Abstract:
Provided is an ion generator that has a potential sensor provided integrally inside the main body of the ion generator, and that can measure, with the potential sensor, an electric field that reaches the potential sensor from a member from which static charge is to be eliminated, without being affected by an electric field between a discharge electrode and an opposing electrode. This ion generator comprises a discharge electrode, an opposing electrode, and a main body part including these electrodes, the ion generator sending out, toward a member from which static charge is to be eliminated, air ions generated by applying a high voltage between the electrodes. A potential sensor that measures the potential of the member from which static charge is to be eliminated is provided integrally to the main body part, and a projecting electrostatic-shielding plate that projects from the main body part is provided between the potential sensor and a discharge part constituted by the discharge electrode and the opposing electrode.

Description:
This is a U.S. national stage of application No. PCT/JP2014/060242, filed on Apr. 9, 2014. Priority under 35 U.S.C. §119(a) and 35 U.S.C. §365(b) is claimed from Japanese Patent Applications No. 2013-083022 filed on Apr. 11, 2013, the disclosure of which is also incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an ion generator that neutralizes charge of an electrically-charged body as an object required to be electrically neutralized (hereinafter “charged member”). The ion generator blows positive or negative air ions generated by corona discharge against the charged member. The present invention relates to, especially an ion generator having an integral potential sensor integrally provided therewith. 
     BACKGROUND ART 
     The ion generator is called ionizer or static charge eliminator as well. The ion generator blows air ions against a charged target and eliminates charge. In a manufacture line in which manufacture and assembly of electronic components are conducted, electronic components and manufacture assembly jigs are charged. The electronic components and manufacture assembly jigs are regarded as a charged member, and the ion generator is used. Blowing air ions against the charged member prevents foreign matters from adhering to electronic components by static electricity, prevents electronic components from being destroyed by static electricity, and prevents foreign matters from adhering to jigs. 
     It is known to measure the potential of the charged member, by using a potential sensor (see, for example, Patent Literatures 1 and 2). If such a potential sensor is used together with the ion generator, it is possible to eliminate charge in the charged member, while measuring the potential of the charged member by using the potential sensor. Such a potential sensor is usually attached separately from the ion generator or externally to the ion generator and used. 
     CITATION LIST 
     Patent Literature 
     {PTL 1} Japan Unexamined Patent Application Publication 2012-242094 
     {PTL 2} Japan Unexamined Patent Application Publication 2010-85393 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In a case where the ion generator and the potential sensor are provided separately, the installation space is large. On the other hand, in a case where the ion generator and the potential sensor are provided integrally, the installation space is small. If the ion generator and the potential sensor are provided integrally, however, a problem is posed. For example, since a discharge electrode and an opposite electrode are disposed near the potential sensor, an electric field between the discharge electrode and the opposite electrode with a high voltage applied reaches the potential sensor. The electric field is superposed on an electric field that reaches the potential sensor from the charged member, i.e., an electric field to be measured, and becomes noise. Therefore, it is not possible to measure the potential of the charged member, accurately. 
     An object of the present invention is to provide, an ion generator that measures potential of a charged member, by using a potential sensor without being influenced by an electric field between a discharge electrode and an opposite electrode, i.e., noise, although the ion generator and the potential sensor are provided integrally. 
     Solution to Problem 
     In order to solve the problem, the present invention provides an ion generator that blows air ions generated by applying a high voltage to a discharge unit including discharge electrodes and opposite electrodes, toward a charged member. The ion generator includes a potential sensor provided integrally in a main body unit to measure potential of the charged member, and a projecting electrostatic shield plate disposed between the discharge unit and the potential sensor to project from the main body unit. 
     It is possible to set a projection length of the projecting electrostatic shield plate in a range of 8 to 10 mm. 
     An aperture window is formed in the potential sensor to take in an electric field from the charged member. It is possible to set a distance from the projecting electrostatic shield plate to the aperture window in the potential sensor equal to 2 mm or less. 
     A blow-off opening is formed in the main body unit to blow off the air ions. A plurality of the discharge electrodes is disposed at intervals along the blow-off opening. It is possible to cause the projecting electrostatic shield plate to intervene between any of the discharge electrodes and the aperture window. 
     It is possible to cause the blow-off opening and the aperture window to be disposed on the same plane in the main body. 
     Advantageous Effects of the Invention 
     In the ion generator according to the present invention, the potential sensor is provided integrally in the main body unit. Potential of the charged member is measured by the potential sensor. The discharge unit includes the discharge electrodes and the opposite electrodes. The projecting electrostatic shield plate is provided between the discharge unit and the potential sensor. The projecting electrostatic shield plate projects toward an ion blow-off direction from the main body unit. The high voltage is applied between the discharge electrode and the opposite electrode, and an electric field is generated. The electric field is shielded by the projecting electrostatic shield plate, and the electric field does not reach the potential sensor. Therefore, the potential of the charged member is measured by the potential sensor without being influenced by the electric field between the discharge electrode and the opposite electrode, i.e., noise. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a general perspective view obtained by viewing an ion generator according to an embodiment of the present invention from a front side. 
         FIG. 2  is a general perspective view obtained by viewing the ion generator illustrated in  FIG. 1  from a rear side; 
         FIG. 3  is a front view of the ion generator. 
         FIG. 4  is a plan view of  FIG. 3 . 
         FIG. 5  is a rear view of  FIG. 3 . 
         FIG. 6  is a perspective view illustrating a single substance of a discharge electrode unit. 
         FIG. 7  is a sectional view taking along line A-A in  FIG. 1 . 
         FIG. 8  is an enlarged view of an X portion in  FIG. 3 . 
         FIG. 9  is a perspective view of  FIG. 8 . 
         FIG. 10  is a perspective view illustrating the whole of a potential sensor. 
         FIG. 11  is a plan view illustrating a configuration of the potential sensor. 
         FIG. 12  is a front sectional view of the potential sensor illustrated in  FIG. 11 , illustrating a state in which the potential sensor is cut along a line indicated by arrows  2 - 2 . 
         FIG. 13  is a side sectional view of the potential sensor illustrated in  FIG. 11 , illustrating a state in which the potential sensor is cut along a line indicated by arrows  3 - 3 . 
         FIG. 14  is a plan view of the potential sensor illustrated in  FIG. 11  in a state in which an electrostatic shield plate is removed. 
         FIG. 15  is a side view of the potential sensor illustrated in  FIG. 11  in the state in which the electrostatic shield plate is removed. 
         FIG. 16  is a diagram illustrating relations between a position of a projecting electrostatic shield plate and a projection length thereof. 
         FIG. 17  is a graphic diagram illustrating relations between the length of the projecting electrostatic shield plate and noise voltage. 
         FIG. 18  is a graphic diagram illustrating relations between the length of the projecting electrostatic shield plate and signal voltage. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereafter, an ion generator according to an embodiment of the present invention will be described in detail with reference to the drawings. “Up-down direction,” “left/right direction (width direction),” and “depth direction” used in the following description refer to directions viewed from a front side where the front side (surface side) is this side in  FIG. 1 . In the embodiment stated hereafter, a product of wide type will be described. The product of wide type blows off generated air ions from a laterally long blow-off opening. 
     The whole of an ion generator  1  is illustrated in  FIGS. 1 to 5 . The ion generator  1  includes a main body unit  10 , a discharge electrode unit  20  (see  FIG. 6 ), and a potential sensor unit  40 . The discharge electrode unit  20  is detachably attached to the main body unit  10  from a blow-off opening  11 . The potential sensor unit  40  is housed in the main body unit  10 . 
     The main body unit  10  is formed into the shape of nearly a rectangular parallelepiped, and the main body unit  10  extends in the left/right direction. As illustrated in  FIGS. 1, 3, 4 and 7 , the blow-off opening  11  is formed in an upper portion of the front of the main body unit  10  on this side. The blow-off opening  11  extends in the left/right direction. 
     As illustrated in  FIG. 7 , a discharge electrode unit mounting portion  12  is formed within the rear portion of the blow-off opening  11 . The discharge electrode unit mounting portion  12  is recessed in the depth direction, and has the same width as that of the blow-off opening  11 . The discharge electrode unit mounting portion  12  is formed into a rectangular recessed shape. The discharge electrode unit  20  (see  FIG. 6 ) is fitted in the discharge electrode unit mounting portion  12 . 
     As illustrated in  FIG. 7 , an air supply chamber  13  is provided further behind the discharge electrode unit mounting portion  12 . The air supply chamber  13  is formed over the whole length of the blow-off opening  11  in the left/right direction. As illustrated in  FIGS. 1 to 5 , the air supply chamber  13  is supplied with jet air from an air supply port  13 A provided on a left side of the main body unit  10  via a tube  13 B. 
     As illustrated in  FIGS. 7 to 9 , an air discharge opening  16  is provided in an upper portion of the front side of the air supply chamber  13 . The air discharge opening  16  communicates from inside of the air supply chamber  13  to a back portion of the discharge electrode unit mounting portion  12 . The air discharge opening  16  takes the shape of a rectangular hole or a round hole. As illustrated in  FIGS. 8 and 9 , two air discharge openings  16  are provided on lower sides of the left and right of each of discharge electrodes  21 . Details of the air discharge openings  16  will be described later. The jet air is jetted forward from the air discharge openings  16 . 
     As illustrated in  FIG. 7 , an air guide portion  17  is provided on an upper portion of the air discharge opening  16 . The air guide portion  17  projects to the upper side of the front of the air discharge opening  16 . The air guide portion  17  enhances the straight advancing property of the jet air blown off from the air discharge opening  16 . 
     A top face cover  14  is provided over the air supply chamber  13  and the discharge electrode unit mounting portion  12 . As illustrated in  FIGS. 1 to 3, 5 and 7 , an air flow path  15  is formed between the top face cover  14  and the air supply chamber  13 . The air flow path  15  is also formed between the top face cover  14  and the discharge electrode unit mounting portion  12 . The air flow path  15  passes through from a back face of the main body unit  10  to a front face. The air flow path  15  is parallel to a direction in which the above-described air guide portion  17  leads the jet air. In other words, a direction of the jet air flow discharged from the air discharge opening  16  becomes the same as a direction of flow of an external air which flows in the air flow path  15 . The external air is an air rolled from the circumference of the ion generator  1  by the flow of the jet air. 
     As illustrated in  FIGS. 2, 5 and 7 , an upper portion of the air supply chamber  13  is formed into a curved shape. As a result, an inlet  15 A on the rear face side of the air flow path  15  is spread toward behind. As a result, it is facilitated to take external air behind the ion generator  1  into the air flow path  15 . 
     On the other hand, as illustrated in  FIG. 6 , a plurality of (four in  FIG. 6 ) discharge electrodes  21  is disposed in the discharge electrode unit  20  at intervals. The discharge electrode  21  is formed in a fine wire form or a needle form. The rectilinear discharge electrode  21  extends toward the blow-off opening  11  on this side in  FIG. 6 . Opening portion  22  are formed on a top face of the discharge electrode unit  20  to correspond to respective discharge electrodes  21 . The respective discharge electrodes  21  is exposed from the top face of the discharge electrode unit  20  via the opening portion  22 . 
     As illustrated in  FIGS. 1, 3 and 6 to 9 , an opposite electrode  23  is provided on the front side of the discharge electrode unit  20 . The opposite electrode  23  is formed of metal having conductivity and formed into the shape of a plate. The opposite electrode  23  is disposed in a lengthwise direction of the discharge electrode unit  20 . 
     As illustrated in  FIGS. 3, 6, 8 and 9 , the opposite electrode  23  is provided on a side lower than the discharge electrode  21  when viewed from the front side of the ion generator  1 . A notched portion  23 A is formed in a nearly half circle shape centered the discharge electrode  21  to correspond to the discharge electrode  21 . In other words, the discharge electrode  21  and the opposite electrode  23  are disposed via a gap  25  having a determinate length interposed therebetween. 
     As illustrated in  FIG. 7 , an air supply path  24  is formed within the discharge electrode unit  20 . The jet air flows through the air supply path  24  which extends from the air discharge opening  16  toward the gap  25 . 
     As illustrated in  FIG. 7 , a separation space  26  is provided in a state in which the discharge electrode unit  20  is attached to the main body unit  10 . The separation space  26  is a space extending from a front side tip portion of the air guide portion  17  to a back end portion of the air supply path  24 . The jet air flows fast from the air discharge opening  16  toward the air supply path  24 . The jet air flowing fast and the external air in the air flow path  15  are brought into contact with each other in the separation space  26  and the opening portion  22 . 
     Power is supplied from an external power supply to the ion generator  1  via a power supply cable  27  (see  FIG. 1 ). A high voltage is applied between the discharge electrode  21  and the opposite electrode  23 . As a result, corona discharge occurs and air ions are generated. As for an internal configuration for applying a high voltage, detailed description thereof will be omitted. 
     As illustrated in  FIG. 7 , a potential sensor unit housing portion  18  is provided on a lower side within the main body unit  10 , i.e., under the air supply chamber  13  and the discharge electrode unit mounting portion  12 . The potential sensor unit housing portion  18  is provided over the left/right direction of the ion generator  1 . A detection window  18 A is provided in a front side wall of the potential sensor unit housing portion  18 . The detection window  18 A communicates with the potential sensor unit housing portion  18 . The potential sensor unit  40  is attached to the potential sensor unit housing portion  18 . The potential sensor unit  40  measures the potential of the charged member P disposed opposite to the blow-off opening  11 . 
     The potential sensor unit  40  includes a potential sensor  41  and a power supply unit (not illustrated) that supplies power to the potential sensor  41 . The potential sensor  41  and the power supply unit are attached within the potential sensor unit housing portion  18 . 
     As illustrated in  FIG. 10 , the potential sensor  41  includes a printed circuit board  111  on which a detection electrode  114  (see  FIG. 12 ) and so forth are mounted, and an electrostatic shield plate  43  to which the printed circuit board  111  is attached. A lengthwise direction of the printed circuit board  111  extends in the left/right direction of the main body unit  10  in the ion generator  1 . By the way, as for the printed circuit board  111  illustrated in  FIGS. 11  to  15 , a portion is illustrated, and other portions are omitted. 
     A rectangular shaped aperture window  113  is formed in the electrostatic shield plate  43 . As illustrated in  FIG. 11 , the aperture window  113  is formed by notching the electrostatic shield plate  43 . All main slits  131  are exposed to the outside via the aperture window  113 , and all main slits  131  is visible from the outside. A position of the aperture window  113  coincides with a position of the detection window  18 A in the potential sensor unit housing portion  18 . 
     As illustrated in  FIGS. 10 to 12 , a projecting electrostatic shield plate  43 A is provided continuously from the electrostatic shield plate  43 . The projecting electrostatic shield plate  43 A is provided over the whole length in the left/right direction of the electrostatic shield plate  43 . The projecting electrostatic shield plate  43 A is projected from the ion generator  1 . Details of the projecting electrostatic shield plate  43 A will be described later. 
     As illustrated in  FIGS. 11 to 13 , the detection electrode  114  is attached to the printed circuit board  111 . A flange portion  114   a  of the detection electrode  114  is fixed to the printed circuit board  111 . In addition, a standing portion  114   b  extending from the flange portion  114   a  is nearly perpendicular to the printed circuit board  111 . An electrode portion  114   c  extends from the standing portion  114   b  in parallel with the printed circuit board  111 . The electrode portion  114   c  opposes to the aperture window  113 . The detection electrode  114  is one of elements included in a detection circuit (not illustrated). At least the electrode portion  114   c  included in the detection electrode  114  forms an electric field between a charged substance and the electrode portion  114   c.    
     A fixed shutter  115  made of a conductive material is attached to the printed circuit board  111 . The fixed shutter  115  covers the detection electrode  114 . A main body portion  116  of the fixed shutter  115  is provided in parallel with the electrode portion  114   c  of the detection electrode  114 . The main body portion  116  is formed into a nearly rectangular shape. Side wall portions  117  and end wall portions  118  are bent from the main body portion  116  at right angles and are integral with the main body portion  116 . As illustrated in  FIG. 12 , tip portions of the side wall portions  117  are inserted into mounting holes formed through the printed circuit board  111 , and the fixed shutter  115  is fixed to the printed circuit board  111 . 
     Aperture slits  119  are formed in the main body portion  116  of the fixed shutter  115 . The aperture slits  119  extend in the lengthwise direction (left/right direction) of the electrostatic shield plate  43 . Five aperture slits  119  are formed in a width direction (up-down direction) of the main body portion  116 . The aperture slits  119  are disposed at constant intervals. 
     As illustrated in  FIGS. 12 to 15 , a movable shutter  121  is provided on the printed circuit board  111 . The movable shutter  121  is provided outside the fixed shutter  115  to cover the fixed shutter  115 . The movable shutter  121  moves between two positions: a full open position in which the slits  131  and slits  132  of the movable shutter  121  coincide with the aperture slits  119  of the fixed shutter  115 , and an interruption position in which the aperture slits  119  of the fixed shutter  115  are closed. A change of an aperture area of the shutters depending upon the full open position and the interruption position gives a change in an electric field formed between a charged substance and the detection electrode  114  (the electrode portion  114   c ). 
     A center line (not illustrated) of the aperture slits  119  in the lengthwise direction (left/right direction) is referred to as aperture slit center line. A center line (not illustrated) of the slits  131  and  132  in the lengthwise direction (left/right direction) is referred to as main slit center line. A position of the movable shutter  121  in which the aperture slit center line and the main slit center line coincide with each other is referred to as “full open position.” 
     A center line in the lengthwise direction (left/right direction) of a shield portion (reference numeral is omitted) existing between two main slits  131  is referred to as main shield portion center line. A center line (not illustrated) in the lengthwise direction (left/right direction) of a shield portion (reference numeral is omitted) existing between a main slit  131  and a subsidiary slit  132  is referred to as subsidiary shield portion center line. A position of the movable shutter  121  in which the aperture slit center line coincides with the main shield portion center line or the subsidiary shield portion center line is referred to as “interruption position.” 
     The movable shutter  121  is formed of a material having conductivity. The movable shutter  121  reciprocates in an open-close direction (up-down direction). The movable shutter  121  includes a fixed end portion  122  fixed to the printed circuit board  111 . Leg pieces  123  are integrally provided on both sides of the fixed end portion  122 . The leg pieces  123  are inserted into mounting holes formed in the printed circuit board  111 . The fixed end portion  122  of the movable shutter  121  is attached to the printed circuit board  111 . 
     An arm portion  124  is provided integrally in each of the leg pieces  123  in the fixed end portion  122 . The arm portion  124  extends to one end side (right side) in the lengthwise direction (left/right direction) of the printed circuit board  111 . As illustrated in  FIG. 14 , two arm portions  124  are provided at a predetermined interval in the present embodiment. Each of the arm portions  124  is formed of a flexible plate shaped member. A main body portion  125  is provided integrally on tips of the arm portions  124 . As illustrated in  FIGS. 12 and 13 , the main body portion  116  of the fixed shutter  115  is disposed outside the detection electrode  114 . The main body portion  116  covers the detection electrode  114 . The main body portion  125  of the movable shutter  121  is disposed outside the fixed shutter  115 . In addition, the main body portion  125  is exposed to the outside via the aperture window  113 . The main body portion  125  of the movable shutter  121  reciprocates in an open-close direction indicated by an arrow N, and opens and closes the aperture slits  119 . 
     At least the main body portion  116  of the fixed shutter  115  is grounded. And at least the main body portion  125  of the movable shutter  121  is also grounded. 
     As illustrated in  FIGS. 13 and 14 , a magnet  127  functioning as a magnetic substance is attached to an end wall  126  provided integrally in the main body portion  125 . The magnet  127  has a function of driving the movable shutter  121  to open and close. As indicated by solid lines and dashed lines in  FIG. 14 , a U-shaped yoke  128  is attached to one end side (right side) of the printed circuit board  111 . One pair of coils  129   a  and  129   b  is wound round the yoke  128  via bobbins  281 . The coils  129   a  and  129   b  are connected to a power supply unit, which is not illustrated. An alternating current is flown through each of the coils  129   a  and  129   b . As a result, magnetic fields that are inverse in direction to each other are formed on two magnetic pole surfaces  128   a  and  128   b . Therefore, the magnet  127  moves between a position opposed to one magnetic pole surface  128   a  and a position opposed to the other magnetic pole surface  128   b . In this way, a drive means that drives the movable shutter  121  in the reciprocation direction N to open and close is formed by the coils  129   a  and  129   b  wound round the yoke  128  and the magnet  127 . 
     Five main slits  131  are formed in the main body portion  125  in the movable shutter  121 . The five main slits  131  correspond to the five aperture slits  119  formed in the fixed shutter  115 . The respective main slits  131  extend in the same direction as that of the aperture slits  119 . Adjacent main slits  131  are disposed at a constant interval. The interval is the same as that of the aperture slits  119 . 
     The movable shutter  121  conducts reciprocal vibration, and moves between the full open position and the interruption position. 
       FIG. 12  illustrates a state in which the movable shutter  121  is in a neutral position. At this time, all of the five main slits  131  are opposed to the aperture slits  119 . One subsidiary slit  132  is formed on each of outsides of main slits  131  located on both ends of the reciprocation direction N (up-down direction). Each subsidiary slit  132  takes the same shape as that of the main slit  131 . Intervals between the five main slits  131  are the same as intervals between the main slits  131  and the subsidiary slits  132 . The five main slits  131  and the two subsidiary slits  132  take the same shape. These seven slits  131  and  132  formed on the movable shutter  121  take the same shape as that of the aperture slits  119  formed on the fixed shutter  115 . When the movable shutter  121  reciprocates, therefore, the aperture slits  119  are opened and closed by the subsidiary slits  132 . 
     In this way, one subsidiary slit  132  is formed on the outside of the two main slits  131  located at both ends of the reciprocation direction N, i.e., in an extension direction of the reciprocation direction N. During one period of movement of the movable shutter  121 , the movable shutter  121  moves from the neutral position illustrated in  FIG. 12  to a reciprocation end in a left direction in  FIG. 12 , then the movable shutter  121  moves to a position of a reciprocation end in a right direction, and returns to the neutral position. During the one period, the five aperture slits  119  on the fixed shutter  115  are opened and closed four times. In other words, the aperture slits  119  are opened and closed with a frequency that is four times the drive frequency of the movable shutter  121 . 
     A current detection circuit is connected to the detection electrode  114 . In a state in which the detection electrode  114  is opposed to a charged substance via the aperture window  113 , an alternating current in the range of, for example, 600 to 800 Hz is applied to the coils  129   a  and  129   b  to cause the movable shutter  121  to conduct reciprocal vibration. As a result, the aperture slits  119  on the fixed shutter  115  are opened and closed with a frequency that is four times the drive frequency of the movable shutter  121 . With this open/close frequency, an electric field between the detection electrode  114  and the charged substance changes, and an alternating voltage is generated in the detection electrode  114 . 
     The projecting electrostatic shield plate  43 A provided on the electrostatic shield plate  43  will now be described. In the ion generator  1  with the potential sensor  41  integrally mounted thereon, charge elimination is conducted by blowing generated air ions against the charged member P (see  FIG. 7 ). At the same time, the potential sensor  41  measures surface potential of the charged member P. In other words, since the ion generator  1  and the potential sensor  41  are provided in the same casing, it is more convenient to use as compared with the case where the ion generator  1  and the potential sensor  41  are provided separately. 
     It is necessary to dispose both the blow-off opening  11  of air ions and the potential sensor  41  to be opposed to the charged member P. Therefore, the discharge unit (the discharge electrodes  21  and the opposite electrodes  23 ) and the aperture window  113  of the potential sensor  41  are provided on the same plane of the main body unit  10 . As a result, not only an electric field from the charged member P but also an electric field between the discharge electrode  21  and the opposite electrode  23 , i.e., a discharge electric field reaches the potential sensor  41 . The discharge electric field becomes noise. In the present embodiment, the blow-off opening  11  and the aperture window  113  of the potential sensor  41  are provided on the same plane of the main body unit  10 . In addition, the projecting electrostatic shield plate  43 A is projected and provided between the blow-off opening  11  and the aperture window  113  to conduct electrostatic shielding between the discharge unit and the potential sensor  41 . 
     A length S 1  of the forward projection of the projecting electrostatic shield plate  43 A influences a noise voltage and a signal voltage of the potential sensor  41 . Graphs in  FIG. 17  represent a ratio of a noise voltage Vn to Vn 0  (Vn/Vn 0 ) and a projection length S 1  of the projecting electrostatic shield plate  43 A, where Vn 0  is the noise voltage in a case where the projection length S 1  of the projecting electrostatic shield plate  43 A is 0 mm. A distance S 2  (2, 4, 6 and 10 mm) between the projecting electrostatic shield plate  43 A and the aperture window  113  is set to be a parameter. The ratio Vn/Vn 0  does not largely depend upon the distance S 2  between the projecting electrostatic shield plate  43 A and the aperture window  113 . The ratio Vn/Vn 0  decreases, the longer the projection length S 1  of the projecting electrostatic shield plate  43 A is made. For example, in a case where the projection length S 1  of the projecting electrostatic shield plate  43 A is set equal to 8 or 10 mm, Vn/Vn 0  decreases to 35% or 50%. 
     Graphs in  FIG. 18  represent a ratio of a sensor signal voltage Vs to Vs 0  (Vs/Vs 0 ) and a function of the projection length S 1  of the projecting electrostatic shield plate  43 A, where Vs 0  is the signal voltage in a case where the projection length S 1  of the projecting electrostatic shield plate  43 A is 0 mm. A distance S 2  (2, 4, 6 and 10 mm) between the projecting electrostatic shield plate  43 A and the aperture window  113  is set to be a parameter. In a case where the distance S 2  between the projecting electrostatic shield plate  43 A and the aperture window  113  is as short as 2 mm, the signal decreases by approximately 20% as compared with a case where the distance is 10 mm. 
     The length of the projecting electrostatic shield plate  43 A in the left/right direction is made long enough to be also effective to a plurality of discharge electrodes  21  disposed at intervals along the lengthwise direction of the blow-off opening  11 . Relations between the length of the projecting electrostatic shield plate  43 A and the distance S 2  between the projecting electrostatic shield plate  43 A and the aperture window  113  will be described hereafter. When the projection length of the projecting electrostatic shield plate  43 A is prolonged gradually from 0 mm to 10 mm, attenuation of the sensor signal is in the range of 0% to at most approximately 20% (the distance S 2 =2 mm). Whereas attenuation of the noise voltage is in the range of 30% (the distance S 2 =10 mm) to 50% (the distance S 2 =2 mm). In other words, the sensor signal attenuates little whereas the attenuation of the noise voltage is large. Especially in a case where the projecting electrostatic shield plate  43 A and the aperture window  113  are made close to each other so as to have the distance S 2  that is approximately 2 mm and the projection length of the projecting electrostatic shield plate  43 A is set equal to 10 mm, attenuation of the sensor signal is approximately 20%. On the other hand, attenuation of the noise voltage is approximately 50%. Therefore, the signal to noise ratio is improved by 0.8÷0.5=1.6, i.e., 60%. 
     In the ion generator according to the embodiment of the present invention, the potential sensor  41 , which measures the potential of the charged member P, is provided integrally in the main body unit  10 . In addition, the projecting electrostatic shield plate  43 A projecting from the main body unit  10  is provided between the discharge unit formed of the discharge electrodes  21  and the opposite electrodes  23 , and the potential sensor  41 . Therefore, the electric field between the discharge electrode  21  and the opposite electrode  23  is electrostatically shielded by the projecting electrostatic shield plate  43 A. Accordingly, the electric field is hard to reach the potential sensor  41 . As a result, superposition of noise caused by the electric field between the discharge electrode  21  and the opposite electrode  23  on a value measured by the potential sensor  41  is suppressed. Therefore, the voltage of the charged member P is measured accurately. 
     The discharge unit formed of the discharge electrodes  21  and the opposite electrodes  23 , and the aperture window  113  of the potential sensor  41  are disposed on the same plane. As a result, a depth dimension L (see  FIG. 7 ) of the ion generator  1  can be made small. Accordingly, it becomes possible to design an ion generator  1  having a smaller size. 
     The projection length S 1  of the projecting electrostatic shield plate  43 A is set in the range of 8 to 10 mm from the aperture window  113  of the potential sensor  41 . As compared with the case where the projecting electrostatic shield plate  43 A is not provided, therefore, it is possible to decrease the ratio of the noise voltage Vn to Vn 0  represented by Vn/Vn 0  to a range of 35 to 50%. In addition, the distance S 2  from the projecting electrostatic shield plate  43 A to the aperture window  113  is set equal to or less than 2 mm. As a result, it is possible to suppress the decrease in the ratio of the signal voltage Vs to Vs 0  represented by Vs/Vs 0  to approximately 20%. 
     In addition, the blow-off opening  11  is formed to be long. And a plurality of discharge electrodes  21  is disposed at intervals along the lengthwise direction of the blow-off opening  11 . In such a configuration, the projecting electrostatic shield plate  43 A is made to intervene between all of the discharge electrodes  21  and the aperture window  113 . As a result, it is possible to suppress noise generation effectively. 
     Heretofore, the ion generator according to the embodiment of the present invention has been described. However, the present invention is not restricted to the embodiment described above, but various modifications and changes can be made on the basis of the technical thought of the present invention. 
     For example, in the present embodiment, the ion generator  1  having a plurality of discharge electrodes  21  along the lengthwise direction has been described. However, the present invention can also be applied to an ion generator having one discharge electrode  21  (an ion generator that blows off air ions in a spot way). 
     REFERENCE SIGNS LIST 
     
         
           1  Ion generator 
           10  Main body unit 
           11  Blow-off opening 
           12  Discharge electrode unit mounting portion 
           13  Air supply chamber 
           13 A Air supply port 
           13 B Tube 
           14  Top face cover 
           15  Air flow path 
           16  Air discharge opening 
           17  Air guide portion 
           18  Potential sensor unit housing portion 
           18 A Detection window 
           20  Discharge electrode unit 
           21  Discharge electrode 
           22  Opening portion 
           23  Opposite electrode 
           24  Air supply path 
           25  Gap 
           26  Separation space 
           27  Power supply cable 
           40  Potential sensor unit 
           41  Potential sensor 
           43  Electrostatic shield plate 
           43 A Projecting electrostatic shield plate 
           111  Printed circuit board 
           113  Aperture window 
           114  Detection electrode 
           114   a  Flange portion 
           114   b  Standing portion 
           114   c  Electrode portion 
           115  Fixed shutter 
           116  Main body portion 
           117  Side wall portion 
           118  End wall portion 
           119  Aperture slit 
           121  Movable shutter 
           122  Fixed end portion 
           123  Leg piece 
           124  Arm portion 
           125  Main body portion 
           126  End wall 
           127  Magnet 
           128  Yoke 
           128   a  Magnetic pole surface 
           128   b  Magnetic pole surface 
           129   a  Coil 
           131  Slit 
           131  Main slit 
           132  Subsidiary slit 
           281  Bobbin 
         S 1  Projection length of projecting electrostatic shield plate 
         S 2  Distance from projecting electrostatic shield plate to aperture window 
         Vn Noise voltage 
         Vn 0  Noise voltage in case where projecting electrostatic shield plate is not provided 
         Vs Signal voltage 
         Vs 0  Signal voltage in case where projecting electrostatic shield plate is not provided 
         L Depth dimension 
         N Reciprocation direction 
         P Charged member