Abstract:
Disclosed in a non-contacting electric potential sensor capable of being readily reduced in its size, which includes a detecting electrode, an electrically-conductive movable shutter, and a driving unit for driving the electrically-conductive movable shutter. The detecting electrode is to be placed facing a measurement object whose electric potential is to be measured. The electrically-conductive movable shutter is disposed so as to be movably located in a spacing formed between the detecting electrode and the measurement object when the detecting electrode is placed facing the measurement object, so that an exposure degree of the detecting electrode against the measurement object can be controlled. The driving unit includes a current injecting unit for selectively injecting current into the electrically-conductive movable shutter in a direction approximately perpendicular to a moving direction of the electrically-conductive movable shutter.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a non-contacting electric potential sensor capable of being fabricated using micro-electro-mechanical-systems (MEMS) technology, and an image forming apparatus using the electric potential sensor. 
   2. Description of the Related Background Art 
   As a sensor for measuring a surface electric potential of an object to be measured (a measurement object), a variable capacitive coupling electric potential sensor of a mechanical type is known.  FIG. 9  shows a principle of the mechanical type variable capacitive coupling potential sensor. A measurement object  1099  has an electric potential V relative to a ground potential. A detecting electrode  1021  is disposed facing the measurement object, and a movable shutter  1025  is arranged right above the detecting electrode  1021 . Upon motion of the movable shutter  1025 , an electrostatic capacitance C between the measurement object  1099  and the detecting electrode  1021  is changed. An electrical charge Q is induced in the detecting electrode  1021  in accordance with V and C. Current flowing between the detecting electrode  1021  and the ground is detected by an ampere meter  1060 . 
   Here, the amount of the electrical charge Q induced in the detecting electrode  1021  satisfies a relation Q=CV. Accordingly, current i flowing into the ampere meter  1060  is given by i=dQ/dt=VdC/dt where t is the time. The potential V can be obtained if dC/dt is known. The sensitivity of the sensor is represented by dC/dt. As can be understood from that relation, the sensitivity increases if a difference between maximum and minimum of C is increased, or changing time t is shortened. 
   In connection with the above-discussed mechanical type variable capacitive coupling potential sensor capable of being fabricated using the MEMS technology, the following construction is disclosed by Japanese Patent Application Laid-Open No. 2000-147035 (its U.S. counterpart is U.S. Pat. No. 6,177,800).  FIG. 10  illustrates an electric potential sensor  1001 . The sensor  1001  is comprised of a driver component  1010  and a sensor component  1020 . Those components can be fabricated on a substrate  1004  by the MEMS technology. 
   The driver component  1010  is comprised of a suspension  1018  having a parallel hinge structure, and a comb electrostatic actuator  1012 . The comb electrostatic actuator  1012  is a general mechanism for driving a minute structure in an electrostatic manner, and is comprised of a movable electrode  1013  supported by the suspension  1018 , and a stationary electrode  1014  attached to the substrate  1004 . The comb electrostatic actuator  1012  is electrically connected to an electrostatic drive signal source  1050 . The movable electrode  1013  is held by the suspension  1018  movably in right and left directions in  FIG. 10 . Comb-shaped electrodes of the movable electrode  1013  are in alternate mesh engagement with those of the stationary electrode  1014  with these two sets of Comb-shaped electrodes being spaced. Upon application of a potential difference between those two sets of Comb-shaped electrodes, electrostatic attractive force acts between those electrodes  1013  and  1014 . 
   The sensor component  1020  is connected to the driver component  1010 . A detecting electrode assembly  1021  is fixed to the substrate  1004 , and is capable of capacitive coupling to a surface of an object to be measured. The detecting electrode assembly  1021  is composed of a set of spaced detecting electrodes  1021   a ,  1021   b  and  1021   c . Those detecting probes are connected together such that individual signals can be combined or superimposed. The sensor component  1020  further includes a movable shutter  1025  which selectively covers the detecting electrode assembly  1021 . The movable shutter  1025  is mechanically connected to the driver component  1010  such that a linear displacement of the driver component  1010  can cause a corresponding displacement of the movable shutter  1025 . 
   The movable shutter  1025  has a plurality of openings  1024 . The movable shutter  1025  is constructed such that the detecting electrode assembly  1021  can be selectively exposed through the openings  1024  when the movable shutter  1025  takes a first position. The openings  1024  are spaced from each other by a distance corresponding to the spacing between the detecting electrodes. When the movable shutter  1025  takes a second position, the detecting electrode assembly  1021  is covered with shielding portions  1026  present between the openings  1024 . 
   In other words, when the movable shutter  1025  takes the first position, capacitive coupling between the detecting electrode assembly  1021  and the measurement object can be established. On the other hand, when the movable shutter  1025  takes the second position, capacitive coupling between the detecting electrode assembly  1021  and the measurement object is masked and prohibited. Current created by the detecting electrode assembly  1021  is output into a takeout electrode  1028 , and is amplified by an amplifier  1060 . 
     FIG. 11  is a cross-sectional view taken along a line  1080  of  FIG. 10 . As can be seen from  FIG. 11 , widths w 1  of the respective detecting electrode of the detecting electrode assembly  1021  must be arranged with being spaced from each other by a distance w 2  corresponding to the distance between the respective shutter openings  1024 . Therefore, the width w 1  is equal to the distance w 2 , and accordingly the effective area of the detecting electrodes is about a half of the area occupied thereby on the substrate. 
   As discussed above, the MEMS electrostatic sensor has the following disadvantages to be solved. In the first place, the driver component  1010  and the sensor component  1020  of the conventional MEMS electrostatic sensor are fabricated at different locations on the substrate  1004 , respectively, and accordingly the chip size is liable to increase irrespective of their arrangement manner. Therefore, there is a limitation to reduction of the size of the conventional MEMS electrostatic sensor, and its cost increases. 
   Further, since the driver component  1010  and the sensor component  1020  move together, mass of a movable portion is likely to increase, and it is hence difficult to increase the driving frequency. The detecting sensitivity dC/dt of the electrostatic sensor is also proportional to the driving frequency, and accordingly the detecting sensitivity is difficult to increase. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention in view of the above-described disadvantages to provide an electric potential sensor which is to be used facing an object whose electric potential is to be measured (a measurement object). 
   According to one aspect of the present invention, there is provided a potential sensor which includes a detecting electrode, an electrically-conductive movable shutter which is disposed such that it can be movably located in a spacing formed between the detecting electrode and a measurement object, when the detecting electrode is placed facing the measurement object, to control an exposure degree of the detecting electrode against the measurement object, and a driving unit for driving the electrically-conductive movable shutter, which includes a current injecting unit for selectively injecting current into the electrically-conductive movable shutter. In such a construction, the electrically-conductive movable shutter itself constitutes a portion of an actuator, so that there is no need to provide an actuator unit in a totally separate form, leading to a decrease in its size. Further, even when a plurality of electrically-conductive movable shutters are arranged, the individual electrically-conductive movable shutters can be independently moved, so that mass of a movable portion can be lightened. Accordingly, its moving speed can be increased, and the sensitivity of the sensor can be hence enhanced. Moreover, no high voltage is needed to drive the electrically-conductive movable shutter, and accordingly the cost of the driving unit can be reduced. 
   The following specific structures are possible on the basis of the above fundamental construction. 
   The driving unit can include a magnetic-field applying unit for applying a magnetic field to the electrically-conductive movable shutter in a direction approximately perpendicular to a direction of the current injection and a moving direction of the electrically-conductive movable shutter. The magnetic-field applying unit can be a permanent magnet, or an electromagnetic coil. 
   It is possible to adopt a construction in which at least two electrically-conductive movable shutters and at least two current injecting units are provided, and the electrically-conductive movable shutters are movable owing to interaction between the currents which are injected into the electrically-conductive movable shutters by the current injecting units, respectively, in a direction approximately perpendicular to a moving direction of the electrically-conductive movable shutter. Also in such a construction, the electrically-conductive movable shutters constitute a portion of the actuator, so that there is no need to provide an actuator unit in a totally separate form, leading to a decrease in its size. Further, the individual electrically-conductive movable shutters can be independently moved, so that mass of the movable portion can be lightened. Accordingly, its moving speed can be increased, and the sensitivity of the sensor can be hence enhanced. Moreover, no high voltage is needed to drive the electrically-conductive movable shutters, and accordingly the cost of the driving unit can be reduced. 
   When the electrically-conductive movable shutter is shaped into an electrically-conductive movable shutter which is elastically supported movably, the movable shutter can be moved without its motion being disturbed by friction. Further, when the structure is constructed such that the driving frequency of the electrically-conductive movable shutter can be approximately equalized with its mechanical resonance frequency, consumption of electrical power for achieving a given amplitude of the motion can be largely reduced. 
   Further, it is possible to construct a structure in which there are arranged first and second detecting electrodes which are to be placed facing the measurement object, and a differential processing unit for differentially processing outputs supplied from the first and second detecting electrodes, the electrically-conductive movable shutter can selectively take a first state or a second state, the first detecting electrode is exposed to the measurement object more (typically, exposed approximately entirely) at the time when the electrically-conductive movable shutter takes the first state than at the time when the electrically-conductive movable shutter takes the second state, and the second detecting electrode is exposed to the measurement object less (typically, shielded approximately entirely) at the time when the electrically-conductive movable shutter takes the first state than at the time when the electrically-conductive movable shutter takes the second state. In such a construction, the first and second detecting electrodes can be disposed close to each other, so that the effective area of the detecting electrode can be increased. Further, since the outputs from the first and second detecting electrodes are differentially processed to obtain a signal, the sensitivity can be increased even if the size is relatively small. 
   Further, it is possible to construct a structure in which there are arranged a substrate, first and second detecting electrode assemblies which are provided on the substrate, and at least one of which is comprised of a plurality of portions, and at least one movable shutter which is disposed above the first and second detecting electrode assemblies with a spacing being provided therebetween, the first detecting electrode assembly is exposed to the measurement object more at the time when the movable shutter takes the first state than at the time when the movable shutter takes the second state, and the second detecting electrode assembly is exposed to the measurement object less at the time when the movable shutter takes the first state than at the time when the movable shutter takes the second state. While each of the first and second detecting electrodes can be composed of a single portion, the effective area of the detecting electrode can be further increased in the event of such a construction. 
   Furthermore, it is possible to construct a structure in which the detecting electrode is comprised of a detecting electrode assembly consisting of a plurality of portions, and a plurality of electrically-conductive movable shutters are arranged. In such a construction, the effective area of the detecting electrode can be increased. 
   According to another aspect of the present invention, there is provided an image forming apparatus which includes the above-described electric potential sensor, and an image forming unit for controlling formation of an image based on an output of the electric potential sensor. In such an image forming apparatus, technical advantages of the above-described electric potential sensor are effectively utilized. The image forming unit has a function of copying, printing, or facsimile, for example. Further, the image forming unit can be constructed as a structure which includes a photosensitive drum, and in which an electrified potential on the photosensitive drum is measured by using the electric potential sensor placed facing the photosensitive drum. 
   These advantages, as well as others, will be more readily understood in connection with the following detailed description of the preferred embodiments and examples of the invention in connection with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a disassembled perspective view illustrating a first embodiment of an electric potential sensor according to the present invention. 
       FIG. 2  is a view explaining the operation of the first embodiment. 
       FIG. 3  is a disassembled perspective view illustrating a second embodiment of an electric potential sensor according to the present invention. 
       FIG. 4  is a view explaining the operation of the second embodiment. 
       FIG. 5  is a disassembled perspective view illustrating a third embodiment of an electric potential sensor according to the present invention. 
       FIG. 6  is a view explaining the operation of the third embodiment. 
       FIG. 7  is a view explaining the operation of a fourth embodiment of an electric potential sensor according to the present invention. 
       FIG. 8  is a schematic view illustrating a fifth embodiment of an image forming apparatus according to the present invention. 
       FIG. 9  is a view explaining a general operational principle of a conventional electric potential sensor of a mechanical type. 
       FIG. 10  is a view explaining a conventional MEMS electric potential sensor. 
       FIG. 11  is a view explaining the problem of a conventional MEMS electric potential sensor. 
       FIG. 12  is a view illustrating a circumferential structure of a photosensitive drum in an electrophotographic developing apparatus using an electric potential sensor according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of an electric potential sensor and an image forming apparatus of the present invention will be hereinafter described with reference to the drawings. 
     FIG. 1  illustrates a disassembled structure of a first embodiment directed to an electric potential sensor. As illustrated in  FIG. 1 , a detecting electrode assembly  121 , a takeout electrode  122  for the detecting electrode assembly  121 , and takeout electrodes  123   a  and  123   b  for driving are patterned on a substrate  104 . The detecting electrode assembly  121  consists of a set of detecting electrodes which are spaced from each other and electrically connected to each other. 
   Movable shutter units  110   a  to  110   d  are comprised of shielding members  110   a  to  110   d , parallel hinge suspensions  112   a  to  112   d , and fixing members  113   a  to  113   d , respectively. Those portions  111   a  to  111   d ,  112   a  to  112   d , and  113   a  to  113   d  are formed in a united form with electrically-conductive material, respectively. Each of the fixing members  113   a  to  113   d  is fixedly bonded to the driving takeout electrodes  123   a  and  123   b . The shielding members  111   a  to  111   d  are supported by the parallel hinge suspensions  112   a  to  112   d  with being spaced from the detecting electrode assembly  121  disposed below the shielding members  111   a  to  111   d , respectively. The movable shutter units  110   a  to  110   d  are electrically connected in a parallel manner through the driving takeout electrodes  123   a  and  123   b.    
   A permanent magnet  130  is disposed under the substrate  104 . The magnet  130  generates a magnetic flux in a direction perpendicular to the substrate  104 . The driving takeout electrodes  123   a  and  123   b  are electrically connected to a driver  150 . 
   Operation of the electric potential sensor of the first embodiment will be described with reference to  FIGS. 2A and 2B . An object to be measured (a measurement object) is placed above the substrate  104  and in a direction perpendicular to the substrate  104 . In the event that current is generated from the driver  150  and is caused to flow from the driving takeout electrode  123   a  to the driving takeout electrode  123   b  as illustrated in  FIG. 2A , the movable shutter units  110   a  to  110   d  (more exactly the shielding members  111   a  to  111   d ) are displaced rightward in  FIG. 2A  under the influence of the magnetic field generated perpendicularly to a sheet of the figure. The detecting electrode assembly  121  is accordingly shielded, so that the electrostatic capacitive coupling between the assembly  121  and the measurement object decreases. 
   Conversely, in the event that current is caused to flow from the driving takeout electrode  123   b  to the driving takeout electrode  123   a  as illustrated in  FIG. 2B , the movable shutter units  110   a  to  110   d  (more exactly the shielding members  111   a  to  111   d ) are displaced leftward in  FIG. 2A  under the influence of the magnetic field. The detecting electrode assembly  121  is accordingly exposed, so that the electrostatic capacitive coupling between the assembly  121  and the measurement object increases. Upon repetition of those motions, electrical charges are alternately induced in the detecting electrode assembly  121 . An electric potential of the measurement object can be measured by detecting the charges. 
   When the driving frequency of the movable shutter units  110   a  to  110   d  is made approximately equal to the mechanical resonance frequency, electrical power needed for the driving can be reduced. 
   In a construction according to the first embodiment, the movable shutter itself constitutes a portion of an actuator, so that there is no need to provide an actuator unit in a separate form on the substrate, leading to a decrease in its size. Therefore, when the size is approximately equivalent to that of a conventional sensor, the sensitivity can be increased. Further, in the event that the sensitivity only needs to be equivalent to that a conventional sensor, the size can be reduced. Moreover, the fabrication cost can be decreased by increasing the number of sensors per a silicon wafer. 
   Furthermore, since the individual movable shutters are independently moved, mass of a movable portion can be lightened. Accordingly, the motion speed can be increased, and hence the sensor sensitivity can be enhanced. In addition, no high voltage is needed to drive the movable shutters, and accordingly the cost of the driver can be reduced. 
     FIG. 3  illustrates a disassembled structure of a second embodiment directed to an electric potential sensor. As illustrated in  FIG. 3 , two detecting electrode assemblies  221   a  and  221   b , takeout electrodes  222   a  and  222   b  for the detecting electrode assemblies  221   a  and  221   b , and takeout electrodes  223   a  and  223   b  for driving are patterned on a substrate  204 . Each of the detecting electrode assemblies  221   a  and  221   b  consists of a set of detecting electrodes which are spaced from each other and electrically connected to each other by each of the takeout electrodes  222   a  and  222   b  for the detecting electrode assemblies  221   a  and  221   b . Detecting electrodes of the detecting electrode assemblies  221   a  and  221   b  are spaced from each other such that they cannot be electrically shorted therebetween. 
   Movable shutter units  210   a  to  210   d  are comprised of shielding members  211   a  to  211   d , parallel hinge suspensions  212   a  to  212   d , and fixing members  213   a  to  213   d , respectively. Those portions  211   a  to  211   d ,  212   a  to  212   d , and  213   a  to  213   d  are formed in a united form with electrically-conductive material, respectively. Each of the fixing members  213   a  to  213   d  is fixedly bonded to the driving takeout electrodes  223   a  and  223   b . The shielding members  211   a  to  211   d  are supported by the parallel hinge suspensions  212   a  to  212   d  with being spaced from the detecting electrode assemblies  221   a  and  221   b  disposed below the shielding members  211   a  to  211   d , respectively. 
   A permanent magnet  230  is disposed under the substrate  204 . The magnet  230  generates a magnetic flux in a direction perpendicular to the substrate  204 . The driving takeout electrodes  223   a  and  223   b  are electrically connected to a driver  250 . The detecting electrode takeout electrodes  222   a  and  222   b  are electrically connected to a differential amplifier  290 . 
   Operation of the electric potential sensor of the second embodiment will be described with reference to  FIGS. 4A and 4B . An object to be measured (a measurement object) is placed above the substrate  204  and in a direction perpendicular to the substrate  204 . Under such a placement condition, in the event that current is generated from the driver  250  and is caused to flow from the driving takeout electrode  223   a  to the driving takeout electrode  223   b  as illustrated in  FIG. 4A , the parallel hinge suspensions  212   a  to  212   d  are flexed, and the shielding members  211   a  to  211   d  are displaced rightward in  FIG. 4A  under the influence of the magnetic field generated upward perpendicularly to a sheet of the figure. The detecting electrode assembly  221   a  is accordingly exposed, so that the electrostatic capacitive coupling between the assembly  221   a  and the measurement object increases. In contrast thereto, the detecting electrode assembly  221   b  is shielded, so that the electrostatic capacitive coupling between the assembly  221   b  and the measurement object decreases. 
   Conversely, in the event that current is caused to flow from the driving takeout electrode  223   b  to the driving takeout electrode  223   a  as illustrated in  FIG. 4B , the shielding members  211   a  to  211   d  are displaced leftward in  FIG. 4B . The detecting electrode assembly  221   b  is accordingly exposed, so that the electrostatic capacitive coupling between the assembly  221   b  and the measurement object increases. In contrast thereto, the detecting electrode assembly  221   a  is shielded, so that the electrostatic capacitive coupling between the assembly  221   a  and the measurement object decreases. 
   Upon repetition of those motions, electrical charges are induced in the detecting electrode assemblies  221   a  and  221   b  in a mutually opposite phase. An electric potential of the measurement object can be measured by differentially amplifying the charges using the differential amplifier  290 . 
   When the driving frequency of the movable shutter units  210   a  to  210   d  is made approximately equal to the mechanical resonance frequency, electrical power needed for the driving can be reduced. 
   Also in a construction according to the second embodiment, the same technical advantages as those of the first embodiment can be obtained. In addition, the area of the detecting electrodes can be widened in the second embodiment. Therefore, when the size is approximately equivalent to that of a conventional sensor, the sensitivity can be improved. Further, in the event that the sensitivity only needs to be equivalent to that a conventional sensor, the size can be reduced. Moreover, the fabrication cost can be decreased by increasing the number of sensors per a silicon wafer. 
     FIG. 5  illustrates a disassembled structure of a third embodiment directed to an electric potential sensor. As illustrated in  FIG. 5 , two detecting electrode assemblies  321   a  and  321   b , takeout electrodes  322   a  and  322   b  for the detecting electrode assemblies  321   a  and  321   b , coupling electrodes  323   a  to  323   c , and takeout electrodes  324   a  and  324   b  for driving are patterned on a substrate  304 . Each of the detecting electrode assemblies  321   a  and  321   b  consists of a set of detecting electrodes (only one detecting electrode of the detecting electrode assembly  321   b  is shown in  FIG. 5 ) which are spaced from each other and electrically connected to each other by each of the takeout electrodes  322   a  and  322   b  for the detecting electrode assemblies  321   a  and  321   b . Detecting electrodes of the detecting electrode assemblies  321   a  and  321   b  are spaced from each other such that they cannot be electrically shorted therebetween. 
   Movable shutter units  310   a  to  310   d  are comprised of shielding members  311   a  to  311   d , parallel hinge suspensions  312   a  to  312   d , and fixing members  313   a  to  313   d , respectively. Those portions  311   a  to  311   d ,  312   a  to  312   d , and  313   a  to  313   d  are formed in a united form with electrically-conductive material, respectively. Each of the fixing members  313   a  to  313   d  is fixedly bonded to the coupling electrodes  323   a  to  323   c  or the driving takeout electrodes  324   a  and  324   b . The shielding members  311   a  to  311   d  are supported by the parallel hinge suspensions  312   a  to  312   d  with being spaced from the detecting electrode assemblies  321   a  and  321   b  disposed below the shielding members  311   a  to  311   d , respectively. The movable shutter units  310   a  to  310   d  are electrically serially connected through the coupling electrodes  323   a  to  323   c  and the driving takeout electrodes  324   a  and  324   b.    
   A coil substrate  361  is arranged under the substrate  304 . A planar coil  362  is patterned on the coil substrate  361 . A magnetic flux in a direction perpendicular to the substrate  304  is generated when current is injected into the planar coil  362  by a coil driver  363 . The driving takeout electrodes  324   a  and  324   b  are electrically connected to a driver  350 . The detecting electrode takeout electrodes  322   a  and  322   b  are electrically connected to a differential amplifier  390 . 
   Operation of the electric potential sensor of the third embodiment will be described with reference to  FIGS. 6A and 6B  illustrating this embodiment viewed from above. An object to be measured (a measurement object) is placed above the substrate  304  and in a direction approximately perpendicular to the substrate  304 . Under such a placement condition, in the event that current is generated from the driver  350  and is caused to flow from the driving takeout electrode  324   a  to the driving takeout electrode  324   b  as illustrated in  FIG. 6A , the shielding members  311   a  and  311   c  are displaced leftward in  FIG. 6A  while the shielding members  311   b  and  311   d  are displaced rightward in  FIG. 6A , under the influence of the magnetic field generated by the planar coil  362  upward perpendicularly to a sheet of the figure. The detecting electrode assembly  321   b  is accordingly exposed, so that the electrostatic capacitive coupling between the assembly  321   b  and the measurement object increases. In contrast thereto, the detecting electrode assembly  321   a  is shielded, so that the electrostatic capacitive coupling between the assembly  321   a  and the measurement object decreases. 
   Conversely, in the event that current is caused to flow from the driving takeout electrode  324   b  to the driving takeout electrode  324   a  as illustrated in  FIG. 6B , the shielding members  311   a  and  311   c  are displaced rightward in  FIG. 6B  while the shielding members  311   b  and  311   d  are displaced leftward in  FIG. 6A . The detecting electrode assembly  321   a  is accordingly exposed, so that the electrostatic capacitive coupling between the assembly  321   a  and the measurement object increases. In contrast thereto, the detecting electrode assembly  321   b  is shielded, so that the electrostatic capacitive coupling between the assembly  321   b  and the measurement object decreases. 
   Upon repetition of those motions, electrical charges are induced in the detecting electrode assemblies  321   a  and  321   b  in a mutually opposite phase. An electric potential of the measurement object can be measured by differentially amplifying the charges using the differential amplifier  390 . 
   When the driving frequency of the movable shutter units  310   a  to  310   d  is made approximately equal to the mechanical resonance frequency, electrical power needed for the driving can be reduced. 
   Also in a construction according to the third embodiment, the same technical advantages as those of the second embodiment can be obtained. In addition, no permanent magnet is needed, and therefore, the entire size can be reduced. 
     FIGS. 7A and 7B  illustrate a fourth embodiment directed to an electric potential sensor. In the fourth embodiment, two detecting electrode assemblies  421   a  and  421   b , and movable shutter units  410   a  to  410   d  are the same as those of the third embodiment. 
   As illustrated in  FIGS. 7A and 7B , the movable shutter units  410   a  and  410   c  are electrically serially connected to a driver  450   a , while the movable shutter units  410   b  and  410   d  are electrically serially connected to a driver  450   b.    
   In the event that current is generated from the drivers  450   a  and  450   b  such that the current is caused to flow in directions as illustrated in  FIG. 7A , current is caused to flow upward in  FIG. 7A  through the movable shutter units  410   a  and  410   d  while current is caused to flow downward in  FIG. 7A  through the movable shutter units  410   b  and  410   c . Since currents flowing in the same direction repel each other and currents flowing in opposite directions attract each other, the shielding members  411   a  and  411   c  are displaced leftward in  FIG. 7A  while the shielding members  411   b  and  411   d  are displaced rightward in  FIG. 7A . Accordingly, the detecting electrode assembly  421   a  is shielded, while the detecting electrode assembly  421   b  is exposed. 
   Conversely, in the event that a direction of current generated by the driver  450   b  is reversed as illustrated in  FIG. 7B , current is caused to flow upward in  FIG. 7B  through the movable shutter units  410   a  and  410   b  while current is caused to flow downward in  FIG. 7B  through the movable shutter units  410   c  and  410   d . Since currents flowing in the same direction repel each other and currents flowing in opposite directions attract each other, the shielding members  411   a  and  411   c  are displaced rightward in  FIG. 7B  while the shielding members  411   b  and  411   d  are displaced leftward in  FIG. 7B . Accordingly, the detecting electrode assembly  421   a  is exposed, while the detecting electrode assembly  421   b  is shielded. Upon measuring current flowing through the detecting electrode assemblies  421   a  and  421   b , an electric potential of the measurement object can be measured similarly to the second and third embodiments. 
   Also in the third embodiment, when the driving frequency of the movable shutter units  410   a  to  410   d  is made approximately equal to the mechanical resonance frequency, electrical power needed for the driving can be reduced. 
   Also in a construction according to the fourth embodiment, the same technical advantages as those of the above embodiments can be obtained. In addition, since two or more than two current generating units are used, no additional magnetic-field generating unit is needed, and therefore, the size and the cost can be further reduced as compared with the above embodiments. 
   In the above-described first to fourth embodiments, a leg portion of the fixing member in the movable shutter unit is fixedly connected to the driving takeout electrode or the coupling electrode. It is, however, possible to construct a structure in which a groove portion serving as a guide portion or a slide terminating portion is formed in each of those electrodes, and the leg portion of the fixing member is slidably fit in the guide portion or the slide terminating portion such that the movable shutter unit can be entirely slid between its detecting electrode shielding position and its detecting electrode exposing position. In this case, there is no need to provide the parallel hinge suspension portion in the movable shutter unit. The same technical advantages as above can also be obtained in such a structure. 
     FIG. 8  illustrates a fifth embodiment directed to an image forming apparatus. In  FIG. 8 , reference numerals  501   a  to  501   c  designate electric potential sensors of the present invention, respectively. Reference numeral  591  designates a photosensitive drum generally used for electrophotographic process. Reference numeral  592  designates an electrostatic charging device. An electric potential distribution on the photosensitive drum  591  can be measured when outputs of the potential sensors  501   a  to  501   c  are monitored in synchronism with the rotation of the photosensitive drum  591 . Unevenness of an image can be reduced when the amount of light exposed to the photosensitive drum  591 , or the electrostatic charging device  592  is controlled based on the thus-measured electric potential distribution. 
     FIG. 12  illustrates a detailed structure around the photosensitive drum of an electrophotographic developing apparatus using the potential sensor of the present invention. As illustrated in  FIG. 12 , an electrostatic charging device  2102 , an electric potential sensor  2101 , a light exposing device  2105 , and a toner supplying device  2106  are arranged around a photosensitive drum  2108 . The electrostatic charging device  2102  electrifies a surface of the drum  2108 , and a surface of the drum  2108  is exposed to light using the exposing device  2105  to form a latent image on the drum  2108 . Toner is attached to the latent image by the toner supplying device  2106  to obtain a toner image. The toner image is then transferred to a transferring material  2109  sandwiched between a transferring material conveying roller  2107  and the photosensitive drum  2108 , and the toner on the transferring material  2109  is fixed. Image formation is achieved by executing those process steps. 
   In the above-discussed structure, a charged condition of the drum  2108  is measured by the potential sensor  2101 , its signal is processed by a signal processing apparatus  2103 , and the electrostatic charging device  2102  is controlled by feeding the processed signal back to a high voltage generating device  2104 . Thus, a stable electrical charging of the drum  2108  is achieved such that a stable image formation can be obtained. 
   As described in the foregoing, in a construction according to the present invention, an electrically-conductive movable shutter of an electric potential sensor constitutes an actuator, so that there is no need to fabricate an actuator unit in a separate form, leading to a decrease in the size. Therefore, in a case where the size is approximately equivalent to that of a conventional sensor, the sensitivity can be increased. Further, in the event that the sensitivity only needs to be equivalent to that of a conventional sensor, the size can be reduced. Moreover, the fabrication cost can be decreased by increasing the number of sensors per a silicon wafer. 
   In addition, since an electric potential sensor of the present invention can be reduced in its size, a lot of potential sensors can be built in an apparatus, leading to achievement of capability of a highly-precise control. 
   While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and the scope of the appended claims.