JP2000056639A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently and correctly initialize sensitivity irrespectively of a change in the bulk density of developer, in a toner concentration detection sensor of a system for detecting a T/C ratio by detecting the change of magnetic permeability. SOLUTION: At the time of initializing the sensor sensitivity, a control voltage for adjusting the output operation point of each of the sensors, X1-X2, is set to such a value that the output value of the sensor is set to the middle of the range of the change in the output of the sensor, and a control voltage, Vcont, is further changed from the value by a specific amount, and the amount of the change in the output of the sensor which corresponds to the specific amount is detected. By the amount of the change in the output of the sensor, the sensor sensitivity, that is, the amount of the change in the output of the toner concentration detection sensor which corresponds to the amount of change in toner concentration is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copier, a printer and a recorded image display for developing a visible image by developing an electrostatic latent image formed on an image carrier by an electrophotographic method or an electrostatic recording method. And an image forming apparatus such as a facsimile.

[0002]

2. Description of the Related Art Conventionally, a dry developer as a developer is carried on the surface of a developer carrier, and the developer is conveyed and supplied to the vicinity of the surface of an image carrier carrying an electrostatic latent image. A developing device that develops an electrostatic latent image to make it visible while applying an alternating (alternating) electric field between a carrier and a developer carrier is well known.

Since the developer carrier generally uses a developing sleeve in many cases, it is referred to as a "developing sleeve" in the following description, and the image carrier generally uses a photosensitive drum. Therefore, in the following description, it is referred to as “photosensitive drum”.

As the above-mentioned developing method, conventionally, for example, 2
A magnetic brush was formed on the surface of a developing sleeve in which a magnet was disposed by using a developer (two-component developer) composed of a component-based composition (carrier particles and toner particles), and opposed to each other while maintaining a minute developing gap. The magnetic brush is rubbed or brought close to the photoconductor drum, and an alternating electric field is continuously applied between the development sleeve and the photoconductor drum (between SD), so that toner particles move from the development sleeve side to the photoconductor drum side. A so-called magnetic brush developing method in which development is performed by repeatedly performing transfer to and reverse transfer to a toner, is known. (For example, JP-A-55-32060 and JP-A-59-16)
No. 5082).

[0006] As shown in FIG. 6, the developing device 18 for developing the two-component magnetic brush has a developing chamber R 1 by a partition wall 19.
And a developing container 18a partitioned into a stirring chamber R2. The developing chamber R1 and the stirring chamber R2 rotatably house stirring and conveying screws 23 and 24, respectively, which are stirring members. At the opening of the developing chamber R1, a developing sleeve 25 that rotates in a predetermined direction is disposed opposite to the photosensitive drum 3 that rotates in a predetermined direction at a small interval, and a magnet roller 29 is fixed inside the developing sleeve 25. Have been. A regulating blade 28 for forming a thin layer of the developer on the surface of the developing sleeve 25 is disposed to face the developing sleeve 25.

The developing container 18a contains a developer 22 in which toner particles and a magnetic carrier are mixed, and the mixing ratio of the toner particles to the magnetic carrier (hereinafter referred to as "T / C ratio") is consumed by development. An amount of toner corresponding to the supplied toner is dropped and replenished from the toner storage chamber 21 in which the replenishing toner 20 is stored, thereby keeping the toner constant.

Various methods have heretofore been proposed for detecting and maintaining the mixing ratio between the toner particles in the developing container 18a and the magnetic carrier. For example, a detection unit is provided around the photosensitive drum 3 to irradiate the toner transferred from the developing sleeve 25 to the photosensitive drum 3 with light, and the amount of toner supply is adjusted based on the transmitted light and reflected light at this time. A method of maintaining a T / C ratio, a method of providing a detection means near the surface layer of the developing sleeve 25, and a method of detecting the T / C ratio from reflected light when light is applied to the developer applied on the developing sleeve 25; A method in which a sensor is provided in the container 18 and a change in the apparent magnetic permeability of the developer in a certain volume near the sensor is detected by utilizing the inductance of the coil and a T / C ratio is detected has been proposed and put to practical use. ing.

However, the method of maintaining the T / C ratio from the amount of the developing toner on the photosensitive drum 3 is based on the variation of the T / C ratio such as the variation of the interval between the photosensitive member and the developing sleeve and the variation of the latent image potential. The amount of toner on the drum may change due to factors other than the above, and as a result, the toner replenishment may malfunction. The method of detecting the / C ratio is
If the detection means is soiled due to toner scattering or the like, there is a problem that the T / C ratio cannot be accurately detected.

On the other hand, a detection means (hereinafter referred to as "toner density detection") for detecting the T / C ratio by detecting the change in the apparent magnetic permeability of the developer 22 in a certain volume near the sensor using the inductance of the coil. Sensor)), the cost of the sensor alone is inexpensive, and the sensor is not affected by the problem of contamination due to toner scattering. Therefore, the optimal T / C is suitable for a low-cost, small-space copier or image forming apparatus.
It can be said to be a C ratio detecting means.

[0010] The toner density detection sensor using the change in the magnetic permeability of the developer means that, for example, when the magnetic permeability increases, the T / C ratio in the developer decreases within a certain volume. Means that the amount of toner in the developer has decreased, so toner replenishment is started. Conversely, if the magnetic permeability decreases, it means that the T / C ratio in the developer has increased within a certain volume. Therefore, the T / C ratio is controlled based on a sequence in which toner supply is stopped or reduced in order to indicate that the amount of toner in the developer has increased.

This will be described in more detail with reference to FIGS.
FIG. 7 shows a DC voltage (hereinafter referred to as a “control voltage”) for adjusting the operating point of the output of the toner density sensor.
t ") and the output of the toner concentration detection sensor (Vo
6 is a graph showing a relationship with the ut).

As described above, the toner density detection sensor utilizing the change in the magnetic permeability of the developer can change the sensor output value within the output fluctuation range of the toner density detection sensor by the control voltage.

FIG. 8 is a graph showing the relationship between the T / C ratio and the output (Vout) of the toner density sensor. As shown in the graph, the output of the toner density sensor changes in response to the change in the T / C ratio, and the amount of the change is most sensitive near the center of the output fluctuation range of the toner density sensor (the sensitivity of the toner density sensor with respect to the change in the T / C ratio). Output change) and is stable.

Accordingly, the output value of the toner density detection sensor is determined by the developer having the central T / C ratio in the T / C ratio range of the developer from which the image forming apparatus can obtain a good image.
By changing the control voltage, the control voltage may be set at the center of the output fluctuation range of the toner density detection sensor.

For example, as shown in FIG. 8, the T / C ratio is 6 to 8%.
The combination of an image forming apparatus and a developer capable of obtaining a good image with the above-described configuration has an output fluctuation range of 0 to 5 V, and a sensor output with respect to the fluctuation of the T / C ratio near the center value (2.5 V) of the output fluctuation range. Fluctuation (sensor sensitivity) is 1% T / C ratio
When a toner density detection sensor that fluctuates by 0.5 V with respect to the change of the T / C ratio is used, the output value of the toner density sensor at a T / C ratio of 7% is set to 2.5 V, and the output value of the sensor is set around 2.5 V. The T / C ratio may be controlled by increasing the output by 0.5 V, replenishing the toner when the output becomes 3 V, and stopping the toner replenishment when the output decreases by 0.5 V to 2 V. As described above, when the T / C ratio is controlled so that the image forming apparatus can obtain a good image, the sensor sensitivity plays an important role.

[0016]

However, such a toner density detecting sensor that detects a change in apparent magnetic permeability of a developer in a certain volume includes sensors such as a transformer variation, a case bobbin variation, and an assembly variation. There is a problem that the variation in the output of the toner density sensor (sensor sensitivity) with respect to the variation in the T / C ratio changes due to variations in the manufacturing method of a single unit, and the T / C ratio cannot be detected accurately.

For example, as shown in FIG. 9, when a sensor having a sensor sensitivity such as A is mounted on an image forming apparatus and mass production is performed, an image forming apparatus equipped with a sensor having a sensor sensitivity characteristic such as B, There is an image forming apparatus equipped with a sensor having a sensor sensitivity characteristic like C.

In this case, when the table of the sensor output with respect to the T / C ratio of all the image forming apparatuses is determined based on the sensor of sensitivity A, when the T / C ratio becomes 6%, the sensor of A accurately detects the T / C ratio. The output a with respect to the T / C ratio can be detected. However, a sensor such as the sensitivity B detects a sensor output such as b, and a sensor such as the sensitivity C detects a sensor output such as c. Will detect the sensor output of 6.5%, C will detect the sensor output of 5.5%,
The sensor output for the T / C ratio cannot be accurately detected.

As a result, a sensor output indicating that the amount of toner has decreased is output without consuming toner, and toner is replenished, or the amount of toner decreases despite the amount of toner decreasing. There is a problem that the sensor output is not generated and toner is not supplied.

In the former case, there is a problem that the image density is increased due to excessive toner replenishment, a problem that the amount of the developer increases with an increase in the amount of toner and the developer overflows from the developing container, or an increase in the toner ratio in the developer. This causes a problem such as toner scattering due to a decrease in toner charge amount.

On the other hand, in the latter case, problems such as image deterioration due to a decrease in the amount of toner in the developer, a low image density, and a low image density due to an increase in toner charge amount are caused.

In order to deal with such a problem, at a production stage in a factory, before installing a toner density detection sensor in a developing device,
In order to examine individual differences in sensor sensitivity, for example, as shown in FIG. 10, a cylindrical jig 100 is prepared, and a sensor surface 1 of the toner density sensor 110 is provided on a bottom surface 101 of the jig 100.
A hole 102 having the same diameter as the hole 11 is opened, and a plurality of developers having different T / C ratios are filled so that the developer surface is the same (the thickness of the developer is constant with respect to the toner density sensor). The sensor 110 is attached to the tool 100 one by one, and the sensor sensitivity is checked from the output value for the T / C ratio of each sensor.
From the table as shown in FIG.
The reference value of the change amount of the sensor output with respect to the change amount of the / C ratio is set.

However, the above-mentioned operation has to be performed a plurality of times for each sensor by using a developer having a different T / C ratio, which leads to a delay in the operation process.
Furthermore, since these developers are different from the developer that is filled (actually used) in the developing device at the time of shipment from the factory,
It also picks up variations due to differences in developer.

Further, a toner density detection sensor of a type that detects a change in the apparent magnetic permeability of a developer in a certain volume, when the bulk density of the developer itself changes, the apparent permeability of the developer changes in accordance with the change in the bulk density. Since the magnetic susceptibility also changes,
There is a problem that the sensor output also changes with the change in magnetic permeability. The change in the bulk density is due to the change in repulsion between the developers due to the change in the toner charge amount of the developer, the developer fluidity due to the change in temperature and humidity, and the change in the toner charge amount. There is no problem if the bulk density of the developer having the C ratio is constant, but there is a difference in the bulk density between the developers having different T / C ratios due to a difference in toner charge amount depending on the storage state. In such a case, there is a problem that the initial setting of the table of the sensor output with respect to the T / C ratio cannot be accurately performed.

Accordingly, an object of the present invention is to efficiently set the initial setting of the sensitivity in the toner density control means for detecting the T / C ratio by detecting the change in the magnetic permeability and to change the bulk density of the developer. It is an object of the present invention to provide an image forming apparatus that can perform an image forming operation accurately and accurately.

[0026]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
In an image forming apparatus including a developing device having a toner density control means for controlling a toner density by detecting a change in magnetic permeability of a developer, when the sensitivity is initially set by the toner density control means, the toner is charged into the developing device. For a stationary or flowing two-component developer, the DC voltage value for adjusting the output operating point of the toner density control means is determined by the output value of the toner density control means. After setting the DC voltage value to a value set at the center of the fluctuation range, the DC voltage value is further changed by a predetermined amount from the above value, and the output value of the toner density control means at that time is detected, whereby the toner density detection is performed. The image forming apparatus is characterized in that an output change amount with respect to a toner density change amount of the means is determined.

It is preferable that the toner density control means has a table for deriving an output voltage with respect to the control voltage change amount, and a table for deriving an output change amount with respect to the toner density change amount from the output voltage change amount. The installation location of the toner concentration control means is preferably a location where the flow rate of the developer flowing on the detection surface of the toner concentration control means during stirring of the developer is constant and the flow is regular.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an image forming apparatus according to the present invention will be described in more detail with reference to the drawings. In the following embodiments, the present invention will be described as being embodied in an electrophotographic image forming apparatus as shown in FIG. 2, for example, but the present invention is not limited to this.

Referring to FIG. 2, in the electrophotographic image forming apparatus, a photosensitive drum 3 serving as an image carrier is rotatably provided, and the photosensitive drum 3 is uniformly charged by a primary charger 1. For example, an information signal is exposed by the light emitting element 2 such as a laser to form an electrostatic latent image, and is visualized by the developing device 31. Next, the visible image is transferred to the transfer paper 5 by the transfer charger 4 and further fixed by the fixing device 6 to obtain a permanent image. The transfer residual toner on the photosensitive drum 3 is removed by the cleaning device 7.

An embodiment of the developing device according to the present invention will be described with reference to FIG. 1 and FIGS.

In FIG. 1, the developing device 31 includes a developing container 32, and the inside thereof is partitioned by a partition 44 into a developing chamber R1 and a stirring chamber R.
2 and a toner storage chamber R3 above the stirring chamber R2.
Is provided, and a supply toner 33 is accommodated therein. From the replenishing port 34 at the lower part of the toner storage chamber R3, an amount of toner corresponding to the toner consumed in the development is supplied to the stirring chamber R3.
It is dropped and replenished into 2. On the other hand, the developer 35 in which the toner particles and the magnetic carrier are mixed is accommodated in the developing chamber R1 and the stirring chamber R2.

A developer conveying screw (stirring member) 36 having a spiral shape and having excellent functions of stirring and conveying the developer is accommodated in the developing chamber R1. Is transported along the longitudinal direction of the developing sleeve 38 as a developer carrier.

Similarly, a spiral developer conveying screw (stirring member) 37 is rotatably accommodated in the stirring chamber R2 in the direction of the arrow.
7 is opposite to that by the developer transport screw 36 in the developing chamber R1. The partition 44 has openings (not shown) on the near side and the far side,
The developer conveyed by the screw 36 is transferred to the screw 37 from one of the openings, and the developer conveyed by the screw 37 is transferred to the screw 3 from the other one of the openings.
Handed over to 6.

Further, an opening is provided in a portion of the developing container 32 close to the photosensitive drum 3, and the opening is made of a material such as aluminum or non-magnetic stainless steel, and has a surface having moderate irregularities. 38 are provided.

In this embodiment, the developing sleeve 38 has a peripheral velocity V in the direction of arrow b (the same direction as the rotation direction of the photosensitive member).
After rotating at b, the layer thickness is regulated to an appropriate layer thickness by a layer thickness regulating blade 41 provided at the upper end of the opening of the developing container 32, and then the developer is carried and transported to the developing area 39. The magnetic brush of the developer carried on the developing sleeve 38 is applied to the developing area 3
9, the photosensitive drum 3 rotating at the peripheral speed Va in the direction of arrow a
, And the electrostatic latent image is developed in the developing area 39.
The peripheral speed Vb of the developing sleeve 38 is the photosensitive drum peripheral speed ratio 1
30-200% is desirable, and 150-180% is even better. Below the above range, a sufficient image density cannot be obtained, and above this range, the developer is scattered.

In the developing sleeve 38, a magnet 42 as a roller-shaped magnetic field generating means is fixedly arranged. The magnet 42 has a development magnetic pole N1 facing the development area 39. The developing magnetic pole N1 forms a magnetic brush of a developer by a developing magnetic field formed in the developing region 39, and the magnetic brush contacts the photosensitive drum 3 to develop an electrostatic latent image. At this time, the toner adhering to the magnetic brush and the toner adhering to the surface of the sleeve are transferred to the image area of the electrostatic latent image and developed. In this embodiment, the magnet is the developing magnetic pole N.
1 and N2, S1, S2, and S3.

With this configuration, the developer applied on the S2 pole by the rotation of the developing sleeve 38 passes through the layer thickness regulating blade 41 to reach the developing magnetic pole N1, as in the related art.
The developer raised in the magnetic field develops the electrostatic latent image on the photosensitive drum 3. Thereafter, the developer on the developing sleeve 38 falls into the stirring chamber R1 due to the repulsive magnetic field between the S2 pole and the S3 pole. The developer dropped into the stirring chamber R1 is the first and second developers.
It is stirred and transported by the screws 36 and 37.

The inductance detecting sensor 43, which is a toner concentration detecting means in this embodiment, is disposed at a position close to the screw 37 on the side surface of the stirring chamber R2 as shown in FIG. The side surface of the screw 37 has a constant flow rate of the developer and is unlikely to stagnate. Therefore, when the inductance detection sensor 43 is disposed in this portion, the detection accuracy is considerably higher than in the other portion in the developing container 32. Go up.

As described above, the toner density detecting inductance detecting sensor 43 detects the change in the magnetic permeability of the developer by using the inductance of the coil. In addition, this sensor 4
3 is located in the vicinity of the screw, where the sensor surface has a thickness of the developer 35 such that the toner concentration can be detected, and the developer 35 shows a constant flow when the screw rotates. If it is, another place is fine.

Next, the features of the present invention will be described with reference to FIGS.
This will be described with reference to FIG.

The control for determining the individual difference in the sensor sensitivity of the toner density detection sensor used in the present embodiment is performed by changing the ΔVcont of the DC voltage (hereinafter referred to as “control voltage”) for adjusting the operating point of the toner density detection sensor. A table for calculating a change amount ΔVout of the sensor output voltage with respect to
A table of the sensor output voltage with respect to the T / C ratio from the output voltage change amount ΔVout is provided in the image forming apparatus, and the sensor output voltage with respect to the T / C ratio change amount of the developer 35 contained in the developing container 32 is provided. The change amount (sensor sensitivity) is obtained.

FIG. 3 is a graph showing the relationship between the control voltage (Vcont) and the output voltage (Vout), and FIG.
4 is a graph showing a relationship between a / C ratio and an output voltage (Vout).

According to the experiment conducted by the present inventors, a sensor having a good sensor sensitivity of the toner concentration sensor (curve X1 in the figure)
In FIG. 3, within the output range of the sensor (0 to 5 V in the figure), the amount of change of the output voltage with respect to the change of the control voltage is large, and from FIG. Is large, that is, the response of the sensor output fluctuation to the fluctuation of the sensor output fluctuation factor is large.

Conversely, a sensor having a poor sensor sensitivity (curve X2 in the figure) has a small change in the output voltage with respect to a change in the control voltage within the output range of the sensor (0 to 5V in the figure). 4, the amount of change in the output voltage with respect to the change in the T / C ratio of the developer is small, that is, the response of the sensor output fluctuation to the fluctuation of the sensor output fluctuation factor is small.

As described above, the change in the sensor output voltage when the control voltage is changed is such that the change in the sensor output voltage when the T / C ratio of the developer is changed can be reflected 1: 1. I understand.

The present embodiment will be further described below with reference to the graphs of FIGS.

(1) Based on the graph of FIG. 3, an initial developer having a predetermined T / C ratio is put in a developing container, and the median value of the output of the toner density sensor (for example, in the present embodiment) When the output range is 0 to 5 V as shown in FIG.
V), the control voltage value (Vcont) is set. At this time, the developing device may be in a stationary state or a driving state.

(2) Using the control voltage value set in (1) as the median value, the control voltage ΔVcon set as shown in FIG.
t is first changed in the decreasing direction from the median value, and the output change value ΔVo of the sensor of the sensor sensitivity X1 at that time is changed.
ut-X11, and then similarly changed in the increasing direction,
By obtaining the output change value ΔVout-X12, (Δ
From Vout-X12)-(ΔVout-X11), the control voltage change amount Δ in the sensor having the sensor sensitivity X1 is obtained.
An output change amount ΔVout1 with respect to Vcont can be obtained.

Similarly, the output change amount of the sensor having the sensor sensitivity X2 is obtained from the output change values ΔVout−X21 and ΔVout−X22 when the control voltage ΔVcont is changed from the median value in the decreasing direction and the increasing direction, respectively. ΔVout2 can be obtained.

In this way, the sensor sensitivities X1 to Xn
Output change amounts ΔVout1 to ΔVout in the sensors of FIG.
tn can be obtained.

(3) FIG. 5 shows ΔVout obtained from (2).
FIG. 7 is a diagram illustrating an output change amount (sensor detection sensitivity) of the toner density detection sensor with respect to a change in the T / C ratio and the T / C ratio of 1%.

ΔVout1, ΔVo obtained from (2)
The detection sensitivity of the toner density detection sensor is obtained from ut2 using FIG. It is Va at ΔVout1 and Vb at ΔVout2.

(4) The initial setting of the sensitivity of the initial toner density sensor is completed by incorporating the Va and Vb into the sequence of the T / C ratio control by the toner density sensor in the image forming apparatus.

As described above, in this embodiment, the sensor sensitivity variation due to the transformer variation, case / bobbin variation, assembly variation, etc. of the toner density detection sensor is set in a state where the toner density sensor is installed in the developing container. Therefore, at the stage of production at the factory, it is possible to omit an operation process for determining a reference value of the sensor output with respect to the T / C ratio before installing the toner density detection sensor in the developing device.

Furthermore, since the control of the reference value of the sensor with respect to the change of the T / C ratio is performed by using the developer actually used, the bulk density when the developers having different T / C ratios are used is used. Variations in sensor sensitivity due to changes can be omitted.

[0056]

As is apparent from the above description, according to the present invention, at the time of initial setting of the sensitivity in the toner density control means, the two-component developing device in the stationary state or the flowing state filled in the developing device is used. DC voltage value for adjusting the output operation point of the toner density control means for the toner, a set value such that the output value of the toner density control means is set at the center of the output fluctuation range of the toner density control means. After doing
The sensitivity at the time of the initial setting is determined by further changing the DC voltage value by a predetermined amount from the set value and detecting the output value of the toner density control unit at that time. Thereby, the initial setting of the sensitivity can be performed efficiently and accurately irrespective of the change in the bulk density of the developer, the productivity can be improved, and the image formation can be performed satisfactorily.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of a developing device according to the present invention.

FIG. 2 is a schematic configuration diagram illustrating an embodiment of an electrophotographic image forming apparatus to which the present invention is applied.

FIG. 3 is a graph showing a relationship between a voltage Vcont and an output voltage Vout in sensors X1 and X2.

FIG. 4 is a graph showing a relationship between a T / C ratio and an output voltage Vout in sensors X1 and X2.

FIG. 5 is a graph showing a relationship between an output voltage change amount ΔVout and a sensor detection sensitivity V /%.

FIG. 6 is a configuration diagram illustrating an example of a conventional developing device.

FIG. 7 shows a control voltage Vcont and a sensor output voltage Vou.
6 is a graph showing the relationship of t.

FIG. 8 is a graph showing a relationship between a T / C ratio and a sensor output voltage Vout.

FIG. 9 shows T / T values of sensors A, B and C having different sensitivities.
9 is a graph showing a relationship between a C ratio and a sensor output voltage Vout.

FIG. 10 is a perspective view showing a jig for checking individual differences in the sensitivity of the toner density sensor at a factory.

[Explanation of symbols]

31 developing device 32 developing container 43 inductance detection sensor (toner density control means)

Claims (4)

[Claims] 1. An image forming apparatus comprising a developing device having a toner density control means for controlling a toner density by detecting a change in magnetic permeability of a developer, wherein an initial setting of sensitivity in the toner density control means is performed.
The DC voltage value for adjusting the output operation point of the toner density control means for the two-component developer in a stationary state or a flowing state filled in the developing device, and the output value of the toner density control means is After setting the value to be set at the center of the output fluctuation range of the toner density control means, the DC voltage value is further changed by a predetermined amount from the above value, and the output value of the toner density control means at that time is detected. By
An image forming apparatus, wherein an initial setting of the sensitivity is performed. 2. The image forming apparatus according to claim 1, wherein the toner density control unit has a table for deriving an output voltage with respect to the change amount of the control voltage, and a table for deriving an output change amount with respect to the toner density change amount from the change amount of the output voltage. The image forming apparatus according to claim 1, wherein: 3. An installation location of the toner concentration control means,
2. The image forming apparatus according to claim 1, wherein the flow rate of the developer flowing on the detection surface of the toner concentration control means during the stirring of the developer is constant and the flow is regular. 4. The image forming apparatus according to claim 1, wherein the sensitivity is an output change amount with respect to a toner density change amount of the toner density detection unit.