
     EXAMPLE 3 
     In Example 3, a rotary stage that can be driven to rotate and also move along the Z-axis and is adapted to be used for an optical microscope was prepared by using an electrostatic actuator as described in Example 1 or 2. FIG. 11 is a schematic illustration of the rotary stage of this example. Note that the lens barrel  208  of an optical microscope is arranged in the Z-direction above the rotary stage in FIG.  11 . 
     The rotary stage of this example comprises a stool  209 , an oscillator  207  rigidly held to the stool  209  and a stator  202  arranged on the oscillator  207 . A rotor  201  is arranged above the stator  202 . A sample holding jig  210  is fitted onto the rotor  201  and a sample  206  is placed on the sample holding jig  210 . 
     For driving the rotary stage of this example, an AC voltage is applied to the oscillator  207  to produce a squeeze film between the stator  202  and the rotor  201  and float the rotor  201 . Then, a voltage is applied to the drive electrodes arranged in the stator  202  to drive the rotor  201  in order to rotate the entire sample  206  for a full examination. The rotor  201  can be moved along the Z-axis by changing the voltage being applied to the oscillator  207 . The rotor  201  is driven to rotate by the electrostatic force acting on the inductive electric charges generated on the rotor  201 . 
     The rotor  201  is driven to move along the Z-axis by the squeeze film produced there. Three-phase drive electrodes are formed in the stator  202  having a glass-made substrate to drive the rotor  201  to move along the axis of rotation and an insulating thin film is arranged on the surfaces of the drive electrodes to cover the latter. 
     Thus, a compact rotary stage having a simple configuration and adapted to align the rotor around the axis of bidirectional rotation and also along the Z-axis in a non-contact way was prepared in this example. Since the rotor is driven to rotate in a non-contact way, no friction occurs between the rotor and the stator and therefore no friction-related problems such as the emission of heat and that of noise arise. Thus, unlike a structural support mechanism such as a ball bearing, the rotary stage of this example that is free from friction provides the advantage of easy maintenance and hence can find applications in strict environments, such as a clean room, because it does not give rise to any dust. 
     EXAMPLE 4 
     In Example 4, an optical scanning device was prepared by using an electrostatic actuator as described in Example 1 or 2. FIG. 12 is a schematic illustration of the optical scanning device of this example. 
     Polygon mirror  303  is driven to rotate at a high rate by means of an electrostatic actuator according to the invention. The laser beam emitted from a light source, which is a semiconductor laser  301 , strikes the polygon mirror  303 . The laser beam reflected (deflected) by the polygon mirror  303  is focussed on a projection surface  304  by means of a scanning lens  302 . As the polygon mirror  303  is driven to rotate by the electrostatic actuator, the laser beam scans the projection surface  304 . 
     A compact polygon mirror drive system that has a simple configuration and is adapted to support a polygon mirror in a non-contact way can be prepared by using an electrostatic actuator according to the invention. Since the rotor carrying the polygon mirror is driven to rotate in a non-contact way, no friction occurs between the rotor and the stator and therefore no friction-related problems such as the emission of heat and that of noise arise. Thus, unlike a structural support mechanism, such as a ball bearing, the polygon mirror drive system of this example that is free from friction provides the advantage of easy maintenance. 
     As described above in detail, according to the invention, there is provided a compact electrostatic actuator having a simple configuration and adapted to align the rotor around the axis of bidirectional rotation and also along the Z-axis in a non-contact way. Additionally, an electrostatic actuator according to the invention can suitably be applied to a stage to be used for an optical microscope that is adapted to move bidirectionally around the axis of rotation and also along the Z-axis or to a drive system to be used for driving a polygon mirror.