Abstract:
A microelectromechanical modulation device comprising: a movable portion, which is supported to be elastically and bi-directionally displaced and which has a modulation function; and a plurality of driving sources operative to apply a physical action force to said movable portion, wherein when said movable portion is displacement-driven in a first direction, a physical action force for suppressing oscillation of said movable portion is applied to said movable portion in a second direction different from the first direction by said drive sources while said movable portion is transited in the first direction.

Description:
This application is based on Japanese Patent application JP 2004-287975, filed Sep. 30, 2004, the entire content of which is hereby incorporated by reference. This claim for priority benefit is being filed concurrently with the filing of this application. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to a microelectromechanical modulation device which has a movable portion that is bi-directionally displaced, to a microelectromechanical modulation device array, and to an image forming apparatus. More particularly, the present invention relates to improved techniques for damping the movable portion. 
     2. Description of the Related Art 
     In recent years, rapid progress of MEMS (MicroElectroMechanical Systems) technology has resulted in extensive development of a microelectromechanical modulation device that electrically displaces or moves a microstructure having a size in the order of micrometers. This microelectromechanical modulation device is, for example, a digital micromirror device (DMD), which deflects light by inclining a micromirror. The DMD is used for various purposes, such as a projection display, a video monitor, a graphic monitor, a television set, and an electrophotographic printing machine, in the field of optical information processing. 
     Generally, the microelectromechanical modulation device has a movable portion that is elastically displaceably supported and is bidirectionally displaced. This movable portion mainly performs a modulation operation. Therefore, it is very important for favorably performing a switching operation to control the damping of the movable portion. 
     For example, a micromirror device disclosed in JP-A-8-334709 (hereinafter referred to as JPA&#39;709) is configured so that a voltage is applied to one of a pair of drive electrodes, and that a movable portion having the mirror difsposed between these electrodes is rotated by an electrostatic attracting force determined according to the difference in electrical potential between the movable portion and the drive electrode and to the electrostatic capacity therebetween. 
     Further, a method of damping ribbon devices in a microelectromechanical grating device disclosed in JP-A-2001-174720 (hereinafter referred to as JPA&#39;720) is a method of damping electromechanical ribbon devices over a channel, which defines a bottom surface and has a bottom conductive layer formed below this bottom surface. This method includes the step of providing at least one constant amplitude voltage pulse to at least one ribbon device and the step of providing at least one damping pulse, which is separated by a narrow tentative gap from the constant amplitude voltage pulse, to the ribbon devices. That is, an electrostatic force is caused by one movable portion electrode and one fixed electrode of a parallel plate type device to act in a single direction. Further, the oscillation of the ribbon devices is controlled by a driving voltage, which used for attracting the ribbon device to a lower electrode, and two damping drive voltages that include an initial damping voltage, which is applied immediately before the application of the driving voltage, and a final damping voltage that is applied immediately after the application of the driving voltage. 
     Furthermore, an optical path switching device disclosed in JP-A-2002-169109 (hereinafter referred to as JPA&#39;109) has a mechanical optical switch, which switches an optical path by applying a signal voltage to an electromagnetically driven actuator, and a control circuit that supplies a signal voltage to the optical switch. Regarding the signal voltage, let V H  and T denote a rise amplitude and a width of a signal, respectively. When a time (T/2) elapses since the rise of the signal, the voltage level of the signal is equal or less than (⅔)V H . Further, when a time (T/2) elapses since the rise of the signal having a width T, the signal voltage applied to the actuator is reduced to a value that is equal to or less than (⅔) times the rise amplitude to thereby suppress the oscillation of a movable portion. 
     Additionally, a method of controlling a micromachine device, which is disclosed in JP-A-2002-36197 (hereinafter referred to as JPA&#39;197), is such that a first control signal and a second control signal are supplied to the micromachine device, that the second control signal sets the micromachine device to be in an active state, and that the first control signal causes this state of the micromachine device to maintain this state. The micromachine device is controlled by using at least two control signals, one of which sets the micromachine device to be in a pull-in state, and the other of which causes the micromachine device to maintain the pull-in state thereof. This enables the control of the micromachine device at a low voltage level. 
     However, in the micromirror device disclosed in JPA&#39;709, a voltage is applied to one of the drive electrodes. An electrostatic attracting force is generated according to the difference in potential between the movable portion and the drive electrode and the electrostatic capacity therebetween. Thus, the movable portion is rotated. Consequently, as illustrated in  FIG. 21A , a micromirror is transited to a contact position by the application of a voltage Va to the drive electrode. Immediately after the micromirror reaches the contact position, the mirror receives a reaction force from a contact member. Thus, the oscillation of the movable portion occurs. Even in a noncontact structure in which the micromirror does not reach the contact position, as illustrated in  FIG. 21B , the movable portion passes over a desired angle (that is, a convergence position), so that an overshoot of the mirror occurs. Consequently, it takes time to stop the oscillation. Such oscillation and overshoots hinder the speeding-up of a switching operation of the microelectromechanical modulation device. 
     Further, in the case of the micromechanical grating device disclosed in JPA&#39;720, as illustrated in  FIG. 22A , the constant amplitude voltage pulse  1  is a function of time. The constant amplitude voltage pulse  1  has a duration of 2 μseconds and also has a voltage value of 10V. Just after the constant amplitude voltage pulse  1 , a narrow damping pulse  5  separated from the constant amplitude voltage pulse  1  by a narrow gap  3  is applied. Furthermore, as illustrated in  FIG. 22B , in a case where an adjacent contact amplitude voltage pulse  7  has an opposite polarity, the polarity of a damping pulse  9  is opposite to that of the associated voltage pulse  7 . However, this microelectromechanical grating device is what is called the parallel plate type microelectromechanical modulation device configured so that a ribbon serving as a movable portion is parallel displaced to a substrate, and that thus, damping is performed by applying pulses to one movable-portion-side electrode and one fixed side electrode facing this movable-portion-side electrode. Consequently, this device has a disadvantage in that this device is poor in diversity of oscillation control methods. For instance, when the movable portion is attracted and displaced to the substrate, an opposite damping force cannot be applied to the movable portion. That is, the oscillation thereof cannot be actively reduced. 
     Moreover, the optical path switching device disclosed in JPA&#39;109 is adapted so that when the movable portion approaches an end of a yoke in the electromagnetically-driven actuator, that is, when the attracting force due to the magnitude of a magnetic field of the permanent magnet is enhanced, the movable portion is moved to a fiber-connection position so as to reduce the magnitude of the attracting force due to a coil magnetic field to thereby prevent the magnitude of a total attracting force from becoming too high. A signal outputted from a signal generation circuit has a waveform representing a signal voltage whose rise voltage V H  is 7V and whose level drastically drops after the rise. The width T of the signal is 5 ms. The voltage at an end of the signal is 0.5V. When a time (T/ 2 ) elapses since the rise of the signal, the voltage level of the signal is 2.8V. In a case illustrated in  FIG. 23B , the rise voltage is 7V. The width T of the signal is 5 ms. A time TO, in which the amplitude changes like a step, is 1.5 ms. In a case illustrated in  FIG. 23C , the rise voltage is 5V. A time T′, which is taken until the reduced amplitude reaches 1V, is 2 ms. The time T′ corresponds to the width T of a signal. The application of the voltage of 1V is continued until the next switching is performed after the lapse of the time T′. In a case illustrated in  FIG. 23D , the rise voltage is 5V. The time TO, in which the amplitude changes like a step, is 3 ms. A waveform is changed like a step, so that the amplitude is reduced to a constant value of 0.5V. The rise amplitudes of the waveforms representing these signal voltages are set to be large thereby to speed up the movement of the movable portion (that is, increase the switching rate thereof). When the movable portion starts to move, the signal voltage is rapidly lowered to thereby reduce the magnitude of the force applied to the movable portion. Thus, chattering can be suppressed. However, this optical path switching device is adapted so that the a block serving as the movable portion is bidirectionally parallel displaced, and that the oscillation of the movable portion is suppressed by changing a driving force acting in a forward direction. Consequently, this optical path switching device has a disadvantage in that this device is poor in diversity of oscillation control methods. Also, this optical path switching device is adapted so that basically, the magnitude of an attracting force due to the coil magnetic field is reduced, and that the signal voltage is reduced so as to prevent the magnitude of a total attracting force from being too high. Thus, similarly to the microelectromechanical grating device, this optical path switching device cannot actively reduce the oscillation of the movable portion. 
     Furthermore, according to the method of controlling a micromachine device, which is disclosed in JPA&#39;197, the micromachine device is controlled by using a single or a plurality of control signals.  FIGS. 24A to 24H  show exemplary waveforms of this control signal. As is seen from  FIGS. 24A and 24   b , the control signal may be a pulse train that changes the state of the micromachine device. Similarly, in the case of using at least two control signals, these signals may be a signal synthesized from superimposed signals respectively shown illustrated in  FIGS. 22C and 22D , an amplitude modulated (AM) signal illustrated in  FIG. 24E , a frequency modulated (FM) signal illustrated in  FIG. 24F , a pulse width modulated (PWM) signal, and a pulse density modulated (PDM) signal illustrated in  FIG. 24H . However, this control method aims at reduction of a holding voltage in the pull-in state, reduction of an on/off delay due to discharge of residual electric charges, and increase in amplitude of an output signal. Thus, this control method cannot actively reduce the oscillation of the movable portion. 
     SUMMARY OF THE INVENTION 
     The invention is accomplished in view of the aforementioned circumstances. An object of the invention is to provide a microelectromechanical modulation device enabled to actively reduce the oscillation of a movable portion by exerting a physically attracting force in a direction opposite to the direction of transition of a movable portion, and is to provide a microelectromechanical modulation device array and is to provide an image forming apparatus, thereby speeding up a switching operation. The foregoing object is achieved by the following configuration. 
     (1). A microelectromechanical modulation device comprising: 
     a movable portion, which is supported to be elastically and bi-directionally displaced and which has a modulation function; and 
     a plurality of driving sources operative to apply a physical action force to said movable portion, 
     wherein when said movable portion is displacement-driven in a first direction, a physical action force for suppressing oscillation of said movable portion is applied to said movable portion in a second direction different from the first direction by said drive sources while said movable portion is transited in the first direction. 
     The modulation device is adapted so that during the transition of the movable portion before reaching the final displacement position, a physical attracting force acts in a direction opposite to the direction of the transition, and that the speed of the movable portion is reduced just before the movable portion reaches the final displacement position. This suppresses occurrence of oscillation due to a collision, which would be caused when the movable portion of the related art device reached the final displacement position at a high speed, and also suppresses occurrence of an overshoot, which would be caused when the movable portion of the related art device reached the final displacement position during the noncontact driving thereof. That is, the oscillation of the movable portion, which is caused when the movable portion is put into contact with a stopping member, can be actively reduced. 
     (2). The microelectromechanical modulation device according to (1), wherein 
     a physical action force is applied to said movable portion by said drive sources in the first direction during said movable portion is transited in the second direction after said movable portion is displacement-driven in the first direction. 
     According to an embodiment of the modulation device of the invention, the movable portion is displacement-driven in the first direction. After the movable portion reaches the final position, a physical action force acting in the first direction is exerted on the movable portion during the movable portion is transited in the second direction by a reaction force or an elastic force caused when the movable portion abuts against the stopping member. Thus, when departing from the final displacement position, the movement of the movable portion is actively braked. 
     (3). The microelectromechanical modulation device according to (1) or (2), wherein 
     the physical action force is exerted to a plurality of points of action of said movable portion. 
     This modulation device of an embodiment of the invention employs a plurality of points of action. Thus, for example, a swing type movable portion, whose center serves as the center of rotation thereof, is adapted so that the physical action force is applied to both sides of the center of rotation. Consequently, damping forces respectively having different magnitudes can be applied to the points of action at different moments. Thus, various damping effects can be obtained. 
     (4). The microelectromechanical modulation device according to any one of ( 1 ) to ( 3 ), 
     wherein when said movable portion reaches a final position of displacement in a specific direction, a speed of said movable portion is substantially 0. 
     According to an embodiment of the modulation device of the invention, when the movable portion reaches the final position of the displacement in the specific direction, the speed of the movable portion is substantially zero at that moment. This prevents occurrence of oscillation due to a collision, which would be caused when the movable portion of the related art device reached the final displacement position at a high speed, and also prevents occurrence of an overshoot, which would be caused when the movable portion of the related device reached the final displacement position during the noncontact driving thereof. 
     (5). The microelectromechanical modulation device according to any one of (1) to (4), 
     wherein the physical action force, which is exerted by said drive sources and which displaces said movable portion in the first direction and the second direction, is an electrostatic force. 
     According to an embodiment of the modulation device of the invention, the physical action force is an electrostatic force. Thus, a high-speed oscillation suppressing force is obtained. 
     (6). The microelectromechanical modulation device according to any one of (1) to (5), wherein each of the physical action forces is applied so that a magnitude of the physical action force varies with time like a pulse waveform whose ordinates represent magnitudes and whose abscissa represent time. 
     According to an embodiment of the modulation device of the invention, the physical action force is generated within a range specified by the pulse waveform. Consequently, various damping effects are obtained. Incidentally, the pulse waveform includes the waveforms of a rectangular wave, a sinusoidal wave, a cosine wave, a saw tooth wave, a triangular wave, and waves synthesized from these waves. 
     (7). The microelectromechanical modulation device according to (6), 
     wherein the physical action forces are generated so that magnitudes of the physical action forces vary with time like a plurality of pulse waveforms. 
     According to an embodiment of the modulation device of the invention, physical action forces having different magnitudes are applied to the points of action at different moments. Thus, various damping effects can be obtained. 
     (8). The microelectromechanical modulation device according to any one of (1) to (7), 
     wherein two or more physical action forces can be set corresponding to each of directions of transition of said movable portion. 
     According to an embodiment of the modulation device of the invention, for instance, the swing type movable portion, whose center serves as the center of rotation thereof, is adapted so that two or more physical action forces are applied to each of both sides of the center of rotation. Consequently, different damping forces respectively having different magnitudes can be applied to each of the sides of the movable portion at different moments. Thus, various damping effects can be obtained. 
     (9). The microelectromechanical modulation device according to any one of (1) to (8), 
     wherein when said movable portion reaches a final position of displacement in a specific direction, said movable portion touches a stopping member and is stopped. 
     According to an embodiment of the modulation device of the invention, when the movable portion reaches the final position, the movable portion touches the stopping member (or the landing site) and is stopped. That is, this modulation device of the embodiment is operated as that of what is called the contact type. In this case, the movable portion receives a reaction force from the stopping member. However, the movable portion is braked by the electrostatic attracting force, and is forcibly damped. 
     (10). A microelectromechanical modulation device array comprising said microelectromechanical modulation devices according to any one of (1) to (9), which are one-dimensionally or two-dimensionally arranged. 
     According to an embodiment of the modulation device array of the invention, the modulation devices enabled to perform high-speed switching operations are formed into an array. This enables the reduction of an oscillation stopping time. Consequently, the writing of address voltages can be performed earlier, as compared with a related array. 
     (11). The microelectromechanical modulation device array according to (10), 
     wherein each of said microelectromechanical modulation devices has a driving circuit including a memory circuit; and 
     one of electrodes provided on said movable portion and at leas two fixed portions facing said movable portion is a signal electrode, to which an device displacement signal outputted from said driving circuit is inputted, and another of said electrodes is a common electrode. 
     This modulation device array of an embodiment of the invention has a memory circuit. The provision of such a memory circuit enables the preliminary writing of a deviced is placement signal to this memory circuit. Further, a constant common voltage is applied to the common electrode, similarly to a related art array. Simultaneously, the device displacement preliminarily written to the memory circuit is applied to the signal electrode. Thus, the plural modulation devices can be actively driven at a high speed. 
     (12). The microelectromechanical modulation device array according to (10) or (11), wherein the modulation device further comprising: 
     a control portion adapted to modulation-drive each of said movable portions. 
     According to an embodiment of this modulation device array of the invention, the movable portion is drive-controlled by the control portion. Thus, before the movable portion reaches the final displacement position, the reduction, or increase, or increase/decrease of the absolute value of the interelectrode voltage between the movable electrode and the fixed electrode is performed. This enables the suppression of oscillation due to a collision, which would be caused when the movable portion of the related device reached the final displacement position at a high speed, and also enables suppression of an overshoot. 
     (13). An image forming apparatus comprising: 
     a light source; 
     the microelectromechanical modulation device array according to any one of (10) to (12); 
     an illuminating optical system adapted to irradiate light, which is outputted from said light source, onto said microelectromechanical modulation device array; and 
     a projection optical system adapted to project light, which is outputted from said microelectromechanical modulation device array, onto an image forming surface. 
     This image forming apparatus of one embodiment of the invention has one of the modulation device arrays according to any one of (10) to (12) as a primary part. Thus, the oscillation of the movable portion can be actively reduced. The driving cycle can be reduced, as compared with a related art apparatus. This enables high-speed exposure of a photosensitive material, and display in a higher-pixel projector. Further, an image forming apparatus (or an exposure apparatus) adapted to perform gradation control by switching on/off of exposing light can reduce anon/off time. Thus, higher gradation can be realized. 
     The microelectromechanical modulation device according to the invention has a plurality of drive sources operative to apply a physical action force to the movable portion. When the movable portion is displacement-driven in a first direction, a physical action force for suppressing the oscillation of the movable portion is applied to the movable portion by the drive sources while the movable portion is transited in the first direction. Thus, a physical attraction force is exerted in a direction opposite to the direction of transition of the movable portion. The oscillation of the movable portion, which is caused when the movable portion is put into contact with a stopping member, can be actively reduced. Consequently, the speeding-up of a switching operation of the microelectromechanical modulation device can be achieved. 
     In accordance with the microelectromechanical modulation device array according to the invention, after the movable portion reaches the final displacement position, the oscillation of the movable portion is suppressed. Therefore, an oscillation stopping time can be eliminated or can be considerably reduced. The writing of address voltages can be performed without necessity for waiting for the stop of the oscillation. Consequently, the speeding-up of a switching operation can be achieved by reducing a driving cycle. 
     The image forming apparatus according to the invention has a light source, the microelectromechanical modulation device array according to any one of (10) to (12), an illuminating optical system adapted to irradiating light, which is outputted from the light source, onto the the microelectromechanical modulation device array, and a projection optical system adapted to project light, which is outputted from the microelectromechanical modulation device array, onto an image forming surface. Thus, as compared with a related art apparatus, the length of time required to perform a switching operation can be reduced. This enables high-speed exposure of a photosensitive material, and display in a higher-pixel projector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual view illustrating a first embodiment of a microelectromechanical modulation device according to the invention. 
         FIGS. 2A to 2C  are operation explanatory views illustrating a damping process of the microelectromechanical modulation device shown in  FIG. 1 . 
         FIGS. 3A to 3C  are explanatory views illustrating a behavior of a movable portion to which a pulse waveform is applied. 
         FIGS. 4A to 4C  are explanatory views illustrating a first modification to which two rectangular pulse waveforms are applied. 
         FIGS. 5A to 5C  are explanatory views illustrating a second modification to which a triangular pulse waveform is applied. 
         FIGS. 6A to 6C  are explanatory views illustrating a third modification to which two triangular pulse waveforms are applied. 
         FIGS. 7A to 7C  are explanatory views illustrating a fourth modification in which a pulse waveform is applied by being superimposed on a driving voltage Va. 
         FIGS. 8A to 8C  are explanatory views illustrating a fifth modification in which a pulse waveform is applied to a damping electrode after a pulse waveform is applied by being superimposed on the driving voltage Va. 
         FIGS. 9A to 9C  are explanatory views illustrating a sixth modification in which the driving voltage Va is reduced in a predetermined interval. 
         FIGS. 10A to 10C  are explanatory views illustrating a seventh modification in which the driving voltage Va is reduced at predetermined intervals. 
         FIGS. 11A to 11C  are explanatory views illustrating an eighth modification in which a constant voltage is applied after a pulse waveform is applied. 
         FIGS. 12A to 12C  are explanatory views illustrating a ninth modification in which a constant voltage is applied after a plurality of pulse waveforms are applied. 
         FIGS. 13A to 13C  are explanatory views illustrating a tenth modification in which a constant voltage is applied before a pulse waveform is applied. 
         FIG. 14  is a conceptual view illustrating a second embodiment of the microelectromechanical modulation device according to the invention. 
         FIG. 15  is a conceptual view illustrating a third embodiment of the microelectromechanical modulation device according to the invention. 
         FIG. 16  is a conceptual view illustrating a fourth embodiment of the microelectromechanical modulation device according to the invention. 
         FIGS. 17A to 17C  are explanatory views illustrating the checking of operations of a microelectromechanical modulation device, whose configuration is equivalent to that of the first embodiment, in a case of utilizing simulation. 
         FIGS. 18A and 18B  are explanatory views illustrating the checking of operations of a microelectromechanical modulation device, whose configuration is equivalent to that of the first embodiment, in a case of actually manufacturing this device. 
         FIGS. 19A to 19C  are explanatory views illustrating the behavior of a movable portion in a case where the invention is applied to a noncontact type microelectromechanical modulation device. 
         FIG. 20  is an explanatory view illustrating the configuration of a microelectromechanical modulation device array having a driving circuit in which each modulation device includes a memory circuit. 
         FIGS. 21A and 21B  are explanatory views illustrating the oscillation of a movable portion, which occurs in a related microelectromechanical modulation device. 
         FIGS. 22A and 22B  are explanatory views illustrating damping pulses applied in a related microelectromechanical grating device. 
         FIGS. 23A to 23D  are explanatory views illustrating signal voltage waveforms applied in a related optical path switching device. 
         FIGS. 24A to 24H  are explanatory views illustrating control signals applied according to a method of controlling a related micromachine device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, preferred embodiments of a microelectromechanical modulation device, a microelectromechanical modulation device array, and an image forming apparatus according to the invention are described with reference to the accompanying drawings. 
       FIG. 1  is a conceptual view illustrating a first embodiment of a microelectromechanical modulation device according to the invention.  FIGS. 2A to 2C  are operation explanatory views illustrating a damping process of the microelectromechanical modulation device shown in  FIG. 1 .  FIGS. 3A to 3C  are explanatory views illustrating a behavior of a movable portion to which a pulse waveform is applied. 
     The microelectromechanical modulation device (hereunder also referred to simply as a “modulation device”)  100  according to this embodiment includes a substrate  21 , a small-piece-like movable portion disposed in parallel to the substrate  21  through a gap  23 , hinges  29 ,  29  respectively extended from both edge parts of the movable portion  27 , and spacers  31 ,  31  supporting the movable portion  27  on the substrate  21  through these hinges  29 ,  29 , as fundamental components. By this configuration, the movable part is capable of being rotated and transited by the distortion of the hinges  29 ,  29 . 
     When the modulation device  100  is used in, for example, a DMD, the movable portion  27  serves as a light reflector (or a mirror portion). Additionally, the modulation device can modulate sound waves, fluids, and heat waves by preferably selecting the material of the movable portion  27 . 
     When the modulation device  100  is used as a light modulation device, the movable portion  27  serves as a light reflector (or mirror portion) and performs light modulation by deflection. However, the present invention is not limited thereto, this modulation method enables light modulation through other type or system of light modulation device by appropriately adopting structure or materials of the movable portion  27 . The Examples of the device include a transmission type, a shutter system, an interference system, a diffraction system, and a total reflection system. 
     In this embodiment, the movable portion  27  is stopped by being put into contact with a stopping member (not shown) when reaching at a final position of displacement in a specific direction. That is, this embodiment constitutes a contact type modulation device. 
     Therefore, when reaching the final position, the movable portion  27  is put into contact with the stopping member (what is called, a landing site) and is then stopped. 
     A first address electrode  35   a  and a second address electrode  35   b  are provided on both sides of the hinges  29 ,  29  on the central part of the top surface of the substrate  21 . Also, a movable electrode (not shown) is provided on a part of the movable portion  27 . The modulation device  100  applies a voltage to each of the first address electrode  35   a , the second address electrode  35   b , and the movable portion  27  as a fundamental operation. Thus, the movable portion  27  is swung and displaced. That is, in a case where the movable portion  27  is a mirror portion, the direction of reflection of light is deflected. 
     When a difference in potential is given between the movable portion  27  and each of the first address electrode  35   a  and the second address electrode  35   b  in the modulation device  100 , an electrostatic force is generated between the movable portion  27  and each of the electrodes, so that torque acts around each of the hinges  29 ,  29 . The electrostatic force generated at that time depends upon a vacuum dielectric constant, the area of the movable portion  27 , an applied voltage, and the distance between the movable portion  27  and each of the address electrodes. 
     Therefore, in a case where the vacuum dielectric constant, the area of the movable portion  27 , the distance between the movable portion  27  and each of the address electrodes, and the elastic coefficient of the hinges  29 ,  29  are constant, the movable portion  27  can be rotation-displaced sideways by controlling the potential at each of the electrodes. For instance, in a case where Va&gt;Vb, the magnitude of the electrostatic force generated between the first address electrode  35   a  and the movable portion  27  is larger than that of the electrostatic force generated between the second address electrode  35   b  and the movable portion  27 , so that the movable portion  27  is leftwardly inclined, as viewed in the figure. Conversely, in a case where Va&lt;Vb, the magnitude of the electrostatic force generated between the second address electrode  35   b  and the movable portion  27  is larger than that of the electrostatic force generated between the first address electrode  35   a  and the movable portion  27 , so that the movable portion  27  is rightwardly inclined, as viewed in the figure. 
     Thus, the movable electrode of the movable portion  27 , the first address electrode  35   a  and the second address electrode  35   b  serve as a drive source for rotation-displacing the movable portion  27 . A physical action force applied from such a drive source to the movable portion  27  is an electrostatic force to thereby enable high-speed rotation-displacement. 
     Incidentally, the physical action force acting upon the movable portion  27  may be a physical action force other than an electrostatic force. For instance, a force due to the effect of a piezoelectric device and an electromagnetic force are cited as the physical action force other than an electrostatic force. In this case, a piezoelectric actuator, which uses a piezoelectric device, and an electromagnetic actuator, which uses a magnet coil, are employed as the drive sources. 
     The modulation device  100  has the movable portion  28  that are bidirectionally displaced. This movable portion  27  has the modulation function. The movable portion  27  is rotation-displaced by a plurality of drive sources (the movable electrode of the movable portion  27 , the first address electrode  35   a , and the second address electrode  35   b ). Incidentally, the modulation device  100  is adapted so that when the movable portion  27  is displacement-driven in the leftward direction (corresponding to the first direction) shown in  FIG. 1 , a physical action force for suppressing the oscillation of the movable portion  27  is applied to the movable portion  27  by the drive sources in a second direction, which differs from the first direction) during the movable portion  27  is transited in the first direction. 
     As shown in  FIG. 2A , first, the driving voltage Va is applied to the first address electrode - 35   a  placed in an anti clockwise direction. Subsequently, as shown in  FIG. 2B , the oscillation suppressing voltage Vb is applied to the second address electrode  35   b  immediately before the left end of the movable portion  27  touches the stopping member. Consequently, as shown in  FIG. 2C , an electrostatic force is generated between the movable portion  27  and the second address electrode  35   b  by the oscillation suppressing voltage Vb. This electrostatic force attracts the right end of the movable portion  27  to the substrate  21 . This electrostatic force has an oscillation absorbing effect. Thus, the movable portion  27  stops simultaneously with the contact between the movable portion  27  and the stopping member. 
     Thus, when the movable portion  27  reaches the final position of the displacement in the specific direction, the speed of the movable portion  27  is substantially zero at that moment. This prevents occurrence of oscillation due to a collision, which would be caused when the movable portion of the related device reached the final displacement position at a high speed. 
     Further, when reaching the final position, the movable portion  27  touches the stopping member (or the landing site). Immediately after touching the stopping member, the movable portion  27  receives a reaction force from the stopping member. However, the movable portion  27  is braked by the electrostatic attracting force, and is forcibly damped. Additionally, because the physical action force having the oscillation absorbing effect is an electrostatic force, a high-speed oscillation suppressing force can be obtained. 
     Moreover, the physical action force is exerted onto a plurality of points of action of the movable portion  27  (in this embodiment, the left-side point and the right-side point of the movable portion  27 ). Thus, for example, a swing type movable portion  27 , whose center serves as the center of rotation thereof, is adapted so that the physical action force is applied to both sides of the center of rotation. Consequently, damping forces respectively having different magnitudes can be applied to the points of action at different moments. Thus, various damping effects can be obtained. 
     An oscillation suppressing voltage Vb for generating an electrostatic attracting force, that is, a voltage applied between the movable portion  27  and the second address electrode  35   b  can be set to have a pulse waveform illustrated in  FIG. 3B  whose ordinates represent the magnitude of the voltage and whose abscissas represent time. In this example, just before the movable portion  27  touches the stopping member, one “backward pulse waveform” p 1  is applied. Hereunder, a pulse waveform to be applied as the waveform of a signal representing an oscillation suppressing voltage Vb between the movable portion  27  and the second address electrode  35   b  is referred to as a “backward pulse wave”. Further, a pulse waveform to be applied as the waveform of a signal representing a driving voltage Va between the movable portion  27  and the first address electrode  35   a  is referred to as a “forward pulse wave”. 
     The waveform of a signal representing the oscillation suppressing voltage Vb is set to be such a pulse waveform. Thus, the electrostatic attracting force is generated within a range specified by the pulse waveform. Consequently, various damping effects are obtained. Incidentally, the pulse waveform includes the waveforms of a rectangular wave, a sinusoidal wave, a cosine wave, a saw tooth wave, a triangular wave, and waves synthesized from these waves. 
     This modulation device  100  is adapted so that during the transition of the movable portion  27  before reaching the final displacement position, a physical attracting force acts in a direction opposite to the direction of the transition, and that the speed of the movable portion  27  is reduced just before the movable portion  27  reaches the final displacement position. This suppresses occurrence of oscillation due to a collision, which would be caused when the movable portion of the related device reached the final displacement position at a high speed, and also suppresses occurrence of an overshoot, which would be caused when the movable portion of the related device reached the final displacement position during the noncontact driving thereof. That is, the oscillation of the movable portion  27 , which is caused when the movable portion is put into contact with a stopping member, can be actively reduced. 
     Next, various modifications of the pulse waveforms applied by being superimposed on the oscillation suppressing voltage Vb and the driving voltage Va so as to generate the electrostatic attracting force having the oscillation absorbing effect. 
       FIGS. 4A to 4C  are explanatory views illustrating a first modification to which two rectangular pulse waveforms are applied. 
     Incidentally, in the descriptions of the following embodiments and modifications, like reference characters designate like members or parts. The duplicate description of such members or parts is omitted herein. 
     This modification is adapted to apply a plurality of “backward pulse waveforms” P 2  and P 3  just before the movable portion  27  touches the stopping member. Although two pulse waveforms P 2  and P 3  are illustrated in  FIGS. 4A to 4C , three or more pulse waveforms may be applied. 
     According to this modification, electrostatic attracting forces respectively serving as physical action forces and having different magnitudes can be applied to the points of action at different moments. Thus, various damping effects can be obtained. 
       FIGS. 5A to 5C  are explanatory views illustrating a second modification to which a triangular pulse waveform is applied. 
     In this modification, the pulse wave P 4  is a triangular wave. Thus, the pulse wave may be a triangular wave, a sinusoidal wave, or the like. 
     According to this modification, an electrostatic attracting force serving as a physical action force is applied with abrupt timing with which a rectangular wave cannot be provided. 
       FIGS. 6A to 6C  are explanatory views illustrating a third modification to which two triangular pulse waveforms are applied. 
     This modification is adapted so that plural backward triangular pulse waves P 5  and P 6  are applied immediately before the movable portion  27  touches the stopping member. Although two pulse waveforms P 5  and P 6  are illustrated in  FIGS. 6A to 6C , three or more pulse waveforms may be applied. 
     According to this modification, electrostatic attracting forces respectively having steep waveforms and different magnitudes can be applied at different moments. 
       FIGS. 7A to 7C  are explanatory views illustrating a fourth modification in which a pulse waveform is applied by being superimposed on a driving voltage Va. 
     This modification is adapted so that after a backward pulse wave P 2  is applied just before the movable portion  27  touches the stopping member, the movable portion  27  touches the stopping member, and that subsequently, when the movable portion  27  is detached by a reaction force from the stopping member, a forward pulse wave P 7  is applied by being superimposed on the driving voltage Va. That is, after the movable portion  27  is displacement-driven in a counterclockwise direction (corresponding to a first direction), a physical action force is applied to the movable portion  27  in the first direction by the drive sources (that is, the first address electrode  35   a  and the movable portion  27 ) during the movable portion  27  is transited in a clockwise direction (corresponding to a second direction). 
     According to this modification, the movable portion  27  is displacement-driven in the first direction. After the movable portion  27  reaches the final position, a physical action force acting in the first direction is exerted on the movable portion  27  during the movable portion  27  is transited in the second direction by a reaction force or an elastic force caused when the movable portion  27  abuts against the stopping member. Thus, when departing from the final displacement position, the movement of the movable portion  27  is actively braked. 
       FIGS. 8A to 8C  are explanatory views illustrating a fifth modification in which a pulse waveform is applied to a damping electrode after a pulse waveform is applied by being superimposed on the driving voltage Va. 
     This modification is adapted so that the backward pulse wave P 2  and the forward pulse wave  7  are applied in order opposite to the order in which these pulse waves are applied in the modification illustrated in  FIGS. 7A to 7C . That is, immediately after the movable portion  27  touches the stopping member and is detached therefrom, the forward pulse wave P 7  is applied. Subsequently, the backward pulse wave P 2  is applied just before the movable portion  27  touches the stopping member again. 
     According to this modification, after the movable portion  27  reaches the final displacement position, a physical action force acting in the first direction is exerted on the movable portion  27  during the movable portion  27  is transited in the second direction by a reaction force or an elastic force caused when the movable portion  27  abuts against the stopping member. Thus, when departing from the final displacement position, the movement of the movable portion  27  is actively braked. Further, just before the movable portion  27 , which cannot be caused by the braking to stand still, touches the stopping member again, the backward pulse wave P 2  is applied, so that the movable portion  27  surely comes to a standstill. 
       FIGS. 9A to 9C  are explanatory views illustrating a sixth modification in which the driving voltage Va is reduced in a predetermined interval. 
     This modification is adapted so that the backward pulse wave P 8  is applied just before the movable portion  27  touches the stopping member, and that simultaneously with this, the pulse wave P 9 , whose voltage level is lowered, is applied. 
     According to this modification, the movable portion  27  is braked by the backward pulse wave P 8  immediately before the movable portion  27  touches the stopping member. At that time, the driving voltage Va is canceled by the pulse wave P 9 . Thus, the movable portion  27  is braked by a force having a larger magnitude. Incidentally, although the voltage level of the pulse wave P 9  is reduced to 0V in this case, the drop of the voltage level of the forward pulse wave P 9  may differ from 0V. 
       FIGS. 10A to 10C  are explanatory views illustrating a seventh modification in which the driving voltage Va is reduced at predetermined intervals. 
     This modification is adapted so that forward pulse waves P 9  and P 10 , which lower the voltage levels thereof, are applied, simultaneously with the application of a plurality of backward pulse waves P 2  and P 3 . 
     According to this modification, the operation of the modification illustrated in  FIGS. 9A to 9C  is repeated, so that the movable portion  27  is more surely braked. 
       FIGS. 11A to 11C  are explanatory views illustrating an eighth modification in which a constant voltage is applied after a pulse waveform is applied. 
     This modification is adapted so that a backward pulse wave P 1  is applied just before the movable portion  27  touches the stopping member, and that thereafter, a constant voltage Vb 1  is kept applied. That is, the normal voltage level of the backward pulse wave is not necessarily set to be 0V. 
     According to this modification, the movable portion is backwardly biased after touching the stopping member. Thus, the movable portion  27  can be driven at a small difference in potential. 
       FIGS. 12A to 12C  are explanatory views illustrating a ninth modification in which a constant voltage is applied after a plurality of pulse waveforms are applied. 
     This modification is adapted so that a constant voltage Vb 1  is kept applied during and after plural backward pulse waves P 2  and P 3  are applied just before the movable portion  27  touches the stopping member. 
     According to this modification, the oscillation absorbing effect can reliably be obtained. Also, the movable portion  27  can be driven at a small difference in potential. 
       FIGS. 13A to 13C  are explanatory views illustrating a tenth modification in which a constant voltage is applied before a pulse waveform is applied. 
     This modification is adapted so that a backward pulse wave P 1  is applied just before the movable portion  27  touches the stopping member, that subsequently, a constant voltage Vb 1  is kept applied, and that even before the backward pulse wave P 1  is applied, the constant voltage Vb 1  is applied as the oscillation suppressing voltage Vb. 
     According to this modification, the movable portion  27  is always backwardly biased, so that the drive portion  27  can be driven at a small difference in potential. The oscillation of the movable portion  27  is caused at all times during counterbalancing between the forward and backward electrostatic forces. In the case of the contact type device, as long as a backward voltage is within a range that allows the movable portion  27  to maintain a contact state by being-pulled-in, such a backward voltage can be always applied without problems. 
     Thus, the aforementioned microelectromechanical modulation device has a plurality of drive sources that exert physical action forces to the movable portion  27 . When the movable portion  27  is displacement-driven in the first direction by the drive sources, the physical action force is applied by the drive sources in the second direction, which differs from the first direction, to the movable portion  27  during the movable portion  27  is transited in the first direction. Consequently, a physical attracting force can be exerted in a direction opposite to the direction of transition of the movable portion  27 . Further, the oscillation of the movable portion  27 , which is caused when the movable portion  27  touches the stopping member, can be actively reduced. Consequently, the speeding-up of the switching operation in the modulation device  100  can be achieved. 
     Next, a second embodiment of the microelectromechanical device according to the invention is described hereinbelow. 
       FIG. 14  is a conceptual view illustrating the second embodiment of the microelectromechanical modulation device according to the invention. 
     A modulation device  200  according to this embodiment is configured so that two or more physical action forces can be set corresponding to the direction of transition of the movable portion  27 . That is, a set of a main first address electrode  35   a   1 , and an auxiliary first address electrode  35   a   2 , and another set of a main second address electrode  35   a   1  and an auxiliary second address electrode  35   b   2  are provided on both sides of the hinges  29 ,  29  on the central part of the top surface of the substrate  21 . A driving voltage Val is applied between the main first address electrode  35   a   1  and the movable portion  27 , while a driving voltage Va 2  is applied between the auxiliary first address electrode  35   a   2  and the movable portion  27 . Further, an oscillation suppressing voltage Vb 1  is applied between a main second address electrode  35   b   1  and the movable portion  27 , while another oscillation suppressing voltage Vb 2  is applied between an auxiliary second address electrode  35   b   2  and the movable portion  27 . 
     According to this modulation device  200 , in the swing type movable portion  27 , whose center serves as the center of rotation thereof, is adapted so that two or more physical action forces are applied to each of both sides of the center of rotation. 
     Consequently, different damping forces respectively having different magnitudes can be applied to each of the sides of the movable portion  27  at different moments. Thus, various damping effects can be obtained. 
     Next, a third embodiment of the microelectromechanical device according to the invention is described hereinbelow. 
       FIG. 15  is a conceptual view illustrating a third embodiment of the microelectromechanical modulation device according to the invention. 
     A modulation device  300  according to this embodiment is configured so that one of ends of a movable portion  41  is supported by and is fixed to the substrate  21  through the hinges  29 ,  29  and spacers  31 ,  31 . That is, the movable portion  41  is configured like a cantilever, so that the other end of the movable portion  41  is a free end. Further, a first address electrode  35   a  facing the free end of the movable portion  41  is provided on the substrate  21 . A second address electrode  35   b  to be formed on an opposed substrate (not shown) is provided on the side opposite to the first address electrode  35   a  so that the movable portion  41  is sandwiched between the first address electrode  35   a  and the second address electrode  35   b.    
     Even in the modulation device  300  having this configuration, a drive voltage Va is applied between the first address electrode  35   a  and the movable portion  41 , while an oscillation suppressing voltage is applied between the second address electrode  35   b  and the movable portion  41 . Thus, during the transition of the movable portion  27  before the movable portion  27  reaches the final displacement position (in this case, the stopping member at the side of the first address electrode  35   a ), an electrostatic attracting force is caused to act in a direction opposite to the direction of the transition. Thus, the speed of the movable portion  41  is reduced just before the movable portion reaches the final displacement position. 
     This suppresses occurrence of oscillation due to a collision, which would be caused when the movable portion of the related device reached the final displacement position at a high speed. That is, the oscillation of the movable portion  27 , which is caused when the movable portion is put into contact with the stopping member, can be actively reduced. 
     Next, a fourth embodiment of the microelectromechanical device according to the invention is described hereinbelow. 
       FIG. 16  is a conceptual view illustrating a fourth embodiment of the microelectromechanical modulation device according to the invention. 
     A modulation device  400  according to this embodiment is what is called a parallel plate type device. Both ends of the flat-plate-like movable portion  43  having both electrical conductivity and flexibility are connected to an insulating film  45  formed on this substrate  21  so that a predetermined gap  47  is provided between movable portion  43  and the insulating from  45 . A first address electrode  35   a  is disposed below the movable portion  43  through the insulating film  45 . A second address electrode  35   b  is disposed above the movable portion  43  through the insulating film  45 . That is, the movable portion  43  is configured like a center impeller type beam supported at both ends thereof between the first address electrode  35   a  and the second address electrode  35   b.    
     In such a parallel plate type modulation device  400 , a driving voltage Va is applied between the first address electrode  35   a  and the movable portion  43 , while an oscillation suppressing voltage is applied between the second address electrode  35   b  and the movable portion  43 . Thus, during the transition of the movable portion  43  before the movable portion  43  reaches the final displacement position (in this case, the stopping member at the side of the first address electrode  35   a ), an electrostatic attracting force is caused to act in a direction opposite to the direction of the transition. Consequently, the speed of the movable portion  43  is reduced just before the movable portion  43  reaches the final displacement position. 
     Further, the configuration of the modulation device according to the invention is not limited to that of each of the modulation devices according to the aforementioned embodiments. The direction, structure, and driving of the modulation device maybe optional. The invention can be applied to all modulation devices that are bidirectionally driven. 
     Next, results of simulation performed on the modulation device having the configuration of the first embodiment are described hereinbelow. 
       FIGS. 17A to 17C  are explanatory views illustrating the checking of operations of a microelectromechanical modulation device, whose configuration is equivalent to that of the first embodiment, in a case of utilizing simulation. 
     The oscillation of the movable portion, which was caused after the transition thereof, in a rotating hinge type micromachine device shown in  FIG. 1  was analyzed by setting a forward electric potential difference and a backward electric potential difference at Va and Vb, respectively. 
     Results of this analysis reveal that the movable portion largely oscillated in a case where the potential difference Va=V 1  was applied after the lapse of time t 1 , and that the oscillation of the movable portion was suppressed in a case where the potential difference Va=V 1  was applied after the lapse of time t 1  and where the potential difference Vb=V 2  was applied between a moment t 2  and a moment t 3 . 
     Next, results of manufacturing a modulation device, which has the configuration of the first embodiment, and actually operating the manufactured modulation device are described hereinbelow. 
       FIGS. 18A and 18B  are explanatory views illustrating the checking of operations of the microelectromechanical modulation device, whose configuration is equivalent to that of the first embodiment, in the case of actually manufacturing this device. 
     The oscillation of the movable portion, which was caused after the transition thereof, in a real rotating hinge type micromachine device shown in  FIG. 1  was analyzed by setting a forward electric potential difference and a backward electric potential difference at Va and Vb, respectively. 
     Results of this analysis reveal that as shown in  FIG. 18A , the movable portion largely oscillated in a case where the potential difference Va=V 1  was applied after the lapse of time t 1 , and that as shown in  FIG. 18B , the oscillation of the movable portion was suppressed in a case where the potential difference Va=V 1  was applied after the lapse of time t 1  and where the potential difference Vb=V 2  was applied between a moment t 2  and a moment t 3  (incidentally, a delay in an input waveform occurred due to the performance limit of a function generator). 
       FIGS. 19A to 19C  are explanatory views illustrating the behavior of a movable portion in a case where the invention is applied to a noncontact type microelectromechanical modulation device. 
     Although the foregoing description of the embodiments and the modifications has described the case where the modulation device is of the contact type, the invention can obtain advantages similar to those of the embodiments and the modifications even when applied to a modulation device of the noncontact type. 
     That is, when the movable portion  27  is displacement-driven in the first direction by the drive sources, a physical action force is applied by the drive sources in the second direction, which differs from the first direction, to the movable portion  27  by applying a pulse wave P 1  during the movable portion  27  is transited in the first direction. Thus, a physical attracting force can be exerted in a direction opposite to the direction of transition of the movable portion  27 , so that an overshoot of the movable portion  27  can be actively reduced. Consequently, the speeding-up of the switching operation in the noncontact-driven modulation device  100  can be achieved. 
     A microelectromechanical modulation device array (hereunder referred to simply as a “modulation device array”) can be configured by one-dimensionally or two-dimensionally arranging the modulation devices  100 ,  200 , or  300  disclosed in the descriptions of the embodiments. 
     In the case of such a modulation device array, the modulation devices  100 ,  200 , or  300  enabled to perform high-speed switching operations are formed into an array. This enables the reduction of an oscillation stopping time. Consequently, the writing of address voltages can be performed earlier, as compared with a related array. 
     Thus, after the movable portion reaches the final displacement position, the oscillation of the movable portion can be suppressed. The oscillation stopping time can be eliminated or can be considerably reduced. The writing of address voltages can be performed without necessity for waiting for the stop of the oscillation. Consequently, the speeding-up of a switching operation can be achieved by reducing a driving cycle. 
     Further, it is preferable for the modulation array that each of the modulation devices thereof has a driving circuit including a memory circuit, similarly to a driving circuit of the modulation device array  100  shown in  FIG. 1 , which circuit is illustrated in  FIG. 20  by way of example. The provision of such a memory circuit enables the preliminary writing of an device displacement signal to the memory circuit. That is, an device displacement signal is preliminarily written to the memory circuit. At the switching of the modulation devices, the device displacement signal stored in the memory circuit of each of the modulation devices, and the drive voltage according to the invention are outputted with desired timing to the signal electrodes of the modulation devices by a driving-voltage control circuit that controls voltages to be applied to the modulation devices. At that time, a predetermined voltage is outputted to the common electrode (or the movable portion). 
     When each of the devices is driven by using the memory circuit in this way, each of the plural modulation devices can easily be operated in an optional driving pattern. Thus, higher-speed active driving can be achieved. Incidentally, although  FIG. 20  shows the modulation devices of the light modulation device array  100  shown in  FIG. 1 , the modulation devices of the array according to the invention are not limited thereto. The modulation devices of the array according to the invention may be those of another modulation device array. 
     Also, preferably, the modulation device array is provided with a control portion that modulation-drives the movable portions. 
     In the modulation device array having such a control portion, the movable portion is drive-controlled. Thus, before the movable portion reaches the final displacement position, the reduction, or increase, or increase/decrease of the absolute value of the interelectrode voltage between the movable electrode and the fixed electrode is performed. This enables the suppression of oscillation due to a collision, which would be caused when the movable portion of the related device reached the final displacement position at a high speed, and also enables suppression of an overshoot. 
     An apparatus having the modulation device array of the aforementioned configuration may include a light source, an illuminating optical system adapted to irradiate light, which is outputted form the light source, to the modulation device array, and a projection optical system adapted to project light, which is outputted from the modulation device array, onto an image forming surface to thereby constitute an image forming apparatus. 
     An image forming apparatus having the aforementioned modulation device array can actively decrease the oscillation of the movable portion. The driving cycle can be reduced, as compared with a related apparatus. This enables high-speed exposure of a photosensitive material, and display in a higher-pixel projector. Further, an image forming apparatus (or an exposure apparatus) adapted to perform gradation control by switching on/off of exposing light can reduce an on/off time. Thus, higher gradation can be realized. 
     Incidentally, the timing, with which the voltage driving of each of the electrodes is performed, and the waveform of a voltage driving signal according to the invention are not limited to those described in the foregoing description, and may be appropriately changed without departing from the scope and spirit of the invention.