JP2015075703A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to improve transfer efficiency regardless of environmental conditions.SOLUTION: A voltage in a transfer nip section 115 is calculated, on the basis of results detected by a transfer current detection part 105 and a current detection part 107, the results obtained when a tip of a sheet 108 reaches the transfer nip section 115. On the basis of the calculated voltage, a voltage or a current to be set to a pre-transfer guide power source 106 is determined. Until the tip of the sheet 108 reaches a fixing nip section 116, the pre-transfer guide power source 106 is constant-voltage controlled or constant-current controlled. A potential in the transfer nip section 115 is calculated on the basis of the results detected by the transfer current detection part 105 and the current detection part 107 when the tip of the sheet 108 reaches the fixing nip section 116. On the basis of the calculated potential, a voltage or a current to be set to the pre-transfer guide power source 106 is determined. Until a rear end of the sheet 108 departs from a pre-transfer guide 102, the pre-transfer guide power source 106 is constant-voltage controlled or constant-current controlled.

Description

  The present invention relates to an image forming apparatus.

  2. Description of the Related Art Conventionally, an image forming apparatus such as a laser printer has a photosensitive drum 101, a transfer roller 103 as a transfer member facing the photosensitive drum 101, a transfer power source 104 that supplies power to the transfer roller 103, and a fixing device 120. The configuration is known (see FIG. 5). In this image forming apparatus, the surface of the photosensitive drum 101 is uniformly charged, and an electrostatic latent image is formed by exposing the charged area. Then, a toner image is formed on the surface of the photosensitive drum 101 by supplying toner to the electrostatic latent image and developing it. The toner image is transferred to a paper 108 as a recording medium at a transfer nip portion 115 where the photosensitive drum 101 and the transfer roller 103 abut. A conductive pre-transfer guide that contacts the paper 108 and guides the conveyance of the paper 108 upstream of the transfer nip 115 in the conveyance direction of the paper 108 (direction from right to left in FIG. 5). 102 is provided.

  As shown in FIG. 5, the fixing device 120 includes a pressure roller 109, a fixing film 110, a ceramic heater 111 that heats the fixing film 110, and an AC power source 112 that supplies electric power to the ceramic heater 111. Then, the toner image transferred onto the paper 108 is heated and fixed at a fixing nip portion 116 that is a contact portion between the pressure roller 109 and the fixing film 110.

  The transfer power supply 104 performs constant voltage control for controlling the voltage applied from the transfer roller 103 to the paper 108 at a constant level, and constant current control for controlling the current flowing from the transfer roller 103 to the paper 108 at a constant level. Here, the current supplied to the transfer nip portion 115 may flow to other than the transfer nip portion 115 via the paper 108. Further, the transfer performance varies greatly depending on changes in temperature, humidity, moisture absorption of paper 108, size of paper 108, and the like.

  When the transfer current flowing from the transfer roller 103 to the paper 108 is controlled by the constant current control, when the paper 108 comes into contact with the pre-transfer guide 102 and the fixing nip 116, the pre-transfer guide 102 and the fixing device 120 are passed through the paper 108. Current will flow through. In particular, when a paper 108 having a small resistance value that has absorbed moisture is conveyed, a large current flows outside the transfer nip 115. As described above, when a current flows to a portion other than the transfer nip portion 115, an amount of charge necessary for transfer in the transfer nip portion 115 is insufficient, and a transfer failure occurs.

  As a solution to such a problem, for example, as shown in FIG. 6, a transfer current detecting unit 105 that detects a current flowing through the transfer roller 103 and a transfer that detects a current (leakage current) flowing through the pre-transfer guide 102. A configuration including the front guide current detection unit 107 has been proposed.

  In such a configuration, before the image formation, the transfer power source 104 performs constant current control at a predetermined current value, and the transfer current detection unit 105 detects the voltage value at the predetermined current value, so that an optimum during image formation is achieved. Calculate the voltage value. Then, the optimum voltage value calculated before the image formation is applied to the transfer roller 103 during the image formation. Further, after the paper 108 enters the transfer nip 115, the pre-transfer guide current detection unit 107 detects a leakage current flowing from the transfer roller 103 to the pre-transfer guide 102 via the paper 108. The transfer voltage applied from the transfer power source 104 to the transfer roller 103 is corrected according to the detection result (see, for example, Patent Document 1).

Furthermore, as a solution to the problem in a high humidity environment, as shown in FIG. 7, a configuration including a power source 106 that applies a voltage having the same polarity as the voltage applied to the transfer roller 103 to the pre-transfer guide 102 is provided. Proposed. According to this configuration, transfer current leakage can be suppressed. Therefore, even when the paper 108 having a small resistance value absorbed in a high humidity environment is transported, the transfer efficiency can be maintained (see, for example, Patent Document 2).

Japanese Patent Laid-Open No. 11-219042 JP 49-112116 A

  In the above-described configuration in which the current leaking to the pre-transfer guide 102 is detected and the transfer voltage applied to the transfer roller 103 is corrected, the transfer voltage is increased according to the current leaking to the pre-transfer guide 102 in a high humidity environment. There is a need to. In that case, the transfer current increases and the voltage drop of the transfer roller 103 increases. Further, when the paper 108 is conveyed in a high humidity environment, the current leaked to the pre-transfer guide 102 via the paper 108 increases because the resistance value of the paper 108 is small.

  For this reason, it is necessary to increase the capacity of the power supply that outputs the transfer voltage in consideration of a voltage drop at the transfer roller 103 and an increase in current leaking to the pre-transfer guide 102, resulting in an increase in cost. If the transfer voltage is increased too much, transfer omission due to excessive charges may occur, and a part of the toner may be charged to the same polarity as the transfer voltage and not transferred to the paper 108. Further, there is a possibility that an excessive current flows through the photosensitive drum 101 and a transfer memory is generated. That is, in the configuration for correcting the transfer voltage, there is a limit to increasing the transfer efficiency with respect to the current leaking to the pre-transfer guide 102.

  Further, in the conventional configuration in which a voltage is applied to the pre-transfer guide 102 provided upstream from the transfer roller 103, a transfer current is transferred from the transfer nip portion 115 through the paper 108 to a portion other than the conveyance path upstream of the transfer nip portion 115. There is a leaking path. For example, current may leak through the fixing nip 116 via the paper 108. As described above, even if the voltage value to be applied to the pre-transfer guide 102 is simply determined only by the environmental conditions, the transfer current leakage path changes depending on the position of the conveyed paper 108, so that optimum transfer performance is exhibited. I can't do it.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus that realizes high transfer efficiency regardless of environmental conditions.

In order to achieve the above object, an image forming apparatus according to the present invention includes:
An image carrier for carrying a toner image;
A transfer member for transferring the toner image to a recording medium in a transfer portion in contact with the image carrier;
A first power source for supplying power to the transfer member;
A conductive member that is positioned upstream of the transfer unit in the transport direction of the recording medium and that can contact the transported recording medium;
A second power source for supplying power to the conductive member;
A first current detector for detecting a current supplied by the first power source;
A second current detector for detecting a current supplied by the second power source;
A controller for controlling the first power source and the second power source;
A fixing member for fixing the toner image transferred to the recording medium to the recording medium in a fixing unit located downstream of the transfer unit in the conveyance direction of the recording medium;
In an image forming apparatus comprising:
The control unit is
Based on the detection result by the first current detection unit and the detection result by the second current detection unit when the leading edge of the recording medium being conveyed reaches the transfer unit, the voltage in the transfer unit is calculated and calculated A set voltage or a set current of the second power source is determined based on the voltage thus determined, and the second power source is controlled at a constant voltage with the set voltage until the leading edge of the conveyed recording medium reaches the fixing unit. Or constant current control with the set current,
The potential in the transfer unit is calculated based on the detection result by the first current detection unit and the detection result by the second current detection unit when the leading edge of the recording medium being conveyed reaches the fixing unit. Determining the set voltage or set current of the second power source based on the measured potential, and controlling the second power source at a constant voltage with the set voltage until the rear end of the conveyed recording medium leaves the conductive member; Alternatively, constant current control is performed with the set current.

An image forming apparatus according to the present invention is
An image carrier for carrying a toner image;
A transfer member for transferring the toner image to a recording medium in a transfer portion in contact with the image carrier;
A first power source for supplying power to the transfer member;
A conductive member that is positioned upstream of the transfer unit in the transport direction of the recording medium and that can contact the transported recording medium;
A second power source for supplying power to the conductive member;
A first current detector for detecting a current supplied by the first power source;
A second current detector for detecting a current supplied by the second power source;
A controller for controlling the first power source and the second power source;
A fixing member for fixing the toner image transferred to the recording medium to the recording medium in a fixing unit located downstream of the transfer unit in the conveyance direction of the recording medium;
In an image forming apparatus comprising:
The control unit is
Based on the detection result by the first current detection unit and the detection result by the second current detection unit when the leading edge of the conveyed recording medium reaches the transfer unit, the resistance value of the recording medium is calculated, A set voltage or set current of the second power source is determined in accordance with the calculated resistance value of the recording medium, and the second power source is set until the leading end of the conveyed recording medium reaches the fixing unit. With constant voltage control, or constant current control with the set current,
A resistance value of the recording medium is calculated based on a detection result by the first current detection unit and a detection result by the second current detection unit when the leading end of the recording medium being conveyed reaches the fixing unit; A setting voltage or a setting current of the second power source is determined according to the calculated resistance value of the recording medium, and the second power source is set until the rear end of the transported recording medium leaves the conductive member. A constant voltage control is performed using a voltage, or a constant current control is performed using the set current.

  According to the present invention, it is possible to provide an image forming apparatus that realizes high transfer efficiency regardless of environmental conditions.

Schematic sectional view showing the configuration of Example 1 The figure which shows the equivalent circuit of FIG. Flowchart of transfer control of embodiment 1 Flowchart of transfer control of embodiment 2 Schematic sectional view showing the configuration of a conventional example Schematic sectional view showing the configuration of a conventional example Schematic sectional view showing the configuration of a conventional example

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be exemplarily described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. Therefore, unless specifically stated otherwise, the scope of the present invention is not intended to be limited thereto.

(Example 1)
A first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic cross-sectional view illustrating the configuration of the first embodiment, and is an enlarged schematic cross-sectional view illustrating the vicinity of the transfer nip portion and the fixing nip portion of the image forming apparatus according to the first embodiment.

<Configuration of Example 1>
As shown in FIG. 1, the image forming apparatus according to the first embodiment includes a photosensitive drum 101 as an image carrier, a pre-transfer guide 102 as a conductive member, a transfer roller 103 as a transfer member, and a fixing member. And a fixing device 120.

  The photosensitive drum 101 carries a toner image as a developer image on its surface. The transfer roller 103 abuts against the photosensitive drum 101 to form a transfer nip portion 115 as a transfer portion. The transfer roller 103 transfers the toner image carried on the photosensitive drum 101 to the paper 108 as a recording medium conveyed to the transfer nip 115. The pre-transfer guide 102 is located upstream of the transfer roller 103 in the transport direction of the paper 108 (in the direction of the arrow in FIG. 1), and is provided so as to be able to contact the transported paper 108. The conveyance to the unit 115 is guided.

  The fixing device 120 is located downstream of the transfer nip portion 115 in the conveyance direction of the paper 108. The fixing device 120 includes a rotatable pressure roller 109 and a cylindrical fixing film 110. The pressure roller 109 and the fixing film 110 are in contact with each other to form a fixing nip portion 116 as a fixing portion. A ceramic heater 111 is provided on the inner peripheral side of the cylindrical fixing film 110 to heat the paper 108 conveyed to the fixing nip portion 116 via the fixing film 110. The fixing device 120 uses the pressure roller 109 and the fixing film 110 to heat and press the paper 108 onto which the toner image has been transferred while being nipped and conveyed, thereby fixing the toner image on the paper 108.

  In addition, the image forming apparatus according to this embodiment includes an AC power source 112 that supplies power to the ceramic heater 111. A resistor 113 and a capacitor 114 are connected in series to the cored bar 109a of the pressure roller 109 and are grounded.

  Further, the image forming apparatus according to the first embodiment includes a transfer power source 104 as a first power source, a transfer current detection unit 105 as a first current detection unit, a pre-transfer guide power source 106 as a second power source, and a second current detection unit. As a pre-transfer guide current detection unit 107.

  The transfer power source 104 applies a voltage (transfer voltage) to the transfer roller 103. The transfer current detection unit 105 detects the current supplied by the transfer power source 104. The pre-transfer guide current detection unit 107 applies a voltage to the pre-transfer guide 102. The pre-transfer guide current detection unit 107 detects the current supplied by the pre-transfer guide power source 106. The transfer power source 104 and the pre-transfer guide power source 106 are controlled by a CPU 117 as a control unit provided in the main body of the image forming apparatus.

<Equivalent circuit>
Next, an equivalent circuit in the configuration of the first embodiment will be described with reference to FIG. FIG. 2 is a diagram illustrating an equivalent circuit in the configuration of the first embodiment. 2A shows a state when an image is formed and before the paper 108 is passed, and FIG. 2B shows a state when the leading end of the paper 108 reaches the transfer nip portion 115. FIG. 2C shows a state when the leading edge of the paper 108 reaches the fixing nip portion 116, and FIG. 2D shows a state when the trailing edge of the paper 108 is separated from the pre-transfer guide 102.

  Here, the impedance of the transfer roller 103 is Ztr, and the impedance of the photosensitive drum 101 is Zc.

  2A, the current that flows from the transfer power supply 104 to the transfer roller 103 in the state before the paper 108 is passed at the time of image formation shown in FIG.

  2B, the current flowing from the transfer power supply 104 to the transfer roller 103 in the state where the leading edge of the paper 108 reaches the transfer nip portion 115 at the time of image formation shown in FIG. Further, the impedance (resistance value) of the paper 108 between the pre-transfer guide 102 and the transfer nip 115 is Zp1, and the current flowing through the paper 108 is Ip2. The current flowing through the photosensitive drum 101 is Ic2.

  In addition, when the image is formed as shown in FIG. 2C and the leading end of the paper 108 reaches the fixing nip portion 116, the current flowing from the transfer power supply 104 to the transfer roller 103 is set to Itr3. The current flowing through the paper 108 via the pre-transfer guide 102 is Ip3. The current flowing through the photosensitive drum 101 is Ic3. Further, the impedance (resistance value) of the paper 108 between the transfer nip portion 115 and the fixing nip portion 116 is Zp2, and the impedance of the pressure roller 109, the resistor 113, and the capacitor 114 is Zf. The current flowing from the transfer nip 115 to the capacitor 114 via the paper 108 is If3. Note that the paper 108 is in contact with the transfer guide 102 in the state shown in FIG.

  2D, the current flowing from the transfer power supply 104 to the transfer roller 103 in the state where the trailing edge of the paper 108 is separated from the pre-transfer guide 102 at the time of image formation shown in FIG. The current flowing through the photosensitive drum 101 is Ic4. The current flowing from the transfer nip 115 to the capacitor 114 via the paper 108 is set to If4.

<Control flow>
Next, with reference to FIG. 3, the flow of transfer control in the first embodiment will be described. FIG. 3 is a flowchart of transfer control according to the first exemplary embodiment. The transfer control is performed by a control unit (CPU) provided in the image forming apparatus.

  First, after detecting a print signal for image formation (YES in S101), a main motor (not shown) and a charging output (not shown) of the image forming apparatus are turned on (S102).

  Next, the control in the state before the paper 108 is passed, that is, the state shown in FIG. The transfer power supply 104 is controlled to a constant voltage to the output voltage Vtr1, and the current Itr1 flowing through the transfer roller 103 and the photosensitive drum 101 is detected by the transfer current detector 105. Based on the detection result, the CPU 117 calculates an impedance Ztr + Zc [Ω], which is the sum of the impedance Ztr of the transfer roller 103 and the impedance Zc of the photosensitive drum 101 (S103). The impedance Ztr + Zc in a state before the paper 108 is passed is expressed by the following (Formula 1).

  Based on the result of the calculated impedance Ztr + Zc, the CPU 117 determines the environmental condition, and determines the set voltage Vtr2 of the transfer power supply 104 and the set voltage Vg1 of the pre-transfer guide power supply 106 during image formation. Then, constant voltage control is performed using these set voltages (S104).

  Further, the control in the state after the leading edge of the paper 108 reaches the transfer nip portion 115, that is, the state shown in FIG. 2B will be described. At the timing when the leading edge of the paper 108 reaches the transfer nip 115, the transfer nip voltage (potential) Vc1 in the transfer nip 115 is determined from the detection results (Itr2, Ip2) of the transfer current detector 105 and the pre-transfer guide current detector 107. Calculated (S105). Here, the following (Formula 2) is established in the state of FIG. Then, when (Expression 2) is solved for Zc, the following (Expression 3) is obtained. Note that Ztr + Zc in (Expression 3) is calculated by (Expression 1).

  Here, the transfer nip voltage Vc1 of the transfer nip portion 115 is expressed by the following (formula 4).

  The transfer nip voltage Vc1 is calculated by storing (Equation 3) and (Equation 4) in the CPU 117 in advance.

  Then, Vg2 for constant voltage control of the pre-transfer guide power supply 106 is determined from the transfer nip voltage Vc1, or Ig2 for constant current control of the pre-transfer guide power supply 106 is determined. The CPU 117 performs constant voltage control on the pre-transfer guide power source 106 with the set voltage Vg2 from when the leading edge of the paper 108 reaches the transfer nip portion 115 until it reaches the fixing nip portion 116, or the pre-transfer guide power source 106. Is controlled with the set current Ig2 (S106).

  Further, control in the state after the leading edge of the paper 108 reaches the fixing nip portion 116, that is, the state shown in FIG. 2C will be described. At the timing when the leading edge of the paper 108 reaches the fixing nip portion 116, the transfer nip voltage (potential) Vc2 in the transfer nip portion 115 is determined from the detection results (Itr3, Ip3) of the transfer current detection portion 105 and the pre-transfer guide current detection portion 107. Calculated (S107). The transfer nip voltage Vc2 is expressed by the following (formula 5).

  On the other hand, Zp1 before the leading edge of the paper 108 reaches the fixing nip portion 116 is expressed by the following (formula 6).

  The transfer nip voltage Vc2 is calculated by storing (Equation 5) and (Equation 6) in the CPU 117 in advance.

  Then, Vg3 for constant voltage control of the pre-transfer guide power supply 106 is determined from the transfer nip voltage Vc2, or Ig3 for constant current control of the pre-transfer guide power supply 106 is determined. The pre-transfer guide power supply 106 is controlled at a constant voltage Vg3 or the pre-transfer guide until the leading end of the paper 108 reaches the fixing nip portion 116 and the rear end of the paper 108 moves away from the pre-transfer guide 102. The power source 106 is subjected to constant current control with the set current Ig3 (S108).

  Further, the control in the state where the trailing edge of the paper 108 is separated from the pre-transfer guide 102, that is, the state shown in FIG. In this embodiment, at the timing when the trailing edge of the paper 108 leaves the pre-transfer guide 102, the set voltage is changed to Vtr3 if the transfer power supply 104 is controlled at constant voltage, and the set current is set to Itr4 if controlled at constant current. change.

  Since Vtr3 or Itr4 is set to a preset value obtained from an experiment in advance from Ztr, Zc and Zp2 + Zf, Zp2 + Zf is calculated according to the following flow.

  First, Zp2 + Zf is expressed by the following (formula 7). The current Ic3 flowing through the photosensitive drum 101 is expressed by the following (formula 8). The current If3 flowing through the capacitor 114 is expressed by the following (formula 9).

  Zp2 + Zf is calculated by storing (Equation 7) to (Equation 9) in the CPU 117 in advance. Then, according to Ztr, Zc, and Zp2 + Zf, which are loads viewed from the transfer power supply 104, Vtr3 for controlling the transfer power supply 104 at constant voltage is determined, or Itr4 for controlling the transfer power supply 104 at constant current is determined. After the trailing edge of the paper 108 leaves the pre-transfer guide 102, the CPU 117 performs constant voltage control of the transfer power source 104 with the set voltage Vtr3 or constant current control of the transfer power source 104 with the set current Vtr4 (S109). After this series of transfer controls, image formation (printing) is terminated (S110).

In the first embodiment, it is possible to calculate in real time the voltage at the transfer nip portion 115 that greatly changes at the transport position of the paper 108 by the configuration and control described above. For this reason, the voltage at the transfer nip 115 can be kept at an optimum value regardless of the environmental conditions, and high transfer efficiency can be maintained even when the low-resistance paper 108 is passed.

  The impedance of the transfer roller 103 under high temperature and high humidity is several tens of MΩ, whereas the impedance when the length between the pre-transfer guide 102 and the transfer nip 115 is about 5 mm is several MΩ. . Therefore, by controlling the pre-transfer guide power source 106, the influence of the voltage drop can be reduced, and the pre-transfer guide power source 106 does not require a large power source capacity.

(Example 2)
Next, Example 2 will be described with reference to FIG. FIG. 4 is a flowchart illustrating transfer control according to the second exemplary embodiment. In the second embodiment, the configuration of the image forming apparatus (see FIG. 1) and its equivalent circuit (see FIG. 2) are the same as those of the first embodiment except that the transfer control is different. For this reason, the same reference numerals are used for the same components, and descriptions thereof are omitted.

  First, after a print signal for image formation is detected (YES in S201), a main motor (not shown) and a charging output (not shown) of the image forming apparatus are turned on (S202).

  Next, the control in the state before the paper 108 is passed, that is, the state shown in FIG. The transfer power supply 104 is controlled to a constant voltage to the output voltage Vtr1, and the current Itr1 flowing through the transfer roller 103 and the photosensitive drum 101 is detected by the transfer current detector 105. Based on the detection result, the CPU 117 calculates an impedance Ztr + Zc [Ω], which is the sum of the impedance Ztr of the transfer roller 103 and the impedance Zc of the photosensitive drum 101 (S203). The impedance Ztr + Zc is expressed by (Equation 1) shown in the first embodiment.

  Based on the result of the calculated impedance Ztr + Zc, the CPU 117 determines the environmental condition, and determines the set voltage Vtr2 of the transfer power supply 104 and the set voltage Vg1 of the pre-transfer guide power supply 106 during image formation. Then, constant voltage control is performed using these set voltages (S204).

  Further, the control in the state after the leading edge of the paper 108 reaches the transfer nip portion 115, that is, the state shown in FIG. 2B will be described. The CPU 117 calculates Zp1 and Zp2 from the detection results (Itr2, Ip2) of the transfer current detection unit 105 and the pre-transfer guide current detection unit 107 at the timing when the leading edge of the paper 108 reaches the transfer nip 115. (S205). The calculation method is as follows.

  The transfer nip voltage Vc1 is the same as (Expression 3) and (Expression 4) shown in Example 1, and Zp1 is expressed by the following (Expression 10).

  From the transport distance between the transfer nip portion 115 and the fixing nip portion 116, the paper 108 impedance Zp2 is calculated. The distance between the pre-transfer guide 102 and the transfer nip 115 and the distance between the transfer nip 115 and the fixing nip 116 are determined by the configuration of the image forming apparatus. Therefore, when the distance between the pre-transfer guide 102 and the transfer nip 115 is A [mm] and the distance between the transfer nip 115 and the fixing nip 116 is B [mm], Zp2 is expressed by the following (formula 11). Is done.

  The impedances Zp1 and Zp2 of the paper 108 between the pre-transfer guide 102 and the transfer roller 103 and between the transfer nip portion 115 and the fixing nip portion 116 are calculated by (Expression 3), (Expression 4), (Expression 10), (Expression). 11) is stored in the CPU 117 in advance (S205).

  If the constant voltage control of the pre-transfer guide power supply 106 is performed until the paper 108 enters the fixing nip portion 116 from a set value obtained in advance by an experiment according to (Equation 1) and the calculation results of Zp1 and Zp2. If Vg2 is constant current control, Ig2 is determined. Further, according to the calculation results of Zp1 and Zp2, Vg3 is set to a constant current for constant voltage control of the pre-transfer guide power supply 106 after the paper 108 enters the fixing nip portion 116 (the state shown in FIG. 2C). If it is control, Ig3 is determined. Further, according to the calculation results of Zp1 and Zp2, Vtr3 is set for constant voltage control of the transfer power source 104 at the timing when the trailing edge of the paper 108 leaves the pre-transfer guide 102 (the state shown in FIG. 2D). If it is constant current control, Itr4 is determined (S206).

  The pre-transfer guide power source 106 determined in the above step is controlled to Vg2 for constant voltage control and Ig2 for constant current control (S207). At the timing when the paper 108 enters the fixing nip portion 116, the pre-transfer guide power supply 106 is controlled to Vg3 for constant voltage control and Ig3 for constant current control (S208). When the trailing edge of the paper 108 leaves the pre-transfer guide 102, the transfer power supply 104 is controlled to Vtr3 for constant voltage control and to Itr4 for constant current control (S209). After this series of transfer controls, the image formation is terminated (S210).

  The transfer control in this embodiment calculates the resistance of the paper 108 between the pre-transfer guide 102 and the transfer nip 115 when the paper 108 enters the transfer nip 115. Then, the pre-transfer guide power supply 106 and the transfer power supply 104 are controlled according to the transport position of the paper 108 using the set values obtained in advance by experiments. Therefore, even in an image forming apparatus with a high printing speed, it is possible to minimize a control response delay.

  When the impedance of the transfer roller 103 in a high temperature and high humidity environment is several tens of MΩ, the impedance of the paper 108 is several MΩ when the distance between the pre-transfer guide 102 and the transfer nip 115 is about 5 mm. Therefore, the influence of the voltage drop can be reduced by controlling the pre-transfer guide power source 106, and a large power source capacity is not required for the pre-transfer guide power source 106. By detecting the impedance of the paper 108 and controlling the pre-transfer guide power supply 106 according to the transport position, it is possible to maintain high transfer efficiency even when the moisture-absorbing low-resistance paper 108 is passed.

  In the above-described embodiment, the pre-transfer guide 102 has been described as an example of the conductive member. However, the conductive member is not limited to this, and can be rotated upstream of the transfer nip 115. You may use the conveyance roller as a conductive member.

  DESCRIPTION OF SYMBOLS 101 ... Photosensitive drum (image carrier), 102 ... Pre-transfer guide (conductive member), 103 ... Transfer roller (transfer member), 104 ... Transfer power source (first power source), 105 ... Transfer current detector (first current detection) ), 106: Pre-transfer guide power supply (second power supply), 107: Current detection section (second current detection section), 108: Paper, 115: Transfer nip section (transfer section), 116: Fixing nip section (fixing section) ) 117... Controller, 120.

Claims (5)

An image carrier for carrying a toner image;
A transfer member for transferring the toner image to a recording medium in a transfer portion in contact with the image carrier;
A first power source for supplying power to the transfer member;
A conductive member that is positioned upstream of the transfer unit in the transport direction of the recording medium and that can contact the transported recording medium;
A second power source for supplying power to the conductive member;
A first current detector for detecting a current supplied by the first power source;
A second current detector for detecting a current supplied by the second power source;
A controller for controlling the first power source and the second power source;
A fixing member for fixing the toner image transferred to the recording medium to the recording medium in a fixing unit located downstream of the transfer unit in the conveyance direction of the recording medium;
In an image forming apparatus comprising:
The control unit is
Based on the detection result by the first current detection unit and the detection result by the second current detection unit when the leading edge of the recording medium being conveyed reaches the transfer unit, the voltage in the transfer unit is calculated and calculated A set voltage or a set current of the second power source is determined based on the voltage thus determined, and the second power source is controlled at a constant voltage with the set voltage until the leading edge of the conveyed recording medium reaches the fixing unit. Or constant current control with the set current,
The potential in the transfer unit is calculated based on the detection result by the first current detection unit and the detection result by the second current detection unit when the leading edge of the recording medium being conveyed reaches the fixing unit. Determining the set voltage or set current of the second power source based on the measured potential, and controlling the second power source at a constant voltage with the set voltage until the rear end of the conveyed recording medium leaves the conductive member; Alternatively, an image forming apparatus that performs constant current control with the set current. An image carrier for carrying a toner image;
A transfer member for transferring the toner image to a recording medium in a transfer portion in contact with the image carrier;
A first power source for supplying power to the transfer member;
A conductive member that is positioned upstream of the transfer unit in the transport direction of the recording medium and that can contact the transported recording medium;
A second power source for supplying power to the conductive member;
A first current detector for detecting a current supplied by the first power source;
A second current detector for detecting a current supplied by the second power source;
A controller for controlling the first power source and the second power source;
A fixing member for fixing the toner image transferred to the recording medium to the recording medium in a fixing unit located downstream of the transfer unit in the conveyance direction of the recording medium;
In an image forming apparatus comprising:
The control unit is
Based on the detection result by the first current detection unit and the detection result by the second current detection unit when the leading edge of the conveyed recording medium reaches the transfer unit, the resistance value of the recording medium is calculated, A set voltage or set current of the second power source is determined in accordance with the calculated resistance value of the recording medium, and the second power source is set until the leading end of the conveyed recording medium reaches the fixing unit. With constant voltage control, or constant current control with the set current,
A resistance value of the recording medium is calculated based on a detection result by the first current detection unit and a detection result by the second current detection unit when the leading end of the recording medium being conveyed reaches the fixing unit; A setting voltage or a setting current of the second power source is determined according to the calculated resistance value of the recording medium, and the second power source is set until the rear end of the transported recording medium leaves the conductive member. An image forming apparatus, wherein constant voltage control is performed using a voltage, or constant current control is performed using the set current. The control means is
Based on the detection result by the first current detection unit and the detection result by the second current detection unit when the leading edge of the recording medium to be conveyed reaches the transfer unit and the fixing unit. Determining a set voltage or set current of the first power supply after the rear end of the recording medium has left the conductive member;
The constant voltage control of the first power source is performed with the set voltage after the rear end of the recording medium to be conveyed leaves the conductive member, or the constant current control is performed with the set current. The image forming apparatus according to 2.   The image forming apparatus according to claim 1, wherein the conductive member is a pre-transfer guide that guides conveyance of the recording medium to the transfer unit.   The image forming apparatus according to claim 1, wherein the conductive member is a conveyance roller that is rotatably provided and conveys the recording medium to the transfer unit.

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