JPH1062390A - Toner concentration detector of image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct the temperature dependency of a device within the device by using a flat coil and a capacitor which have reverse temperature characteristics. SOLUTION: A toner concentration detector is mounted on a stirring chamber to supply toner on the basis of a detection result. A resonance circuit is formed of a typical Colpitts oscillating circuit formed of an inductance coil L, capacitors C1, C2, a transistor Q, and a resistance R. The inductance coil L is formed of a flat coil. When a voltage V is applied to the resonance circuit, an oscillation signal is outputted from the source of the transistor Q. In the toner concentration detector, those having reverse temperature characteristic are used for the flat coil of the resonance circuit and the capacitors C1, C2 to perform a temperature compensation within the toner concentration detector. The reverse temperature characteristics mean that the absolute values of the temperature coefficients are equal to each other, and positive and negative are mutually reversed. As the capacitor, for example, a multilayer ceramic capacitor is considered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, and more particularly to a toner density detecting apparatus using an LC resonance circuit having a planar coil, and more particularly to an improved technique for improving performance.

[0002]

2. Description of the Related Art Conventionally, an electrostatic latent image is formed on a photosensitive drum by an exposure optical system, and the electrostatic latent image is developed with toner by a developing device. An image forming apparatus for fixing is known (Japanese Unexamined Patent Publication No.
7-38482, JP-A-62-157070, etc.).

[0003] Developers used for development in the above-mentioned developing apparatus include a one-component developer and a two-component developer. The two-component developer is formed by mixing magnetic carrier particles and non-magnetic synthetic resin toner particles at an appropriate mixing ratio. The toner particles having an appropriate mixing ratio mixed with the carrier particles are developed. It is carried by the developing sleeve of the device and adheres to the latent image area on the photosensitive drum by sliding or jumping,
A toner image is formed.

When a two-component developer is used, only the toner is consumed and the carrier is circulated and used repeatedly in the developing device. Therefore, a detecting device for detecting the toner concentration in the developing device is provided. The toner is replenished from the toner replenishing device as needed based on the result of detection by the detecting device, and is appropriately charged in the developing device and stirred to have a uniform mixing ratio.

Several methods for detecting the toner concentration have been proposed. One of the methods is to use a change in the resonance frequency of an LC resonance circuit. The apparent magnetic permeability of the developer, which is a mixture of iron powder as a carrier and toner, changes depending on the toner concentration. When the coil is approached by a method such as embedding in the developer, the inductance of the coil changes depending on the magnetic permeability of the atmosphere. Since the oscillation frequency of the LC oscillation circuit is given by f = 1 / 2π (LC) ^1/2 ... (1), it is necessary to detect the toner concentration by measuring the frequency f. Can be.

By the way, when a winding coil in which a thin wire is wound around a rod-shaped core such as iron or ferrite having a high magnetic permeability is used as an inductance, only the tip of the winding coil can be brought into contact with the developer. However, there is a problem that the sensitivity becomes low. Therefore, a toner concentration detecting device in which the detecting coil is a planar coil has been proposed by the present applicant (see Japanese Utility Model Laid-Open No. 6-79661).

[0007] By using a planar coil in this way, not only the sensitivity is improved but also the size can be reduced.

[0008]

However, even in the toner density detecting device of the image forming apparatus using such a planar coil, there is a point to be further improved in order to improve the performance such as the detection sensitivity. That is, when the temperature inside the device changes, the oscillation frequency of the LC oscillation circuit also changes, so that the temperature must be compensated. Further, it is necessary to further improve the detection level. Furthermore, it is necessary to provide a structure that allows the device to be easily mounted, and it is necessary to reduce the size of the detection unit between the planar coil and the capacitor.

The present invention has been made in view of such conventional problems, and has as its object to provide a further improved toner density detecting device of an image forming apparatus in order to improve performance.

[0010]

According to a first aspect of the present invention, there is provided an apparatus having a resonance circuit based on the inductance of a planar coil and the capacitance of a capacitor which are spirally arranged on a flat substrate, and is provided in a developing container. In the toner concentration detecting device of the image forming apparatus for detecting the toner concentration in the stored developer as a change in the resonance frequency of the resonance circuit, the planar coil and the capacitor having opposite temperature characteristics are used. .

According to this configuration, the temperature dependency of the device is corrected in the device. The device according to claim 2 is:
Image formation comprising a resonance circuit based on the inductance of a planar coil and the capacitance of a capacitor spirally arranged on a flat substrate, and detecting the toner concentration in the developer contained in a developing container as a change in the resonance frequency of the resonance circuit. In the toner concentration detecting device of the device, the developing container includes:
The substrate of the resonance circuit was abutted against the outer wall of the developing container so that the substrate was of a substantially cylindrical shape parallel to the direction in which the developer was transported, and the surface of the planar coil was in close contact with the outer wall of the developing container.

According to this configuration, the flat coil surface can be easily mounted so as to be parallel to the developing container. According to a third aspect of the present invention, there is provided an apparatus having a resonance circuit based on the inductance of a planar coil and the capacitance of a capacitor which are spirally arranged on a flat substrate, and determines the toner concentration in a developer contained in a developing container by the resonance. In a toner concentration detecting device of an image forming apparatus that detects a change in resonance frequency of a circuit, an opening is provided in the developing container, a protective layer is formed on a surface of a planar coil, and the surface of the planar coil is directed toward the inside of the developing container. Thus, the flat substrate of the resonance circuit is attached to the opening of the developing container.

According to this structure, the planar coil comes into contact with the developer through the protective layer, and the toner concentration can be detected with high sensitivity. Protected. In the apparatus according to claim 4, the protective layer is formed by applying a coating material.

According to this structure, the protective layer is formed by the applied coating material. In the apparatus according to claim 5, the coating material is polyimide. According to such a configuration, the protective layer is formed by applying polyimide.

In the apparatus according to the invention of claim 6, the coating material is polycarbonate. According to this configuration, the protective layer is formed by applying polycarbonate. In the apparatus according to the invention of claim 7, the coating material is aramid. According to such a configuration, the protective layer is formed by applying aramid.

In the apparatus according to the present invention, the protective layer is formed by attaching a protective sheet with an adhesive. According to such a configuration, the protective layer is formed by the protective sheet attached with the adhesive. In the apparatus according to the ninth aspect, the protection sheet is Kapton.

According to this configuration, the protective sheet of Kapton is attached with the adhesive to form the protective layer. In the apparatus according to the tenth aspect, the protective sheet is a mylar. According to such a configuration, the protective sheet of Mylar is stuck with an adhesive to form a protective layer.

[0018] In the apparatus according to the eleventh aspect, the protective sheet is made of polyethylene terephthalate. According to such a configuration, the protective sheet of polyethylene terephthalate is attached with the adhesive to form the protective layer. The apparatus according to the twelfth aspect of the present invention includes a resonance circuit based on the inductance of a planar coil and the capacitance of a capacitor which are spirally arranged on a flat substrate, and determines the toner concentration in a developer contained in a developing container by the resonance circuit. In a toner density detection device of an image forming apparatus that detects a change in resonance frequency of a circuit, the resonance circuit is configured such that a planar coil and a capacitor are arranged close to each other separately from other circuits.

According to this configuration, the planar coil and the capacitor are arranged close to each other and are separated from other circuits, so that the size of the resonance circuit is reduced. The apparatus according to the thirteenth aspect of the present invention is provided with a resonance circuit based on the inductance of a planar coil and the capacitance of a capacitor spirally arranged on a flat substrate, and adjusts a toner concentration in a developer contained in a developing container by the resonance circuit. In a toner concentration detecting device of an image forming apparatus which detects a change in a resonance frequency of a circuit, the resonance circuit is formed by forming a ground plate in a region substantially corresponding to the plane coil on the back surface of the plane coil arrangement surface of the plane substrate. ing.

According to this configuration, the radio wave noise is absorbed by the ground plate, and the influence on the back surface of the detecting flat coil is reduced. The apparatus according to claim 14 is provided with a resonance circuit based on the inductance of a planar coil and the capacitance of a capacitor which are spirally arranged on a flat substrate, and adjusts the toner concentration in the developer contained in a developing container to the resonance circuit. In a toner concentration detecting device of an image forming apparatus that detects a change in resonance frequency of a circuit, the developing container is disposed in a long stirring chamber containing a developer, and the developer container is transported while stirring the developer. A longitudinal stirring member, wherein the planar coil is configured by connecting a plurality of planar coils side by side along the longitudinal stirring member.

According to this configuration, since the plurality of planar coils of the resonance circuit are connected side by side along the rotation axis of the stirring member of the developing container, the inductance is secured and the detection level of the planar coil is increased. In the apparatus according to claim 15, the stirring member is a transport screw. According to such a configuration, a plurality of planar coils are arranged along the transport screw.

[0022]

FIG. 1 is a block diagram showing an embodiment of the present invention.
This will be described with reference to FIG. First, a first embodiment will be described. This is a toner concentration detecting device of a developing device of an image forming apparatus, in which a planar coil and a capacitor of a resonance circuit having opposite temperature characteristics are used.
The temperature compensation is performed in the apparatus.

First, the configuration of the developing device of the image forming apparatus will be described. FIGS. 3, 4, and 5 are a perspective view, a sectional view, and a plan view of the developing device, respectively. This developing device 1
For example, it is built in an image forming apparatus such as a copying machine and develops an electrostatic latent image formed on a photosensitive drum (not shown) by an exposure optical system with toner. The developing sleeves 3 are disposed so as to be close to each other.

In the developing container 2 of the developing device 1, two stirring chambers 4a and 4b are provided, and each stirring chamber 4a and 4b has
Transport screws 5a and 5b as stirring members are provided. The transport screws 5 a and 5 b are arranged such that the rotation axis is substantially parallel to the rotation axis of the developing sleeve 3. The two-component developer containing the magnetic carrier particles and the nonmagnetic synthetic resin toner particles is accommodated in the stirring chambers 4a and 4b, respectively, and conveyed while being stirred by the rotation of the conveying screws 5a and 5b. You.

The stirring roller 6 rotates in the direction of the arrow (however, may be in the opposite direction) to supply the developer from the stirring chamber 4a to the developing sleeve 3. The developing sleeve 3 is close to the photosensitive drum and rotates in the direction of the arrow to fly and attach the developer to the photosensitive drum. The developer regulating plate 7 is for scraping excess developer adhered to the developing sleeve 3, and the scraped developer is returned to the stirring chamber 4 a by the stirring roller 6.

The toner concentration detecting device 11 is attached to the stirring chamber 4a or 4b, and replenishes toner based on the detection result by the detecting device 11. FIG. 1 shows an example of the resonance circuit 12 of the toner concentration detecting device 11. In this figure, a resonance circuit 12 includes an inductance coil L, a capacitor C ₁ ,
This is a typical Colpitts oscillation circuit composed of C ₂ , transistor Q, and resistor R.
Are composed of planar coils. When a voltage V is applied to this circuit, an oscillation signal is output from the source of the transistor Q. Note that the resonance circuit 12 is not limited to the Colpitts oscillation circuit.

FIGS. 2A and 2B are a plan view and a side view, respectively, of the toner concentration detecting device 11 of the present embodiment. As shown in FIG. 2A, the planar coil 21 is spirally disposed from a coil electrode 23 to a coil electrode 24 on a surface of a printed board 22 made of, for example, glass epoxy.
The planar coil 21 is made of copper foil or aluminum, and has a width of about 50 to 100 μm and a pitch of about 100 to 200 μm. This surface becomes the detection coil surface 28. Note that a copper wire may be spirally wound and arranged on the printed circuit board 22, and this may be used as the planar coil 21.

As shown in FIG. 2B, the upper portion of the planar coil 21 is covered with a resist 25 such as an insulating resin. On the back surface of the printed circuit board 22, components 26 such as capacitors C ₁ and C ₂ of an oscillation circuit, a transistor Q, and a resistor R are provided.
a, 26b and 26c are mounted. The coil electrode 23,
24 are both through-holes and have a flat coil 21
Are connected to the component parts 26a to 26c via the coil electrodes 23 and 24. In addition, the voltage V
And electrodes 27a and 27b for outputting a detection signal to the outside.

Here, the oscillation frequency of the resonance circuit 12 is basically calculated based on the equation (1), but considering the temperature characteristics, the oscillation frequency is calculated by the following equation (2). . f = 1 / (2π ((L (1 + αt)) (C (1 + βt))) ^1/2 (1 + γt)) (2) where L: Inductance of the planar coil 21 C: Capacities of the capacitors C ₁ and C ₂ α: Temperature coefficient of the planar coil 21 β: Temperature coefficient of the capacitors C ₁ and C ₂ γ: Total value of temperature coefficients of components constituting the resonance circuit 12 t The temperature coefficient γ is, for example, an integrated circuit (I
C) is the total value of the temperature coefficient due to the delay time, bypass capacitor, feedback resistance and the like.

The ideal relationship among the temperature coefficients α, β, and γ is expressed by the following equation (3). (Α + αβ + β) ^1/2 + γ = 0 (3) However, in an ideal LC oscillation circuit, γ = 0. It is considered that only the area of the planar coil 21 changes due to the temperature change. The change in the area is basically determined only by the thermal expansion coefficient of the printed circuit board 22.

Since the temperature coefficient of a standard substrate is about 50 ppm / ° C., the rate at which the area of the planar coil 21 changes is
It is about 100 ppm / ° C. Further, since the inductance L is substantially proportional to the area of the planar coil 21, the temperature coefficient of the inductance L is about 100 ppm / ° C. That is, when a standard board is used as the printed board 22, α = about 100 ppm / ° C.

Therefore, capacitors for temperature compensation having a temperature characteristic opposite to that of the inductance L are used as the capacitors C ₁ and C ₂ . Here, the opposite temperature characteristics mean that the absolute values of the temperature coefficients are equal to each other and that the positive and negative are opposite. In this embodiment, the temperature characteristic of the inductance L is α = 100 ppm / ° C., and two capacitors are used, so that the capacitors C ₁ and C ₂ have α = −150, respectively.
Use a capacitor for temperature compensation of about 220 ppm / ° C.
As such a capacitor, for example, a multilayer ceramic capacitor or the like can be considered.

[0033] Incidentally, instead of the temperature compensating capacitor C _1, C _2, may be provided separately from the temperature compensating capacitor is a capacitor C _1, C _2. According to such a configuration, even when the temperature changes and the inductance L changes,
With the capacitors C ₁ and C ₂ , the temperature dependency of the toner concentration detecting device 11 can be compensated in the device.

The first embodiment can be applied to the second to sixth embodiments to be described later.
Next, a second embodiment will be described. This one is
Circuit components other than the plane coil for detection of the resonance circuit are provided on the surface (rear surface) opposite to the surface of the coil for detection, and the resonance circuit is abutted so that the plane coil is in close contact with the developing container. .

6 and 7 are a sectional view showing the second embodiment and a sectional view taken along line XX 'of FIG. 6, respectively. As shown in these drawings, since the detection coil surface is substantially flat, the detection coil surface can be abutted against the developing container 2.
As a result, the detection coil surface comes into close contact with the developing container 2. The toner concentration of the developer in the stirring chamber 4 a is detected from outside the developing container 2.

In this case, if the thickness of the developing container 2 is large, the sensitivity is reduced. Therefore, it is desirable to set the thickness to about 0.3 to 1.0 mm. Further, since the outer wall of the developing container has a substantially cylindrical shape parallel to the axis of the transport screw 5a which is the developer transport direction, the detection coil surface is transported by abutting the detection coil surface against the developing container 2. It is parallel to the screw 5a.

According to this configuration, the detection coil surface can be easily attached in parallel to the transport screw 5a. In addition, since no modification is made to the developing container, it is not necessary to take measures against leakage of the developing container, and the configuration is simplified. Next, a third embodiment will be described. In this device, a protective layer is provided on the surface of a planar coil so that the surface of a detecting coil faces a developer to further improve the sensitivity.

Conventionally, as shown in FIG. 2B, the surface of the planar coil 21 is merely coated with a resist 25 for pattern insulation, and the protection is insufficient. In other words, the sliding contact of the developer may cause the resist 25 to peel off and damage the planar coil 21. For this reason,
In the present embodiment, as shown in FIG. 8, a protective layer 31 is formed on the surface of the sensing coil surface.

The protective layer 31 is made of a resist 25 as shown in FIG.
By applying a coating material on top, or
As shown in FIG. 10, the protective sheet 32 is resisted with an adhesive 33.
It is formed by bonding on 25. In the former case, a polymer material such as Kevlar, which is a kind of aramid, is used for the coating material, for example, polyimide, polycarbonate (PC), aramid and the like.

In the latter case, Kapton, Myra, polyethylene terephthalate (PE)
T) etc. are used. If the thickness of the protective layer 31 is small, the detection level is high, but the planar coil 21 may be damaged. Conversely, if the thickness is large, the detection level is low. The level will be good.
More preferably, the thickness is about 0.05 to 0.1 mm. Further, when a void is generated between the planar coil 21 and the protective layer 31, the magnetic permeability is reduced, the output is increased, and the sensitivity is reduced, so that the occurrence of the void is prevented as much as possible.

FIG. 11 shows the printed circuit board 22 in the stirring chamber 4a.
FIG. 12 is a cross-sectional view when attached to FIG.
It is Y 'sectional drawing. As shown in these figures, in order to attach the printed circuit board 22, an opening is provided in the stirring chamber 4a,
The printed circuit board 22 is arranged in this opening so that the detection coil surface 28 faces the developer, and is covered with a lid and screwed.
When the distance S between the transport screw 5a and the protective layer 31 is about 0.1 to 1.4 mm, the detection level becomes good, and more preferably, about 0.1 to 0.7 mm for stabilizing the control of the toner concentration. .

On the other hand, since the distance S is small, the detection coil surface 28 and the conveying screw 5a are close to each other, and the detection coil surface 28
And the transport screw 5a may come into contact with each other, and the sliding screw also contacts the developer. Even in such a case, the detection coil surface 28 is protected by the protective layer 31. in this way,
Since the detection coil surface 28 is protected by the protective layer 31, the planar coil 21 can be brought close to the developer, and the detection level of the toner density is improved.

In the present embodiment, the protective layer 31 is formed on the resist 25. However, when forming the protective layer 31, FIG.
The resist 25 can be omitted as shown in FIG. Then the fourth
An embodiment will be described. In this device, a detection circuit including a planar coil and a capacitor is separated from other circuits.

A planar coil 21 having an inductance L and capacitors C ₁ and C ₂ shown in FIG.
13, and separates the planar coil 21 and the capacitors C ₁ and C ₂ from each other as shown in FIG.
The detection circuit 41 is arranged on the developing device 1 side. Also,
Other components are provided in the control circuit 42, and the detection circuit 41 and the control circuit 42 are wired and electrically connected.

According to this configuration, the size of the detection circuit 41 is reduced, which is advantageous in terms of space and the like for a small-sized image forming apparatus or a color image forming apparatus having a large number of components. Also, if the detection circuit 41 is configured integrally with other circuits,
The output is a rectangular wave, and the radiation noise may be increased due to the routing of the wiring. However, by separating the detection circuit 41 and the control circuit 42 in this manner, the output of the resonance circuit and the output of the wiring section become triangular waves, and radiation noise is reduced.

Next, a fifth embodiment will be described.
In this device, an earth plate is provided on the back surface of the surface on which the planar coil is arranged. FIG. 14 (A), (B)
As shown in the figure, the detection coil surface on which the planar coil 21 is disposed
An earth plate 51 is formed on the back surface of the wiring board 28 in a region corresponding to the plane coil 21 except for a wiring pattern to the component part 26.

As the ground plate 51, a conductive member such as a metal is used. The ground plate 51 is grounded via a ground pattern 52. Thus, the earth plate 51
Is formed on the back surface of the detection coil surface 28, the planar coil 21 is magnetically shielded by the ground plate 51, and even if the planar coil 21 generates radio noise, the ground plate 51
As a result, the radio noise is absorbed. Further, even when the magnetic body is brought close to the back surface of the detection coil surface 28, the influence of the magnetic material on the detection coil surface 28 is extremely small.

For example, FIG. 15 shows the frequency change rate of the resonance circuit with and without the ground plate 51 when a metal plate as a magnetic material is approached from the back surface of the detection coil surface 28.
The horizontal axis indicates the distance from the planar coil 21 to the back surface of the detection coil surface 28, and the vertical axis indicates the frequency change rate at that distance, regardless of the distance between the metal plate and the planar coil 21. The frequency change rate of the resonance circuit at a certain time is suppressed to 1/100 or less as compared with the case where there is no resonance circuit.

Next, a sixth embodiment will be described.
As a conventional technique, as shown in FIG. 16, the planar coil 21 is composed of, for example, two planar coils 21a and 21b so as to sufficiently exhibit high-accuracy detection capability.
21b are electrically connected in series, and adjacent planar coils 21a and 21b are wound so that the directions of magnetic fluxes formed are opposite to each other (Japanese Patent Laid-Open No. 7-248676). Reference).

In the present embodiment, as shown in FIG. 17, for example, three planar coils 61a to 61c are connected side by side along the axial direction of the conveying screw 5a as a stirring member.
Note that, as in the related art, the adjacent planar coils 61a to 61c are
It may be wound so that the directions of the formed magnetic fluxes are opposite to each other. As shown in FIG. 19, three planar coils 61
Assuming that the direction of arrangement of a to 61c is perpendicular to the rotation axis of the transport screw 5a, even if the total inductance of the planar coils 61a to 61c increases,
a, 61c and the distance between the conveying screw 5a are increased,
Since the detection level of each of the planar coils 61a to 61c is different, the detection level cannot be sufficiently increased as a result.

On the other hand, as shown in FIG. 18, which is a cross-sectional view taken along the line ZZ ′ in FIG. 17, when the planar coils 61a to 61c are arranged along the conveying screw 5a, the planar coils 61a to 61c
Is smaller, and the toner density can be detected while keeping the magnetic permeability of the developer facing each of the planar coils 61a to 61c constant. Further, it is possible to secure the inductance L of the planar coil 21 and minimize the influence of other magnetic materials and the like.

[0052]

As described above, according to the device of the first aspect of the present invention, the temperature dependency of the device can be compensated in the device. According to the apparatus according to the second aspect of the present invention, the flat coil surface can be easily attached in parallel with the developing container.

According to the apparatus according to the third to eleventh aspects of the present invention, the planar coil comes into contact with the developer via the protective layer, so that the toner density can be detected with high sensitivity, and Can protect the planar coil from sliding contact. According to the device of the twelfth aspect, the size of the resonance circuit can be reduced. According to the device of the thirteenth aspect, it is possible to prevent the occurrence of radio noise and the influence on the back surface of the detecting flat coil.

According to the apparatus according to the fourteenth and fifteenth aspects, the detection level of the planar coil can be increased.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a shape of the toner concentration detecting device of FIG. 1;

FIG. 3 is a perspective view of a developing device.

FIG. 4 is a sectional view of a developing device.

FIG. 5 is a plan view showing the inside of the developing device.

FIG. 6 is a structural diagram showing a second embodiment of the present invention.

FIG. 7 is a sectional view taken along line X-X ′ of FIG. 6;

FIG. 8 is a structural diagram showing a third embodiment of the present invention.

9 is a cross-sectional view when a coating material is used for the protective layer in FIG.

10 is a cross-sectional view when a protective sheet is used for the protective layer in FIG.

11 is a cross-sectional view when the structure shown in FIG. 8 is attached to a stirring chamber.

FIG. 12 is a sectional view taken along line Y-Y ′ of FIG. 11;

FIG. 13 is a block diagram showing a fourth embodiment of the present invention.

FIG. 14 is a plan view showing a fifth embodiment of the present invention.

FIG. 15 is a characteristic explanatory diagram of FIG. 14;

FIG. 16 is a plan view for explaining a sixth embodiment of the present invention.

FIG. 17 is a cross-sectional view of the sixth embodiment.

18 is a sectional view taken along the line Z-Z 'of FIG.

FIG. 19 is an explanatory diagram of FIG. 17;

[Explanation of symbols]

1 developing devices 4a, 4b stirring chamber 5a, 5b transport screw 21 planar coil 31 protective layer 41 detection circuit 51 ground plate C _1, C ₂ capacitors

Claims (15)

[Claims] A resonance circuit based on an inductance of a planar coil and a capacitance of a capacitor which are spirally arranged on a plane substrate, wherein a toner concentration in a developer contained in a developing container is changed by a resonance frequency of the resonance circuit. A toner density detecting device for an image forming apparatus, wherein the planar coil and the capacitor have opposite temperature characteristics. 2. A resonance circuit comprising an inductance of a planar coil spirally arranged on a flat substrate and a capacitance of a capacitor, wherein a resonance frequency of the resonance circuit is determined by changing a toner concentration in a developer contained in a developing container. In the toner concentration detecting device of the image forming apparatus, the developing container has a substantially cylindrical shape parallel to the direction of transport of the developer, and the surface of the planar coil is in close contact with the outer wall of the developing container.
A toner concentration detecting device for an image forming apparatus, wherein a substrate of a resonance circuit is abutted against an outer wall of a developing container. 3. A resonance circuit comprising an inductance of a planar coil spirally arranged on a flat substrate and a capacitance of a capacitor, wherein a toner concentration in a developer contained in a developing container is changed by a resonance frequency of the resonance circuit. In the toner concentration detecting device of the image forming apparatus, an opening is provided in the developing container, a protective layer is formed on the surface of the planar coil, and the plane of the planar coil faces the inside of the developing container. A toner concentration detecting device for an image forming apparatus, wherein a substrate is attached to an opening of a developing container. 4. The toner density detecting device according to claim 3, wherein the protective layer is formed by applying a coating material. 5. The apparatus according to claim 4, wherein said coating material is polyimide. 6. The apparatus according to claim 4, wherein said coating material is polycarbonate. 7. The toner density detecting device according to claim 4, wherein the coating material is aramid. 8. The apparatus according to claim 3, wherein said protective layer is formed by attaching a protective sheet with an adhesive. 9. The apparatus according to claim 8, wherein said protective sheet is Kapton. 10. The toner density detecting device according to claim 8, wherein the protective sheet is a mylar. 11. The toner density detecting device according to claim 8, wherein the protective sheet is made of polyethylene terephthalate. 12. A resonance circuit based on an inductance of a planar coil and a capacitance of a capacitor which are spirally arranged on a flat substrate, wherein a toner concentration in a developer contained in a developing container is changed by a resonance frequency of the resonance circuit. Wherein the resonance circuit is configured such that a planar coil and a capacitor are disposed in proximity to each other and separated from other circuits. apparatus. 13. A resonance circuit based on an inductance of a planar coil and a capacitance of a capacitor spirally arranged on a flat substrate, wherein a toner concentration in a developer contained in a developing container is changed by a resonance frequency of the resonance circuit. Wherein the resonance circuit is formed by forming an earth plate in a region substantially corresponding to the plane coil on the back surface of the plane coil arrangement surface of the plane substrate. And a toner concentration detecting device of the image forming apparatus. 14. A resonance circuit comprising an inductance of a planar coil spirally arranged on a flat substrate and a capacitance of a capacitor, wherein a toner concentration in a developer contained in a developing container is changed by a resonance frequency of the resonance circuit. In the toner concentration detecting device of the image forming apparatus, the developing container includes a longitudinal stirring chamber containing the developer, and a longitudinal stirring member that is disposed in the stirring chamber and transports the developer while stirring the developer. Wherein the planar coil has a plurality of planar coils arranged and connected along the longitudinal stirring member. 15. The apparatus according to claim 14, wherein the stirring member is a conveying screw.