Developing device and image forming apparatus

Patent metadata

Field	Value
Publication number	US-9599924-B2
Application number	US-201615063597-A
Country	US
Kind code	B2
Filing date	Mar 8, 2016
Priority date	Mar 19, 2015
Publication date	Mar 21, 2017
Grant date	Mar 21, 2017

Abstract

There are provided a developing device including a developing roller to develop an image carrier by use of a two-component developer and a developing bias power source, and an image forming apparatus using the developing device. The developing device further includes a variable resistance unit provided on a bias apply path to apply a bias from the developing power source to the developing roller, and a resistance controller to control a resistance value of the variable resistance unit based on a charge amount signal indicating a toner's charge amount in a developer layer on the developing roller. The resistance controller provides a low mode to lower the resistance of the variable resistance unit and a high mode to enhance the resistance. The low mode is set if the toner charge amount is low, and the high mode is set if the toner charge amount is high.

First claim

What is claimed is:

1. A developing device comprising: a developing roller configured to carry a two-component developer on its surface to develop an electrostatic latent image on an image carrier; and a developing bias power source to apply a developing bias to the developing roller, wherein
the developing device further includes:
a variable resistance unit provided on a bias applying path through which the bias is applied to the developing roller from the developing bias power source, the variable resistance unit being configured to selectively obtain a resistance value at two or more levels; and
a resistance control unit to control the resistance value of the variable resistance unit based on a charge amount signal indicating a toner's charge amount in the two-component developer which is carried on the developing roller,

the resistance control unit controls the resistance value in one of two control modes including:
a low mode to reduce the resistance value of the variable resistance unit; and
a high mode to enhance the resistance value of the variable resistance unit,

the low mode is selected if the toner's charge amount represented by the charge amount signal is low, and
the high mode is selected if the toner's charge amount represented by the charge amount signal is high.

2. The developing device according to claim 1 , wherein
the variable resistance unit includes:
a fixed resistor;
a bypass path bypassing the fixed resistor; and
a switch to selectively make operative any one of the fixed resistor and the bypass path, and

the resistance control unit is configured to control the switch to:
make operative the bypass path in the low mode; and
make operative the fixed resistor in the high mode.

3. The developing device according to claim 1 , wherein
the variable resistance unit includes a variable resistor, and
the resistance control unit is configured to:
control the resistance value of the variable resistance unit to be a smallest value in the low mode; and
control the resistance value of the variable resistance unit to be an enhanced value which is larger than the smallest value and to be larger when a toner's charge amount represented by the charge amount signal is higher in the high mode.

4. The developing device according to claim 3 comprising an ammeter to measure a value of current flowing in the variable resistance unit, wherein
the resistance control unit is configured to:
obtain a current table representing a relationship between the resistance value of the variable resistance unit and a total amount of current flowing in the variable resistance unit during developing by varying the resistance value of the variable resistance unit to a plurality of levels and performing an image forming operation with a fixed image density at each of the levels;
read out a developing current which is the smallest value among total current values in the current table, the developing current flowing in association with movement of the toner from the developing roller to the image carrier during developing;
obtain the charge amount signal based on an electric charge corresponding to the developing current and a toner amount corresponding to an image density of a formed image;
obtain excess current as a current amount corresponding to an excess amount of the toner's charge amount represented by the obtained charge amount signal with respect to a predetermined upper limit charge amount of the toner;
read out a resistance value of the variable resistance unit from the current table so that the total current is a value calculated by subtracting the excess current value from a current value obtained for the smallest resistance value of the variable resistance unit, and
adjust subsequent resistance values of the variable resistance unit to the read-out value so that the high mode is set.

5. The developing device according to claim 4 , wherein the resistance control unit is configured to:
adjust the resistance value of the variable resistance unit to be the read-out value only if the toner's charge amount represented by the obtained charge amount signal exceeds the upper limit charge amount; and
set the resistance value of the variable resistance unit to be the smallest value within a variable range which is the low mode if the toner's charge amount represented by the obtained charge amount signal is equal to or less than the upper limited charge amount.

6. The developing device according to claim 4 , wherein the developing bias power source is configured to:
apply a developing bias in which an alternating-current component is superimposed with a direct-current component in a normal operation; and
apply a developing bias in which the alternating-current component is reduced or removed from the normal operation if the resistance control unit is under the high mode and the toner's charge amount represented by the obtained charge amount signal is equal to or more than a predetermined supreme limit charge amount which is further higher than the upper limit charge amount.

7. The developing device according to claim 6 , wherein the developing bias power source is configured to apply the developing bias in which the alternating-current component is removed or reduced from the normal operation only if an image to be formed is a line drawing.

8. The developing device according to claim 3 comprising an ammeter to measure a value of current flowing in the variable resistance unit and an image density sensor to measure an image density of an image developed and formed by the developing roller, wherein
the resistance control unit is configured to:
vary the resistance value of the variable resistance unit to a plurality of levels so that an image forming operation and measurement of an image density are performed at each of the levels to obtain a current table representing a relationship between the resistance value of the variable resistance unit and a total amount of current flowing in the variable resistance unit during developing;
read out a developing current which is the smallest value among total current values in the current table, the developing current flowing in association with movement of the toner from the developing roller to the image carrier during developing and designate an image density of the image formed when the current value of the developing current is obtained as a selected image density;
obtain the charge amount signal from an electric charge corresponding to the developing current and a toner amount corresponding to the selected image density;
calculate excess current as a current amount corresponding to an excess amount of a toner's charge amount represented by the obtained charge amount signal with respect to a predetermined upper limit charge amount of the toner;
read out a resistance value of the variable resistance unit from the current table so that the total current is a value calculated by subtracting the excess current value from a current value obtained for the smallest resistance value of the variable resistance unit; and
adjust subsequent resistance value of the variable resistance unit to the read-out value so that the high mode is set.

9. The developing unit according to claim 8 , wherein the resistance control unit is configured to:
adjust the resistance value of the variable resistance unit to be the read-out value only if the toner's charge amount represented by the obtained charge amount signal exceeds the upper limit charge amount; and
set the resistance value of the variable resistance unit to be the smallest value within a variable range which is the low mode if the toner's charge amount represented by the obtained charge amount signal is equal to or less th

Assignees

• Konica Minolta Inc

Inventors

• Mandai Yuusuke
• Inada Yasuyuki
• Mukobayashi Yu
• Yoshimoto Shinichi
• Kanda Junji
• Nakayama Kanji

Classifications

• G03G15/0907

  with bias voltage (G03G15/065 takes precedence) · CPC title

• G03G15/065 Primary

  Arrangements for controlling the potential of the developing electrode · CPC title

• G03G2215/0132

  vertical medium transport path at the secondary transfer · CPC title