JP2007078448A - Electric potential measuring apparatus and image formation apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a noise current which occurs due to changes in parasitic capacitance between a detection electrode and a shield case made of an electrically conductive material in an electric potential measuring apparatus to be oscillated by the sensing electrode. <P>SOLUTION: In the electric potential measuring apparatus, an oscillating member 4 capable of oscillation and provided with sensing electrodes 2a and 2b in surfaces opposed to an object to be measured 1 is housed in the shield case 7 made of an electrically conductive material except a part opposed to the object to be measured 1. The inner wall shape of the shield case 7 in the vicinity of a region including the range of oscillation of the oscillating member 4 has such a shape as to maintain an approximately constant minimum distance (d) between the sensing electrodes 2a and 2b and inner walls of the shield case 7 regardless of displacements of the sensing electrode 4. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a potential measuring device and an image forming apparatus having the potential measuring device.

2. Description of the Related Art Conventionally, for example, in an image forming apparatus that has a photosensitive drum and forms an image by electrophotography, the potential distribution on the surface of the photosensitive drum is appropriately (typically) in any environment in order to obtain a stable image quality. It is necessary to charge the surface of the photosensitive drum so as to be uniform. Therefore, the potential on the surface of the photosensitive drum is measured using a potential measuring device, and feedback control is performed using the result to keep the potential on the surface of the photosensitive drum uniform.

One of the functions that are often required for the potential measuring apparatus used for such a purpose is a function of measuring the surface potential of the measurement target (object to be measured) in a non-contact manner. This is because when the potential measuring device comes into contact with the surface of the photosensitive drum, the potential distribution on the surface of the photosensitive drum is not uniform, which causes a disorder in the formed image.

The principle of such a potential measuring apparatus will be described below. Due to the electric field generated between the surface of the measurement object and the detection electrode built in the potential measuring device, an electric charge of an electric quantity Q proportional to the surface potential V of the measurement object is induced in the detection electrode. The relationship between Q and V is Q = CV (1)
It is expressed by the formula. Here, C is a capacitance between the detection electrode and the surface of the measurement target. From the equation (1), it is possible to obtain the surface potential of the measurement object by measuring the electric quantity Q induced in the detection electrode.

However, it is difficult to directly measure the amount of electricity Q induced in the detection electrode at high speed and accurately. Therefore, as a practical method, by periodically changing the magnitude of the capacitance C between the detection electrode and the surface to be measured, and measuring the alternating current generated at the detection electrode as a potential detection signal current, In general, a method of obtaining the surface potential of the measurement object is employed.

It will be shown below that the surface potential of the measurement object can be obtained by the above method. Assuming that the capacitance C is a function of time t, the potential detection signal current i generated at the detection electrode is a time differential value of the quantity of electricity induced at the detection electrode. It is expressed by a formula.
i (t) = dQ / dt = d (CV) / dt (2)
Here, when the rate of change of the surface potential V of the measurement target is sufficiently slow with respect to the rate of change of the capacitance C, V can be regarded as being constant in the minute time dt, and therefore equation (2) is It is expressed by the following formula.
i (t) = dQ (t) / dt = V · dC (t) / dt (3)

From the equation (3), the magnitude of the potential detection signal current i generated at the detection electrode is a linear function of the surface potential V of the measurement target. Therefore, the surface potential of the measurement target is measured by measuring the amplitude of the alternating current signal. It is possible to obtain Further, according to the equation (3), if the change rate of the capacitance C is the same, the magnitude of the potential detection signal current i with respect to the surface potential of the measurement object, that is, the sensitivity of the potential measuring device is the amount of change in the capacitance. You can see that they are proportional.

As one method of periodically changing the capacitance C between the detection electrode and the measurement target surface, the capacitance C is changed periodically by changing the distance between the detection electrode and the measurement target surface. The method of making it include. The capacitance C between the detection electrode and the surface of the measurement object is approximately C = A · S / x (4)
It is expressed by the following formula. Here, A is a proportional constant related to the dielectric constant of the substance, S is the area of the detection electrode, and x is the distance between the detection electrode and the surface of the measurement object. From equation (4), it can be seen that when the distance x changes periodically, the capacitance C also changes periodically.

One means for periodically changing the distance between the detection electrode and the surface of the measurement target is a means for vibrating the detection electrode in a direction substantially perpendicular to the surface of the measurement target. Examples of means for vibrating the detection electrode include a method of vibrating an elastic body having the detection electrode arranged on the surface facing the measurement object using alternating electrostatic force or electromagnetic force.

When using a potential measuring apparatus of the above-described type, at least the detection electrode is made of a conductive material in order to reduce the potential of external objects other than the surface to be measured and the influence of external electromagnetic noise. Often used in a box-shaped shield case. An opening is provided in a part of the surface of the shield case facing the object to be measured, and this shield case with an opening measures only the potential of the object to be measured while reducing the influence of external charged objects and electromagnetic noise. Can do.

A specific example of a potential measuring device using the above method will be described with reference to FIG. 6 (see Patent Document 1). An air-core electromagnetic coil 8 in which the detection electrode 2 is arranged on the surface facing the measurement object 1 is fixed to the support member 10 via a diaphragm 4 made of an elastic body. The support member 10 is fixed to the bottom of the shield case 11. A permanent magnet 13 is fixed to the inner wall of the shield case 11, and a part of the permanent magnet 13 is inserted into the air core portion of the electromagnetic coil 8. An opening 12 for introducing an electric field stretched by the measurement object 1 is provided in a portion of the surface of the shield case 11 facing the measurement object 1 that is directly above the detection electrode 2.

In this configuration, the electromagnetic coil 8 receives an attractive force or a repulsive force from the permanent magnet 13 by applying an alternating current to the electromagnetic coil 8 from a drive current source (omitted in FIG. 6). Thereby, the detection electrode 2 periodically vibrates in a direction substantially perpendicular to the measurement target 1, and the distance between the detection electrode 2 and the measurement target 1 changes periodically. In this way, the capacitance between the detection electrode 2 and the measurement object 1 changes periodically, and a potential detection signal current having an amplitude proportional to the potential of the measurement object 1 is obtained from the detection electrode 2. This potential detection signal current is voltage signal converted, filtered, amplified, and rectified in the signal processing circuit unit 14 and output as a potential measurement signal.
JP-A-8-110361

In recent years, with the miniaturization of image forming apparatuses, miniaturization is also required for potential measuring apparatuses that measure the potential of a photosensitive drum. If the conventional potential measuring device as mentioned above is downsized without changing the area of the detection electrode, the distance between the vibrating detection electrode and the inner wall of the shield case is reduced, and the distance between the detection electrode and the shield case is reduced. Parasitic capacitance increases. The parasitic capacitance between the detection electrode and the shield case mainly depends on the distance between the detection electrode and the shield case.

In a box-shaped shield case with an opening, which is often used in conventional potential measuring devices, when the vibration angle or vibration displacement of the diaphragm 4 is zero (FIGS. 1A-2 and 2A-2) If the shortest distance between the detection electrode 2 and the shield case 11 is d, the following occurs. When the vibration angle or vibration displacement of the diaphragm 4 becomes maximum in the direction approaching the opening 12 of the shield case 11 (see FIGS. 1B-2 and 2B-2), the detection electrode 2 and the shield The shortest distance to the case 11 changes to d ′ shorter than d.

Thereby, the parasitic capacitance between the detection electrode 2 and the shield case 11 changes according to the displacement of the diaphragm 4. Furthermore, since the shortest distance between the detection electrode 2 and the shield case 11 changes in the same cycle as the vibration of the diaphragm 4, the parasitic capacitance between the detection electrode 2 and the shield case 11 is the vibration of the diaphragm 4. It changes with the same period.

Here, if a potential difference is generated between the detection electrode 2 and the shield case 11 due to adhesion of a charged substance or poor connection to the ground wire, the parasitic capacitance between the detection electrode 2 and the shield case 11 as described above is reduced. A noise current is output from the detection electrode 2 due to the change. Furthermore, since this noise current has the same frequency as the potential detection signal current, it is difficult to remove this noise current from the potential detection signal current even if signal processing means is used.

In view of the above problems, the potential measuring device of the present invention is a potential measuring device having a vibrating member having a detection electrode and a shield case. The vibration member exists inside the shield case, and the shortest distance between the detection electrode and the inner wall of the shield case is maintained substantially constant regardless of the displacement of the detection electrode due to the vibration of the vibration member. It is characterized by that. More specifically, an oscillating vibration member having a detection electrode on the surface facing the measurement target is stored inside a shield case made of a conductive material except for a portion facing the measurement target. The inner wall shape of the shield case in the vicinity of the region including at least the vibration range of the vibrating member is such that the shortest distance between the detection electrode and the inner wall of the shield case is substantially constant regardless of the displacement of the detection electrode. Features. In the electric potential measurement apparatus of the present invention, the following configuration is adopted in which the fixed surface of the detection electrode is configured to always face the shield case inner wall at the shortest distance. That is, the inner wall of the shield case and the locus surface due to the vibration displacement of the fixed surface of the detection electrode are substantially parallel within the vibration range of the vibration member.

In view of the above problems, the potential measuring device of the present invention includes a potential measuring device, a signal processing device that processes an output signal obtained from the potential measuring device, and an image forming unit. The portion where the detection electrode of the potential measuring device is formed is arranged opposite to the potential measurement target of the image forming means, and the image forming means controls the image formation using the signal detection result of the signal processing device. It is characterized by.

In the present invention, the inner wall shape of the shield case is made such that the shortest distance between the detection electrode and the inner wall of the shield case is substantially constant with respect to the vibration of the vibration member. Thereby, the noise current generated by the parasitic capacitance change between the detection electrode and the shield case can be suppressed, and the accuracy of potential measurement can be improved.

Hereinafter, an embodiment of the present invention will be described while explaining the operation principle of the present invention.
FIG. 1 (a-1) is a sectional view showing an embodiment of the present invention. In the potential measuring device of this embodiment, the diaphragm 4 having the two detection electrodes 2a and 2b vibrates in the rotation direction with a straight line existing on a plane including itself as a central axis (extends in a direction perpendicular to the paper surface) ( (Rotational vibration). The inner wall shape of the shield case 7 made of a conductive material is determined so as to be substantially axisymmetric with respect to the central axis of the rotational vibration of the detection electrodes 2a and 2b (in FIG. 1 (a-1)). It is determined to be the side surface of a cylinder with a radius R). The two detection electrodes 2 a and 2 b are opposed to the measuring object 1 through the opening 12 of the shield case 7.

In the present invention, the inner wall shape that reduces the change in parasitic capacitance between the detection electrode and the shield case due to the vibration of the vibration member is that the shortest distance between the detection electrode and the shield case inner wall is always substantially constant. The shape is different. The reason will be described below.

The parameters for determining the parasitic capacitance between the detection electrode and the shield case include the distance between the detection electrode and the inner wall of the shield case (in particular, the shortest distance), the facing area, and the dielectric constant (the above formula ( 4)). Of these three members, the facing area between the detection electrode and the inner wall of the shield case (in this embodiment, the area of the side surface of the detection electrode 2) and the dielectric constant (in this embodiment, the inner wall of the detection electrode 2 and the shield case 7) The dielectric constant of the atmosphere in between is substantially constant with respect to the vibration of the sensing electrode. In other words, in the present invention, the constant surface forming the above-mentioned facing area of the detection electrode seems to always face the shield case inner wall at the shortest distance as compared with other portions of the detection electrode. Mainly intended for the configuration. Therefore, the shortest distance between the detection electrode (a certain surface forming the above-mentioned facing area) and the inner wall of the shield case is a parameter that mainly determines the parasitic capacitance between the detection electrode and the shield case. It is important to suppress the parasitic capacitance change by making it almost constant.

Regardless of the displacement of the detection electrode due to the vibration of the vibration member, the form in which the shortest distance between the detection electrode and the inner wall of the shield case is maintained substantially constant depends on the vibration form of the vibration member as follows. Can be determined. Here, “substantially constant” includes not only the case where it is completely constant, but also the case where it differs within a range that does not adversely affect the measurement accuracy (such as a dimensional error).

When the diaphragm 4 vibrates in the rotational direction about a straight line existing on a plane including itself as in the present embodiment, the inner wall shape of the shield case 7 is changed to the detection electrode 2 as shown in FIG. What is necessary is just to determine so that it may become axisymmetric with respect to the center axis | shaft of this rotational vibration. Thereby, the shortest distance between the shield case 7 and the detection electrode 2 is always as follows. That is, “the point farthest from the central axis of the rotational vibration of the diaphragm 4 in the outer periphery and inside of the detection electrode 2” and “the point that passes through the point on the detection electrode 2 and is orthogonal to the central axis of the rotational vibration of the diaphragm 2. The distance between the straight line to intersect with the point where the inner wall surface of the shield case 7 intersects. It is clear from FIGS. 1 (a-1) and 1 (b-1) that this distance is constant regardless of the rotation angle of the diaphragm 4.

Here, if all the portions of the inner wall of the shield case 7 are formed symmetrically with respect to the central axis of the rotational vibration of the diaphragm 4, it becomes impossible to introduce the electric field stretched by the measuring object 1. It is necessary to provide the opening 12 in a part of the surface of the shield case 7 that faces the measurement object 1. However, the elevation angle ψ at the end of the opening 12 of the shield case 7 as viewed from a plane including the central axis of the rotational vibration of the diaphragm 4 and parallel to the surface of the measurement object 1 is as follows. That is, if the size of the opening 12 of the shield case 7 is determined so that the elevation angle ψ becomes larger than the maximum rotation angle θ of the diaphragm 4 (see FIG. 1 (b-1)), the above object is achieved. . In the embodiment shown in FIGS. 1 (a-1) and 1 (b-1), a method of processing a potential detection signal current generated at the sensing electrode 2 and outputting a potential measurement signal will be described in Example 1 described later. Is the same as

FIG. 2 shows another embodiment. Here, the diaphragm 4 vibrates linearly in a direction substantially perpendicular to the measurement object 1. In this case, at least a portion of the inner wall surface of the shield case 7 between the vicinity of the diaphragm 4 and the measurement target 1 is a plane perpendicular to the vibration direction of the diaphragm 4 as shown in FIG. What is necessary is just to determine so that the cross section of the inner wall surface of the cut | disconnected shield case 7 may become congruent. Thus, the shortest distance between the shield case 7 and the detection electrode 2 is always “the distance between the two points on the plane including the detection electrode 2 and the point on the inner wall surface of the shield case 7 is minimum. The distance between the two points you choose. It is clear from FIGS. 2A-1 and 2B-1 that this distance is constant regardless of the displacement of the diaphragm 4. FIG. In the embodiment of FIGS. 2 (a-1) and 2 (b-1), a method of processing the potential detection signal current generated at the detection electrode 2 and outputting the potential measurement signal will be described in Example 2 described later. Is the same as

The above can be summarized as follows for a configuration in which the above-described certain surface of the detection electrode is always opposed to the inner wall of the shield case at the shortest distance. If the inner wall shape of the shield case in the vicinity including the vibration range of the vibration member is substantially parallel to the locus plane due to vibration displacement of the fixed surface of the detection electrode, the space between the detection electrode and the shield case due to vibration of the vibration member It is possible to suppress a change in parasitic capacitance.

Hereinafter, more specific embodiments of the present invention will be described with reference to the drawings.

Example 1
A first embodiment of the present invention is shown in FIGS. 3 (a) and 3 (b) ((a) is a partially broken schematic perspective view, and (b) is a sectional view taken along a broken line AA ′). In the potential measuring apparatus of the present embodiment, detection electrodes 2a and 2b are arranged on the diaphragm 4 on the surface facing the surface of the measuring object 1, and magnetized in the direction shown in FIG. A magnetic body 3 (the polarity of magnetization may be opposite to that shown in the figure) is arranged. The diaphragm 4 is pivotally supported on the support plates 6a and 6b by two torsion springs 5a and 5b. The support plates 6a and 6b are attached to the protrusions 9a and 9b on the inner wall of the shield case 7 made of a conductive material, and the electromagnetic coil 8 is disposed on the bottom portion of the shield case 7 immediately below the magnetic body 3. Has been.

Of the inner wall of the shield case 7, the region in the vicinity of the vibration range of the diaphragm 4 is made to be a cylindrical side surface having a radius R (R is a constant). The central axis of the cylindrical side surface of the inner wall of the shield case 7 substantially coincides with the central axis of rotational vibration of the diaphragm 4 (perpendicular to the paper surface of FIG. 3B). That is, of the inner wall of the shield case 7, the inner wall shape of the shield case in the vicinity of the region is substantially axisymmetric with respect to the central axis of the rotational vibration of the diaphragm 4. The elevation angle ψ at the point where the cylindrical side surface shape of the inner wall of the shield case 7 is interrupted as viewed from the central axis of the rotational vibration of the diaphragm 4 is designed to be larger than the maximum rotation angle of the diaphragm 4.

In the present embodiment, the fixed surfaces (side surfaces facing the inner wall of the shield case) of the detection electrodes 2a and 2b are displaced by the rotational vibration of the diaphragm 4, and always form the shortest distance d with the inner wall of the shield case. It is comprised so that it may oppose substantially. Based on such a configuration, in more general terms, the shape of the inner wall of the shield case in the vicinity of the region seems to be substantially parallel to the trajectory plane due to the vibration displacement of the fixed surface of the detection electrodes 2a and 2b. It has a shape. Furthermore, in the present embodiment, the inner wall shape of the shield case 7 in the vicinity of the region has a substantially constant shortest distance from each part of the detection electrodes 2a and 2b to the inner wall of the shield case regardless of the displacement of the detection electrode. It also has a shape like This configuration is a configuration that contributes to extremely reducing the amount of change in parasitic capacitance between the detection electrode 2 and the shield case 7.

The operation is performed as follows. When an alternating current is applied to the electromagnetic coil 8 from an external drive current source (omitted in FIG. 3), the magnetic body 3 receives an attractive force or a repulsive force from the magnetic field stretched by the electromagnetic coil 8, and the diaphragm 4 It vibrates in the rotational direction about the torsion springs 5a and 5b. The potential detection signal current generated at the detection electrode 2 is converted into a voltage signal, filtered and amplified in a signal processing circuit (omitted in FIG. 3) (signals from the two detection electrodes 2a and 2b are differentially amplified). Rectified and output as a potential measurement signal.

Due to the inner wall shape of the shield case 7 as described above, the shortest distance d between the detection electrode 2 and the inner wall of the shield case 7 is always substantially constant regardless of the rotation angle of the diaphragm 4. Therefore, the amount of change in the parasitic capacitance between the detection electrode 2 and the shield case 7 is reduced as compared with the case where the conventional shield case is used, and even when a potential difference is generated between the detection electrode 2 and the shield case 7. The noise current generated at the detection electrode 2 is reduced.

As described above, by using the shield case 7 in this embodiment, it is possible to reduce the noise current generated due to the parasitic capacitance change between the detection electrode 2 and the shield case 7 and improve the accuracy of the potential measurement. .

A modification will be described. In the present embodiment, FIGS. 3C and 3D ((c) is a cross-sectional view taken along a broken line AA ′ in a modified example, and (d) is a cross-sectional view taken along a broken line BB ′ in the modified example). Thus, substantially the same effect can be obtained by making the inner wall shape of the shield case 7 in the vicinity of the diaphragm 4 spherical. The center of the spherical surface of the inner wall of the shield case 7 substantially coincides with a point where a perpendicular drawn from the center of each of the detection electrodes 2 a and 2 b with respect to the rotation axis of the diaphragm 4 intersects with the rotation axis of the diaphragm 4. That is, the center of the spherical surface is substantially on the rotation axis of the diaphragm 4. The minimum value of the elevation angle ψ at the point where the spherical shape of the inner wall of the shield case 7 is interrupted as viewed from the center of the spherical surface of the inner wall of the shield case 7 is designed to be larger than the maximum rotation angle of the diaphragm 4. . In this modification as well, the radius R of the spherical surface is set larger than a certain amount with respect to the dimensions of the diaphragm 4 so that the inner wall shape of the shield case 7 in the vicinity of the region substantially satisfies the above condition.

(Example 2)
A second embodiment of the present invention is shown in FIGS. 4A and 4B ((a) is a schematic cross-sectional view, and (b) is a cross-sectional view taken along a broken line CC ′). In the second embodiment, the diaphragm 4 in which the detection electrode 2 is arranged on the surface facing the surface of the measuring object 1 and the magnetic body 3 is arranged on the opposite surface is provided at the bottom of the shield case 7 at one end thereof. It is being fixed to the support member 10 arrange | positioned. The diaphragm 4 is bent at two locations, whereby the detection electrode 2 is supported so as to be able to vibrate at a position closer to the measurement object 1 than the support member 10. The diaphragm 4 is supported by the support member 10 in a cantilever manner, and the detection electrode 2 is supported at the other end. The electromagnetic coil 8 is disposed in a portion of the bottom portion of the shield case 7 made of a conductive material that is directly below the magnetic body 3.

Of the inner wall of the shield case 7, the shape of the portion existing between the plane including the detection electrode 2 and the plane including the surface of the measuring object 1 is a side surface of the cylinder, and the central axis of the cylinder is the diaphragm 4. It is almost parallel to the vibration direction. From another point of view, a cross section of the inner wall shape of the shield case 7 between the plane including at least the detection electrode 2 and the plane including the surface to be measured is a plane perpendicular to the vibration direction of the detection electrode 2. The shape of is almost the same. Since the length of the diaphragm 4 from the portion fixed to the support member 10 is sufficiently long, the vibration direction of the detection electrode 2 supported by the diaphragm 4 is substantially linear. Further, the portion of the shield case 7 that faces the measurement object 1 is not protruding from the portion of the shield case 7 that faces the measurement object 1, and the entire surface that faces the measurement object 1 is an opening without a magnetic material. Yes. The shape of the opening, the cross-sectional shape of the shield case 7, the shape of the detection electrode 2, and the shape of the tip of the diaphragm 4 are as shown in FIG.

In the above configuration, when an alternating voltage is applied to the electromagnetic coil 8 from a drive voltage source (omitted in FIG. 4), the magnetic body 3 periodically receives an attractive force from the magnetic field stretched by the electromagnetic coil 8, and the diaphragm 4 Oscillates in a direction substantially perpendicular to the measuring object 1. The potential detection signal current generated at the detection electrode 2 is converted into a voltage signal, filtered, amplified, and rectified in a signal processing circuit (omitted in FIG. 4) and output as a potential measurement signal.

Also in this embodiment, a certain surface (side surface facing the inner wall of the shield case) of the detection electrode 2 is displaced by the linear vibration of the diaphragm 4 and is always substantially opposed to the inner wall of the shield case with a shortest distance d. It is configured as follows. In more general terms, the shape of the inner wall of the shield case 7 is such that it is substantially parallel to the trajectory plane due to the vibration displacement of the fixed surface of the detection electrode 2. Furthermore, in this embodiment, the inner wall shape of the shield case 7 is also shaped so that the shortest distance from each part of the detection electrode 2 to the inner wall of the shield case is substantially constant in each part regardless of the displacement of the detection electrode. It has become.

Due to the inner wall shape of the shield case 7 as described above, the shortest distance between the detection electrode 2 and the shield case 7 is almost always constant regardless of the position of the detection electrode 2. Therefore, the amount of change in the parasitic capacitance between the detection electrode 2 and the shield case 7 is reduced as compared with the case where the conventional shield case is used, and even when a potential difference is generated between the detection electrode and the shield case. The noise current generated at the electrode 2 is reduced.

As described above, also in the present embodiment, by using the shield case 7, it is possible to suppress the noise current generated due to the parasitic capacitance change between the detection electrode 2 and the shield case 7.

In the said Example, the shape of the part which exists between the plane containing the detection electrode 2 and the plane containing the surface of the measuring object 1 among the inner walls of the shield case 7 should just be the side surface shape of a column, In addition to the side surfaces, various side surface shapes are possible corresponding to the shape of the detection electrode. As shown in FIG. 4C, various side surface shapes of column cylinders such as a side surface of a quadrangular column corresponding to the quadrangular detection electrode 4 can be applied.

(Example 3)
As a third embodiment of the present invention, a configuration example of an image forming apparatus using the potential measuring device of the present invention will be described.

An image forming apparatus of this embodiment is shown in FIG. Around the photosensitive drum 21, a charger 16 that can be controlled by the charger controller 17, an exposure device 18, the potential measuring device 15 of the present invention, and a toner supplier 19 are installed. A latent image is obtained by charging the surface of the photosensitive drum 21 with the charger 16 and exposing the surface of the photosensitive drum 21 with the exposure device 18. Toner is attached to the latent image by the toner supplier 19 to obtain a toner image in which the latent image is developed. This toner image is transferred to a printing object 22 sandwiched between the feed roller 20 and the photosensitive drum 21, and the toner on the printing object 22 is fixed. Image formation is achieved through these steps. The charger controller 17 constitutes a signal processing device, and the charger 16, the exposure device 18, the photosensitive drum 21, and the like constitute image forming means.

In this configuration, the charged state of the photosensitive drum 21 is measured by the potential measuring device 15 of the present invention, and a measurement signal of the surface potential of the photosensitive drum 21 is output to the charger controller 17. Based on this measurement signal, the charger controller 17 feedback-controls the charging voltage of the charger 16 so that the surface potential of the photosensitive drum 21 after charging becomes a desired value (measurement by the potential measuring device 15 of the present invention). The signal can also be fed back to the exposure unit 18 to control it). Thereby, stable charging of the photosensitive drum 21 is realized, and stable image formation is realized.

In the potential measuring device in which the diaphragm vibrates in the rotation direction, (a-1) a cross-sectional view when the rotation angle of the diaphragm of one embodiment to which the shield case of the present invention is applied is 0, (b-1) the present invention. Sectional drawing when the rotation angle of the diaphragm of the embodiment to which the shield case of the embodiment is applied is the maximum, (a-2) Sectional view when the rotation angle of the diaphragm of the example of applying the conventional shielding case is 0, (B-2) It is sectional drawing when the rotation angle of the diaphragm of the example which applied the conventional shield case is the maximum. In the potential measuring device in which the diaphragm vibrates linearly, (a-1) a cross-sectional view when the displacement of the diaphragm of another embodiment to which the shield case of the present invention is applied is 0, (b-1) the present invention. Sectional drawing when the displacement of the diaphragm of another embodiment to which the shield case of the present embodiment is applied is the maximum, (a-2) Sectional view when the displacement of the diaphragm of another example to which the conventional shielding case is applied is zero (B-2) It is sectional drawing when the displacement of the diaphragm of the other example to which the conventional shield case is applied is the maximum. 1A is a partially broken schematic perspective view, FIG. 1B is a sectional view taken along a broken line AA ′, and FIG. 1C is a sectional view taken along a broken line AA ′ in a modification. (D) It is sectional drawing in broken line BB 'of a modification. 4A is a schematic cross-sectional view of a potential measuring apparatus according to a second embodiment of the present invention, FIG. 4B is a cross-sectional view taken along a broken line C-C ′, and FIG. It is the schematic which shows the image forming apparatus in the 3rd Example of this invention. It is a schematic cross section of a conventional potential measuring device.

Explanation of symbols

1,21 ... Measurement target (photosensitive drum)
2, 2a, 2b ... detection electrode 4 ... vibrating member (vibrating plate)
7 ... Shield case 12 of the present invention ... Opening 15 of the shield case ... Potential measuring device 16, 18, 21 of the present invention ... Image forming means (charger, exposure device, photosensitive drum)
17 ... Signal processing device (charger controller)

Claims (9)

A potential measuring device having a vibrating member having a detection electrode and a shield case,
The vibration member exists inside the shield case,
The potential measuring device, wherein the shortest distance between the detection electrode and the inner wall of the shield case is maintained substantially constant regardless of the displacement of the detection electrode caused by the vibration of the vibration member. In the electric potential measurement device according to claim 1, the fixed surface of the detection electrode is configured to always face substantially the shortest distance from the inner wall of the shield case,
An electric potential measuring apparatus, wherein an inner wall of the shield case and a locus plane due to vibration displacement of the fixed surface of the detection electrode are substantially parallel within a vibration range of the vibration member. 3. The potential measuring apparatus according to claim 1, wherein the shortest distance between the inner wall of the shield case in the vicinity of the region and each part of the detection electrode is maintained substantially constant regardless of the displacement of the detection electrode. A potential measuring device characterized by that. The potential measuring device according to any one of claims 1 to 3, wherein the vibrating member vibrates in a rotation direction with a torsion spring as a central axis,
A potential measuring device, wherein a distance between an inner wall of the shield case and a central axis of rotational vibration of the vibration member is substantially constant within a vibration range of the vibration member. In the potential measuring device according to claim 4, the inner wall shape of the shield case in the vicinity of the region is a cylindrical side surface,
A potential measuring device characterized in that the central axis of the cylindrical side surface substantially coincides with the central axis of rotational vibration of the vibrating member. In the potential measuring device according to claim 4, the inner wall shape of the shield case in the vicinity of the region is a spherical surface,
The potential measuring device is characterized in that the center of the spherical surface is substantially on the central axis of rotational vibration of the vibrating member. The potential measuring device according to any one of claims 1 to 3, wherein the vibrating member vibrates in a direction substantially perpendicular to the surface to be measured.
Of the inner wall shape of the shield case, at least a portion existing between a plane including the detection electrode and a plane including the measurement target surface has substantially the same cross-sectional shape by a plane perpendicular to the vibration direction of the detection electrode. A potential measuring device characterized by the above. In the potential measuring device according to claim 7, the inner wall shape of the shield case in the vicinity of the region is a side surface of the columnar cylinder,
A potential measuring device characterized in that the central axis of the cylinder is substantially parallel to the vibration direction of the vibrating member. A potential measuring device according to any one of claims 1 to 8, a signal processing device for processing an output signal obtained from the potential measuring device, and an image forming means,
The portion where the detection electrode of the potential measuring device is formed is arranged opposite to the potential measurement target of the image forming means, and the image forming means controls the image formation using the signal detection result of the signal processing device. An image forming apparatus.

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