JPH09106142A - Device and method for electrifying image carrier for image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To electrify an image carrier so that its surface potential may be within a required range even when dispersion is found out on a contact type electrifier and the image carrier or impressed voltage is changed by the change of temperature, humidity, actuating time and the number of image forming times. SOLUTION: The temperature and the humidity at or near the image carrier 1 are measured by a temperature detection means 10 and a humidity detection means 12. In the case impressed voltage at the measured temperature and humidity has been already stored in a storage means 14, the image carrier 1 is electrified by the impressed voltage stored, and in the case it has not been stored, the image carrier 1 is electrified by optional impressed voltage. The surface potential of the image carrier 1 is measured by a surface potential detection means 11 and compared with a target range. When it is out of the target range, the impressed voltage is adjusted so as to make the surface potential the voltage within the target range, and the impressed voltage in reaching the target range is stored in a matrix state corresponding to the temperature and used next time and succeedingly.

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 using an electrostatic latent image, and more particularly to controlling the surface potential of a contact charging type image carrier.

[0002]

2. Description of the Related Art After an image bearing member such as a photosensitive drum is charged by a charging device, the image bearing member is exposed to an image to form an electrostatic latent image, and the electrostatic latent image is developed and visualized. In electrophotographic devices, in order to keep the image quality above a certain level,
It is important to keep the surface potential of the image carrier constant after charging.

Taking an analog copying machine using a general electrophotographic system as an example, if the surface potential before exposure (hereinafter referred to as "dark portion potential Vd") is high, the image becomes dark, and background fog occurs. On the contrary, if it is low, problems such as a decrease in image density and an image that is too bright occur.

Further, a reference latent image is formed on the image bearing member, the density of the reference pattern visualized by the developing device is detected by using a reflection type photosensor, and toner replenishment is controlled according to the detected value. In the image forming apparatus using the two-component development that is performed, if the surface potential of the reference latent image (hereinafter referred to as "pattern potential Vp") is high, the toner density is lowered, the image density is lowered, carrier adhesion, etc. If it is low, the toner density increases, and problems such as background stain and toner scattering occur.

In particular, when the contact charging system is used as the charging device, the charging ability is largely changed by the environment such as temperature and humidity as compared with the conventional wire type charging device. So,
Japanese Patent Laid-Open No. 4-186381 proposes to measure the temperature of a contact type charger (charging roller) and change the applied voltage to keep the surface potential constant. Further, JP-A-5-46001 proposes to measure the humidity in the vicinity of the contact type charger and change the applied voltage to keep the surface potential constant.

[0006]

However, variations in the contact type charger (for example, variations in the resistance of the charging roller) and variations in the image bearing member cannot be absorbed by the above-mentioned prior art, and the image bearing is carried out as it is. It had become a variation of the surface potential in the body.

Further, in the prior art, since a charging device different from the predetermined environmental characteristics cannot be used at all, strict inspection is required, and the yield tends to deteriorate.
The present invention has been invented in view of the above points, and requires the surface potential of the image carrier even if the contact type charger or the image carrier has variations or the applied voltage changes due to temperature changes. A charging device capable of charging within a range of
The purpose is to provide a charging method.

Another object of the present invention is to provide a charging device and a charging method capable of coping with changes in applied voltage due to humidity other than temperature, operating time, and the number of image forming sheets.

[0009]

In order to achieve the above object, a charging device of the present invention comprises a contact type charger which contacts an image bearing member to charge it, and a voltage which applies a voltage to the contact type charger. An application unit, a temperature detection unit that detects the temperature of the contact type charger or the vicinity thereof, a surface potential detection unit that detects the surface potential of the image carrier, a storage unit, and a control unit for these, The control means calculates an appropriate applied voltage based on the surface potential of the image bearing member detected by the surface potential detecting means to control the voltage applying means, and the storage means controls the temperature detected by the temperature detecting means. It is characterized in that the appropriate applied voltage is stored in a matrix.

Further, it has humidity detecting means for detecting the humidity of the contact type charger or its vicinity, and the storage means stores the appropriate applied voltage corresponding to each temperature and humidity in a matrix form. You can

Further, a timer for detecting the drive time of the image carrier is provided, and the appropriate applied voltage for each temperature or each temperature / humidity is obtained each time a predetermined drive time elapses, and the storage means is provided. May store the applied voltage for each temperature or each temperature and humidity again.

The charging method of the present invention is a method of charging an image carrier in an image forming apparatus in which an image carrier is charged by a contact type charger, by measuring the temperature of the image carrier or in the vicinity thereof and applying at that temperature. It is determined whether the voltage is already stored in the storage means, and if it is stored,
The image-carrying member is charged by the contact type charger with the stored applied voltage value, and when not stored, the image-carrying member is charged with an arbitrary applied voltage to determine the surface potential of the image-carrying member. Measure and compare with the target range.If it is outside the target range, adjust the applied voltage to bring the surface potential to a voltage within the target range, and the applied voltage when it reaches the target range is the temperature. Correspondingly, the surface potential of the image carrier is adjusted within a target range by storing it in a matrix form.

Alternatively, the humidity of the image bearing member or its vicinity is measured, each temperature and humidity and the applied voltage are stored in a matrix form, and when the temperature and humidity change, an appropriate applied voltage is applied by the above matrix. The surface potential of the image carrier may be adjusted within a target range.

Alternatively, the operating time of the image carrier may be measured, and every time a predetermined time is reached, the applied voltage for each temperature or temperature and humidity stored in the matrix is readjusted and stored again. it can.

Further, the number of images formed by the image forming apparatus may be measured, and whenever a predetermined number is reached, the applied voltage for each temperature or temperature and humidity stored in the matrix may be readjusted and stored again. .

Alternatively, the toner replenishment amount is obtained by charging the image bearing member with a contact type charger, and from the visualized image of the image bearing member, the ratio of the output of the pattern portion to the output of the non-pattern portion (background portion). In the charging method of the image carrier in the image forming apparatus for determining, the number of image formations of the image forming apparatus is counted, and when a predetermined number is reached, an electrostatic latent image of the reference pattern is formed on the image carrier. The pattern potential is measured from the latent image of the reference pattern, and when the pattern potential is out of the predetermined range, the applied voltage applied to the contact type charger is adjusted, and the image carrier or its vicinity at that time is adjusted. It is also possible to measure the temperature of, and store the applied voltage for each temperature again in a matrix, and adjust the surface potential of the image carrier within the target range.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of an image forming unit in the image forming apparatus of the present invention. In the figure, 1 is an image carrier composed of a photosensitive drum, 2 is a contact type charger, 3 is a voltage applying means of the contact type charger, 4 is erase, 5 is a developing device, 6 is a transfer / separation device, 7 Is a reflective photo sensor, 8 is a cleaning device,
9 is a static eliminator.

When an appropriate voltage is applied to the contact type charger 2 by the voltage applying means 3, the contact type charger 2 is operated by the image carrier 1.
, And uniformly charges the image carrier 1. The charged image carrier 1 is exposed in the direction of the arrow by an optical system (not shown) to form an electrostatic latent image. The formed electrostatic latent image is visualized by the developing device 5 and transferred to the transfer / separation device 6
Thus, the visualized image is transferred onto a transfer paper (not shown). The toner remaining on the image carrier 1 after the transfer is removed by the cleaning device 8, and the static eliminator 9 removes the static electricity on the surface of the image carrier 1.

The reflection type photo sensor 7 is for controlling the toner density, and determines the toner replenishment amount based on the ratio of the output of the pattern portion and the output of the non-pattern portion (background portion) for the visualized image.

The above construction is the same as the conventional one.
In the present invention, the contact type charger 2 is further provided with the temperature detecting means 10 including a thermistor, and the surface potential detecting means 11 and the humidity detecting means 12 are provided in front of the developing device 5. Then, the temperature detecting means 10, the surface potential detecting means 11, and the humidity detecting means 12 are connected to a control means 13 including a CPU of a computer, and the control means 13 further includes R
A storage unit 14, a timer 15 and a counter 16 using an AM, a magnetic recording medium or the like are connected.

FIG. 2 is a flowchart of the surface potential setting mode for charging the surface potential of the image carrier 1 to a predetermined potential. When the main switch of the image forming apparatus is turned on (S1), temperature detection by the temperature detection means 10 is started (S2).

For example, if the detected temperature when the power is turned on is 19.7 ° C., in the embodiment of FIG. 2, every 1 ° C., at an integer temperature (1, 2 ... 60 ° C.). Since the potential is set, the temperature inside the machine is raised by heat of a fixing heater (not shown) or the like, and waits until it reaches n ° C. (20 ° C.) (S3, S4).

When the temperature reaches 20 ° C., it is checked whether or not the surface potential is set (S5). And E (20)
If = 0, it is determined that the potential has not been set and the potential is set (S
6). If E (20) = 1, the potential setting mode is not executed because the potential has already been set.

In this embodiment, detection is performed every 1 ° C. and at an integer temperature, but this is of course 0.5 ° C.
Every time, every 2 ° C, etc.
Even if the temperature is not ℃, 2, 3 ... 60 ℃, for example, the temperature when the power is turned on (19.7 ℃ in the above example)
May be changed according to the system, such as every 1 ° C.

FIG. 3 shows an embodiment of the surface potential setting mode. When the surface potential setting is executed in the flowchart of FIG. 2, the flow of FIG. 3 is executed.

First, a voltage of -1600 V is applied to the charging device to charge the image carrier 1 (S7), and the surface potential Vd of the image carrier 1 at that time is detected by the surface potential sensor (S8). Then, the target value of Vd is compared with -900V (S9), and if it does not match the target value of -900V,
Further, it is checked whether or not the voltage is within −900 ± 40V (S10). For example, at a detection temperature of 20 ° C., Vd = −68
Assuming that the voltage is 0V, the voltage does not reach −900 ± 40V, so according to the flow of FIG. 3, the applied voltage = −1600.
It is charged once again at −900 − (− 680) = 1820V (S11) and detected by the surface potential detecting means 11 (S9, 10). As a result, if Vd = 910V, the applied voltage f (20) = when the detection temperature is 20 ° C. =
1820-900-(-910) = 1810V is calculated (S12), and the answer, 1810V, is stored in the storage means 1.
4 is stored (S13). Then, since the potential setting at 20 ° C. is completed, E (20) is stored from 0 to 1 (S14).

FIG. 4 shows the matrix stored in the RAM of the storage means 14 in which the voltage to be applied is set for each temperature in this way.

When the applied voltage is determined in this way, E (n) = 1 at the next temperature, so that the stored applied voltage may be applied. Also, if you first create the matrix shown in Figure 4,
After that, the charging potential during image formation can be controlled to be constant according to the applied voltage for each temperature stored in FIGS.

Further, in this embodiment, since the applied voltage is set every 1 ° C., the potential setting at 26 ° C. and 27 ° C. is completed as shown in FIG. 5, and the detected temperature is 26.5 ° C. In that case, the temperature of 26 ° C. and 27 ° C. and the applied voltage are linearly approximated, and when the setting of 28 ° C. is not set and the detected temperature is 27.5 ° C., 26 ° C. and 27 The applied voltage can be obtained and controlled by the extrapolation method from the temperature of ° C and the applied voltage.

Further, in FIG. 3, the first applied voltage is -160.
Although it is fixed at 0 V, this applied voltage may be changed for each temperature, and in this case, the effect of shortening the time for potential setting is produced.

When the contact type charger is used, variations in the charging member, environmental characteristics such as temperature and humidity, and the like are larger than those in the conventional wire type charging device, and the potential controllability is poor. FIG. 6 shows an example of temperature characteristics of the charging member used in this example. The charging rollers A and B as the contact type chargers have almost the same temperature characteristics, but have the same resistance Vd because they have different resistance values.
The applied voltage for obtaining = -900V is different. Further, the rollers A and C have different temperature characteristics.

In this embodiment, since the applied voltage which becomes the predetermined surface potential is set and stored for each temperature, the above-mentioned variation of the surface potential due to the difference of the charging members is eliminated, and a stable image of the same brightness is always displayed. Obtainable.

When the image carrier 1 has a dependency not only on temperature but also on humidity as shown in FIG. 7, the potential is not stable and a good image may not be obtained in the above invention. is there.

The embodiment of FIG. 8 shows an embodiment effective in such a case. In this embodiment, as shown in FIG. 1, a humidity detecting means 12 is provided. First, the temperature and humidity are detected (S21). Then, the temperature is set to every 1 ° C. as in the above embodiment, and the humidity is, for example, less than 30% or 30%.
The applied voltage f (n, w) at each temperature is set in each of the three temperature categories of less than 60% and less than 60%, and it is checked whether or not the potential is set (S22). ),
If not, start the surface potential setting mode (S2
3) The applied voltage is stored (S24) and the matrix of FIG. 9 is created. The subsequent image formation is controlled based on f (n, w). As a result, a stable potential can be obtained and a good image can be obtained even if the temperature / humidity characteristics of the charging member greatly change.

FIG. 10 shows that the image carrier has V
The applied voltage required for d = −900V is shown. The solid line shows the driving time of 0 hours, and the alternate long and short dash line shows after 100 hours. Since the film thickness of the image carrier changes depending on the driving time of the image carrier due to wear or the like, the required applied voltage is also shown in FIG.
It changes as shown in FIG. Therefore, the surface potential deviates from the target value with the applied voltage initially stored, and the image quality deteriorates over time.

Therefore, in the present invention, as shown in FIG. 11, the surface potential is set for each driving time of the image carrier, and the applied voltage is controlled so that the surface potential becomes constant to keep a good image. I have to.

As shown in FIG. 11, in this embodiment, the driving time of the image carrier 1 is measured by the timer 16 (see FIG. 1),
When the driving time exceeds 40 hours and 100 hours, the potential is set again (S31, 32). As a specific method, if it is within 40 hours, it is judged whether or not the potential is set at each temperature (S33), and if not, the surface potential setting mode is executed as in FIG. 3 (S34).

When it is determined that the driving time exceeds 40 hours and is less than 100 hours (S32), it is checked whether the matrix so far is canceled and a new potential is set (S3).
5) If not so, the surface potential mode is executed in the same manner as in FIG. 3 (S36). In memory, E (n,
40) and the matrix of f (n, 40) is newly created.

When the driving time exceeds 100 hours, the matrix up to that time is canceled again and it is checked whether new potential setting has been performed (S37), and the surface potential mode is executed in the same manner as in FIG. 3 (S38). Then, a matrix of E (n, 100) and f (n, 100) portions of FIG. 12 is created on the memory.

Normally, in such an image forming apparatus, the number of image formed sheets is counted by the counter 16 (see FIG. 1), and in order to maintain the image quality for every fixed number of sheets, the adjustment mode as shown in S41 to S45 of FIG. Although the (program control mode) is performed, in the present embodiment, whether or not the potential setting can be executed is determined at the same timing as in the adjustment mode.

FIG. 14 shows, as an example, V for every 1000 sheets.
The flowchart which detects d is shown. That is,
It is checked whether 1000 sheets (1k sheets) have passed since the last execution of the process control (S51). If it exceeds Vd, Vd is detected (S52) and it is checked whether Vd = -900 ± 40V is entered (S53). If so, the normal process mode (S41 to S45) is executed as it is, and Vd ≠ 900 ± 4.
If it is 0 V, the adjustment mode is executed after resetting the surface potential.

For example, 100 after the last execution of the programming mode.
After 0 or more sheets have passed, first, Vd is detected. When the detected temperature at this time is 26.5 ° C., f obtained by the interpolation method from the temperature-applied voltage matrix of FIG.
(N = 26.5) = − 1700.5V is applied to detect Vd. As a result, assuming that Vd = −790.5 V, the applied voltage is newly set to f ′ (t
= 26.5) =-1700.5-900-(-790.
5) = 1810V is calculated (S54) and stored (S5)
5) Erase the memory so far (the matrix of FIG. 15B). Then, after executing the programming mode (S
41 to S45), the electric potential is set for each temperature as in the above-described embodiment (S56, S57), and the blanks in the matrix of FIG. 15 (b) are filled to form the matrix of FIG. 15 (c). To go.

Further, here, the stored value f (n) so far is
Is added to the difference between f '(t) and f (n) to obtain a temporary f' (t)
Until the potential is adjusted and f ′ (t) is determined as
By substituting it (FIG. 15D), the time required for potential adjustment can be shortened. For example, in FIG.
F (n) = 1734 V at ℃, f (n) = 171 at 26 ° C.
Since it is 1 V, the intermediate value of 25.5 ° C. is (1
734 + 1711) /2=1722.5V can be calculated. Similarly, 26.5 ° C. is 1700.5 V, 27.5
C is calculated to be 1678.5V. On the other hand, at 26.5 ° C., the result of detecting Vd is 1810 V as described above.
I know that Therefore, the difference between the two is 1810
−1700.5 = 109.5V is obtained, and the above 172
In addition to the correction of 2.5V and 1678.5V, FIG.
A temporary f '(t) shown in (d) is calculated. As described above, since the provisional f ′ (t) can be calculated in advance, the time for adjusting the potential can be shortened.

When the change in the film thickness of the image carrier 1 is more or less than expected, the method of correcting by the driving time adjusts at an inappropriate timing, so that the potential changes with time. However, in the present embodiment, the adjustment is performed when the potential fluctuates by a predetermined amount or more, so that such a case does not occur. Further, even if there is no humidity detecting means, the potential is adjusted even when the potential changes due to the humidity change, so that the humidity detecting means can be omitted and a good image without potential change can be obtained. .

FIG. 16 shows an embodiment in which whether or not to reset the surface potential setting (matrix) is determined by the pattern potential Vp being created for toner density control.

Conventionally, in such an image forming apparatus, a reference pattern is prepared for every suitable number of sheets, for example, every 10 sheets (S61), and the density of the visible image is reflected by the reflection type photosensor 7 (see FIG. 1). Detect and output Vs of pattern part of sensor
The toner replenishment was controlled by the ratio between p and the output Vsg of the non-pattern portion (background portion) (S62 to S64).

In this embodiment, when Vp deviates from the target value of -600 V by ± 25 V or more (S65), the surface potential adjustment is stored again. Surface potential adjustment (S66-
S69) is the development of Vd adjustment up to now, and
Here, Vp adjustment is also performed by the same method.

According to the present invention, the potential is adjusted only when the Vp potential is deviated. Therefore, it is not necessary to take the timing for detecting the potential as in the embodiment in which the potential is adjusted according to the driving time and the number of image formations. The consumption and the load on cleaning can be reduced. In addition, since the potential change with respect to the humidity change can be detected similarly to the driving time, a good image can be maintained without the humidity detecting means. Obviously, in the embodiment having only the temperature detecting means, the humidity detecting means may be additionally provided.

[0049]

According to the present invention as described above,
The applied voltage for obtaining a predetermined surface potential for each predetermined temperature is stored, and thereafter, since the control is performed based on the stored applied voltage, it is possible to cancel the variation of the charging member, the variation of the surface potential due to the environment, etc. A stable image is always obtained.

By storing the applied voltage for obtaining a predetermined surface potential for each predetermined temperature and humidity including humidity,
It can handle changes in humidity. If the temperature and the applied voltage are stored again each time the driving time of the image carrier reaches a certain time, it is possible to eliminate the potential fluctuation due to the change of the film thickness of the image carrier.

If the surface potential is detected for each predetermined number of sheets and the applied voltage is re-stored when the surface potential is deviated by a predetermined amount or more, it is stable even if there is no correction by the driving time or a temperature / humidity detecting means. You can get an image.

Since the applied voltage for obtaining the predetermined surface potential is re-stored, the judgment as to whether or not it is made is made by the surface potential of the reference pattern used for the toner replenishment control, so that the surface potential can be set a minimum number of times. Therefore, it is possible to reduce the toner consumption, the load on the cleaning, and the like that accompany the potential setting, and always obtain a stable image, and improve the reliability of the entire system.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an image carrier charging device in an image forming apparatus of the present invention.

FIG. 2 is a flowchart for starting a surface potential setting mode.

FIG. 3 is a flowchart for executing a surface potential setting mode.

FIG. 4 is a diagram showing a temperature-applied voltage matrix stored in a storage unit.

FIG. 5 is a diagram showing an example of obtaining an applied voltage by an extrapolation method.

FIG. 6 is a diagram showing that the applied voltage changes due to the difference in resistance even for rollers having substantially the same temperature characteristics.

FIG. 7 is a diagram showing a state in which an applied voltage changes depending on humidity.

FIG. 8 is a flowchart for detecting temperature and humidity and setting an applied voltage.

FIG. 9 is a diagram showing a temperature / humidity-applied voltage matrix.

FIG. 10 is a diagram showing that the temperature-applied voltage characteristic changes due to the difference in driving time.

FIG. 11 is a flowchart for recreating the matrix and updating the memory with the elapse of the driving time.

FIG. 12 is a diagram showing a matrix recreated with the passage of drive time.

FIG. 13 is a flowchart of the process control mode execution.

FIG. 14 is a flow chart in which the surface potential is detected based on the number of images formed by the image forming apparatus after execution of the process control mode, and the matrix is recreated.

15 (a) to 15 (d) are diagrams showing the matrix recreated in FIG.

FIG. 16 is a flow chart for measuring the pattern potential from the visualized reference pattern image on the image carrier and adjusting the applied voltage each time the predetermined number is reached.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image carrier 2 Contact type charger 3 Voltage application means 10 Temperature detection means 11 Surface potential detection means 12 Humidity detection means 13 Control means 14 Storage means 15 Timer 16 Counter

Front page continued (72) Inventor Naomi Sugimoto 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd.

Claims (8)

[Claims] 1. A contact type charger for charging the image bearing member by charging, a voltage applying means for applying a voltage to the contact type charger, and a temperature detector for detecting the temperature of the contact type charger or its vicinity. Means, surface potential detection means for detecting the surface potential of the image carrier, storage means, and these control means, the control means to the surface potential of the image carrier detected by the surface potential detection means. An image forming apparatus characterized in that an appropriate applied voltage is calculated based on the calculated voltage to control the voltage applying means, and the memory means stores the appropriate applied voltage corresponding to the temperature detected by the temperature detecting means in a matrix form. Image carrier charging device in. 2. The apparatus further comprises humidity detecting means for detecting the humidity of the contact type charger or the vicinity thereof, and the storage means stores the appropriate applied voltage corresponding to each temperature and humidity in a matrix form. The charging device for the image carrier in the image forming apparatus according to claim 1. 3. A timer is provided for detecting the drive time of the image carrier, and each time a predetermined drive time elapses,
3. The image forming apparatus according to claim 1, wherein the proper applied voltage for each temperature or each temperature / humidity is obtained, and the storage unit stores the applied voltage for each temperature or each temperature / humidity again. Image carrier charging device. 4. A method of charging an image carrier in an image forming apparatus, wherein an image carrier is charged by a contact type charger, the temperature of the image carrier or the vicinity thereof is measured, and an applied voltage at the temperature is stored in a storage means. It is determined whether or not the image carrier is already stored, and if stored, the contact type charger is used to charge the image carrier according to the stored applied voltage value. The surface potential of the image carrier is measured by comparing it with the target range by adjusting the applied voltage and then the surface potential is targeted. Image formation characterized by adjusting the surface potential of the image carrier within the target range by setting the voltage within the range and storing the applied voltage when reaching the target range in a matrix corresponding to the temperature Image bearing in equipment Charging method. 5. Further, the humidity of the image carrier or in the vicinity thereof is measured, each temperature and humidity and the applied voltage are stored in a matrix form, and when the temperature and humidity change, an appropriate applied voltage is applied by the matrix. 6. The method for charging an image carrier in an image forming apparatus according to claim 5, wherein the surface potential of the image carrier is adjusted within a target range. 6. The operating time of the image carrier is measured, and every time a predetermined time is reached, the applied voltage for each temperature or temperature and humidity stored in the matrix is readjusted and stored again. A method for charging an image carrier in the image forming apparatus according to claim 4. 7. The number of images formed by the image forming apparatus is measured,
Each time the predetermined number of sheets is reached, the applied voltage for each temperature or temperature and humidity stored in the matrix is readjusted,
6. The method for charging an image carrier in an image forming apparatus according to claim 4, wherein the image is re-stored. 8. A toner replenishment amount based on a ratio of an output of a pattern portion and an output of a non-pattern portion (background portion) from an image visualized on the image bearing member by charging the image bearing member with a contact type charger. In the charging method of the image carrier in the image forming apparatus for determining, the number of image formations of the image forming apparatus is counted, and an electrostatic latent image of a reference pattern is formed on the image carrier every time the predetermined number is reached, The pattern potential is measured from the latent image of the reference pattern, and when the pattern potential is out of the predetermined range, the applied voltage applied to the contact type charger is adjusted, and the image carrier or its vicinity at that time is adjusted. Method for charging an image carrier in an image forming apparatus, characterized in that the temperature of the image carrier is measured, the applied voltage for each temperature is stored again in a matrix, and the surface potential of the image carrier is adjusted within a target range.