An image forming apparatus

Abstract: An image forming apparatus comprises a photoreceptor, a developing roller (4) for forming a layer of developer (D) on a surface thereof to supply toner to the photoreceptor at a developing position. A bias supply (7) applies a bias voltage between the developing roller and a ground. A conductive line (5,8,R1) connects the developer layer with ground, and the toner density of the developer layer is detected by monitoring the current flowing through the conductive line (5,8,R1). A voltage controller (10) prevents the surface voltage of the developer layer from falling below a predetermined level, at least during a developing operation, despite the current flowing into the conductive line. Accordingly, the formation of fog on a copy can be prevented because the surface voltage of the developer layer (D) is maintained above the specified level at least during the developer operation.

Description: This invention relates to an image forming apparatus having a toner density detecting device. Conventionally, in an image forming apparatus using bicomponent developer consisting of toner and carrier, a developing unit is provided with a toner density sensor for detecting toner density of the developer. Supply of the toner is controlled in accordance with a sensor signal from the toner density sensor to maintain a proper toner density, whereby preventing the toner from being excessively or deficiently fed to a photosensitive drum. A low cost image forming apparatus requires assemblage from low cost parts. However, toner density sensors are generally relatively expensive because they include a magnetic sensor or like sensor. Accordingly, there has been an increasing demand for a toner density sensor which can be fabricated at low costs, instead of the existing expensive toner density sensors. It is a well-known concept that a resistance value of developer varies according to a mixing ratio of toner to carrier, namely toner density, in the developer. In view of the above concept, Unexamined Japanese Patent Publication 58-169161 (US-A-4 538 897) has proposed an image forming apparatus including a detector circuit in which a resistor is connected between a regulating member for regulating thickness of a layer of developer formed on the surface of a developing roller and a ground to detect the toner density. More specifically, bias voltage is applied to the developing roller in order to prevent formation of fog on a copy. The detector circuit detects an electric current caused by the bias voltage and flowing into the resistor through the developing roller, developer layer and regulating member, whereby to detect the toner density. However, in the above detector circuit, the bias voltage is divided by resistance of the developer and the resistor. The surface voltage of the developer layer formed on the developer roller is reduced, thereby making it difficult to reliably prevent formation of the fog on the copy. In order to avoid the above drawback, it may be considered to increase the level of the bias voltage. However, this necessitates fabrication of the developing roller having improved resistance against voltage at an increased cost. It is an object of the present invention to provide an image forming apparatus having a toner density detecting device having a simple construction and capable of detecting toner density of developer without reducing surface voltage of a layer of developer formed on a developing roller during a developing operation. Accordingly, an image forming apparatus of the invention comprises a photoreceptor, a developing roller for forming a layer of developer on a surface thereof to supply toner to the photoreceptor at a developing position, bias supply means for applying bias voltage between the developing roller and ground, a conductive line for electrically connecting the developer layer with ground, detector means for detecting toner density of the developer layer based on an amount of electric current flowing through the conductive line, and voltage control means for preventing surface voltage of the developer layer from falling below a predetermined level, despite the electric current flowing into the conductive line, at least during a developing operation. With the image forming apparatus thus constructed, the bias voltage is applied between the developing roller and ground, whereby causing electric current to flow through the conductive line. The toner density of the developer layer is detected based on the amount of electric current. Further, the reduction in the surface voltage of the developer roller is controlled by the voltage control means. Accordingly, the toner density can be detected while reliably preventing formation of fog on a copy. The voltage control means may be provided with switch means for disconnecting the developer layer from ground during the developing operation, second detector means for detecting whether the developing operation is currently executed, and control means for driving the switch means to disconnect the developer layer from ground in the case where the second detector means detects that the developing operation is currently executed. With this arrangement, the switch means disconnects the developer layer from ground during the developing operation, whereby preventing the surface voltage of the developer layer from falling below the specified level during the developing operation. Accordingly, formation of fog on a copy can be reliably prevented at lower costs. The second detector means may include charge signal generator means for detecting start and end of the driving of the charger means to generate a charge start signal and a charge end signal, and third detector means for detecting start and end of the developing operation based on a predetermined period required for a specified position of the photoreceptor to move from the charging position to the developing position, the charge start signal and the charge end signal. With this arrangement, a period during which the developing operation is executed is detected accurately with a simple construction. Accordingly, formation of fog on a copy can be prevented reliably at lower costs. Further, the voltage control means may include constant voltage generator means for applying constant voltage between the developer layer and ground. With this arrangement, since the constant voltage is applied to the developer layer, detection of the toner density can always be conducted and formation of fog on a copy can be reliably prevented. Also, the voltage control means can be constructed more simply. The constant voltage generator means may be a voltage-regulator diode. Further, the conductive line may have a connecting member in contact with the developer layer and serving as a regulating member for regulating thickness of the developer layer. These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings. In the drawings : Fig. 1 is a schematic front view in section showing an image forming apparatus embodying the invention; Fig. 2 is a diagram showing a first toner density detecting device for use in the image forming apparatus; Fig. 3 is a schematic front view in section showing an exemplary developing unit provided in the image forming apparatus; Fig. 4 is a flow chart showing a toner density detecting operation; and Fig. 5 is a diagram showing a second toner density detecting device for use in the image forming apparatus. Fig. 1 is a schematic front view in section showing an image forming apparatus embodying the invention. The image forming apparatus has at a top thereof a movable document table including a platen glass 11 and a document holder 12, and a main body below the movable table. A document to be copied is placed on an upper surface of the platen glass 11. An exposure unit, image forming unit, and fixing unit 19 are provided in the interior of the apparatus main body. The exposure unit includes light source 13 such as a lamp and a condenser lens 14. The image forming unit includes a photosensitive drum 15, charger 16, developing unit 1, transfer device 17a, separating device 17b, cleaning device 18, etc. Further, a sheet insertion guide 20 and a discharge tray 21 are provided at right and left side of the apparatus main body in the drawing of Fig. 1. Upon start of a copying operation, light emitted from the light source 13 is reflected by a document surface placed on the platen glass 11. The reflected light is projected onto the drum 15 by way of the condenser lens 14. On the other hand, the surface of the drum 15 is charged by the charger 16 and exposed to the light from the condenser lens 14, thereby forming an electrostatic latent image thereon. The electrostatic latent image is developed by developing unit 1 into a toner image, which is in turn transferred onto a copy sheet inserted along the sheet insertion guide 20 by the transfer device 17a. Then, the copy sheet is separated from the surface of the drum 15 by the separating device 17b. The copy sheet having a document image transferred thereto is transported to the fixing unit 19 wherein the transferred document image is fixed to the copy sheet. Then, the copy sheet is discharged onto the discharge tray 21. The residual toner on the surface of the drum 15 after an image forming operation is cleaned by the cleaning device 18. Next, there will be described an example of a construction of the developing unit 1 with reference to Fig. 3. The developing unit 1 is provided with a toner container 2 and developing device 6 including an agitating roller 3, developing roller 4. regulating member 5 and the like. The toner container 2 stores toner supplied from a toner cartridge 2a. A toner supply roller 2b provided in a lower portion of the toner container 2 is driven in accordance with a drive signal sent from a control unit 10 to be described later, whereby the toner is supplied to the developing device 6. The agitating roller 3 agitates and mixes bicomponent developer consisting of toner and carrier in the developing device 6. The developing roller 4 supplies the toner to the photosensitive drum 15 by forming a layer of developer D on the surface thereof. The regulating member 5 is disposed with spaced away from the surface of the developing roller 4 by a specified distance, and regulates the developer layer D to attain proper thickness. The regulating member 5, providing conductor, serves as an electrode for detecting the toner density as will be described later. Next, there will be described a first toner density detecting device provided in the developing unit 1 with reference to Fig. 2. A bias source 7 applies bias voltage to the developer layer D by way of the developing roller 4 in order to prevent formation of fog on a copy. A switch 8 and a resistor R1 are connected between the regulating member 5 and a ground. The bias voltage applied to the developing roller 4 causes an electric current to flow into the resistor R1 through the developer layer D and switch 8. As a result, a voltage is developed by the resistor R1. Since the resistance of the developer layer D varies according to the toner density of the developer layer D as mentioned above, the electric current flowing into the resistor R1 also varies according thereto. An amplifier 9, connected to the resistor R1, amplifies the voltage developed by the resistor R1 using a specified amplification factor and outputs the amplified voltage to the control unit 10 in the form of a voltage signal. The switch 8 is turned on or off in accordance with a control signal from the control unit 10. The control unit 10 includes a CPU (central processing unit) and the like, and detects the toner density of the developer layer D in accordance with the voltage signal from the amplifier 9. In the case where the detected toner density is not in excess of a predetermined value, the control unit 10 sends a drive signal to a drive unit 2c including a drive roller or the like for driving the toner supply roller 2b of the toner container 2. The toner supply roller 2b is driven in response to the drive signal from the control unit 10, whereby the toner is supplied to the developing device 6. The control unit 10 includes a detector means 10a for detecting a developing period. The developing period is a period during which the toner in the developer depositing on the developing roller 4 is attracted to the electrostatic latent image formed on the surface of the drum 15. The detector means 10a includes a charging detector and a determining device. The charging detector detects a charging start timing and a charging end timing at which driving of the charger 16 is started and completed. The determining device, driven together with the charging detector, determines a developing start timing and a developing end timing moments at which the developing operation is started and completed. The determining device includes two counters for measuring a delay period calculated based on a distance between the charging position and developing position on the surface of the drum 15, and the rotating speed of the drum 15. The one counter is started at the charging start timing while the other is started at the charging end timing. The determining device determines timings at which the respective counters complete counting at the developing start and end timings. In this way, the developing start and end timings are detected. Further, the developing period may be detected more approximately using simpler detector means. For example, detector means detects timings at which the driving of the charger 16 is started and the driving of the transfer device 17a is completed and determines these timings as developing start timing and developing end timing respectively. Alternatively, the detector means detects timings at which the moving of the document table is started and completed and determines these timings as developing start timing and developing end timing respectively. There will be described a toner density detecting operation with reference to a flow chart shown in Fig. 4. At the start of this operation flow, the switch 8 is in ON state. In Step S1, the bias supply 7 applies the bias voltage to the developing roller 4. Subsequently, the developing roller is drivingly rotated in Step S2, whereby the developer deposits on the surface of the developing roller 4 forming the developer layer D thereon. The developer layer D is regulated by the regulating member 5 so as to attain the specified thickness. In Step S3, it is distinguished whether the developing operation has been started. If the developing operation has not been started yet (NO in Step S3), this operation flow waits in Step S3. If the start of the developing operation is detected (YES in Step S3), the switch 8 is turned off upon receipt of the control signal from the control unit 10 in Step S4, whereby the resistor R1 and regulating member 5 are disconnected from each other. In other words, the voltage applied to the developer layer D on the developing roller 4 is not to be divided by the resistor R1, therefore preventing reduction in the surface voltage of the developer layer D. Thereafter, it is distinguished whether the developing operation has been completed in Step S5. If the developing operation has not been completed yet (NO in Step S5), this operation flow waits in Step S5. On the other hand, if the developing operation has been completed (YES in Step S5), the switch 8 is turned on in Step S6, whereby the resistor R1 and regulating member 5 are connected to each other. Then, the electric current caused by the bias voltage flows into the resistor R1 through the developer layer D and switch 8. The voltage developed by the resistor R1 is input to the control unit 10 in the form of a voltage signal after being amplified by the amplifier 9. The control unit 10 detects the toner density of the developer layer D in accordance with the voltage signal from the amplifier 9, and distinguishes whether the detected toner density is not in excess of the predetermined value in Step S7. If the detected toner density is not in excess of the predetermined value (YES in Step S7), in Step S8, the control unit 10 sends the drive signal to the drive unit 2c to drive the toner supply roller 2b, whereby the toner is supplied to the developing device 6 from the toner container 2. If, on the other hand, the detected toner density is in excess of the predetermined value (NO in Step S7), this flow returns to Step S2. As described above, the switch 8 is turned off to disconnect the resistor R1 and regulating member 5 from each other during the developing operation. Accordingly, it can be prevented that the voltage applied to the developer layer D on the developing roller 4 is divided by the resistor R1 to be reduced to a lower level; the surface voltage of the developer layer D is not to decrease. Therefore, formation of fog on the copy can be reliably prevented. In the first toner density detecting device, the switch 8 is kept off only during the developing operation. However, since the toner density is not to change drastically, it may be appropriate that the toner density of the developer layer D be detected at regular intervals. In this case, the switch 8 is kept on only when the toner density is detected. Even in this case, the switch 8 is kept off during the developing operation in order to prevent formation of fog on the copy. Next, there will be described a second toner detecting device of the developing unit 1 with reference to Fig. 5. wherein same numerals designate same parts shown in Fig. 2. In the second toner detecting device, a zener diode D1 is used in place of the switch 8 of the first detecting device. The zener diode D1 maintains the surface voltage of a developer layer D formed on a developing roller 4 at or above a predetermined ON-state voltage determined by characteristics thereof. Formation of fog on a copy can be prevented by setting the ON-state voltage of the zener diode D1 at or above such a voltage as not to cause fog on the copy. Further, an electric current constantly flows into a resistor R1 through the developer layer D, regulating member 5, and zener diode D1 as long as bias voltage is applied to the developing roller 4. Therefore, the toner density of the developer layer D can be detected constantly. As described above, since the toner density of the developer layer D is constantly detected by the second toner density detecting device, it is not necessary for a control unit 10 to distinguish whether the developing operation is currently executed, thereby obviating the need for a detector 10a for detecting a developing period. Accordingly, when the second toner density detecting device is adopted, operations of Steps S3 to S6 of the aforementioned flow chart can be omitted. Moreover, in the second toner density detecting device, the zener diode D1 is connected in series between the regulating member 5 and resistor R1. However, the zener diode D1 may be connected in series between the resistor R1 and ground. Even in this case, the surface voltage of the developer layer D is maintained at a voltage which is a sum of the ON-state voltage of the zener diode D1 and the voltage developed by the resistor R1. Accordingly, the surface voltage of the developer layer D is not to fall below the ON-state voltage. In the above description, the regulating member 5 serves as an electrode for detecting the toner density. However, it may be appropriate to provide a special electrode in contact with the developer layer D and to cause an electric current to flow into the resistor R1 through the electrode. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention as claimed, they should be construed as being included therein.

Claims: An image forming apparatus comprising: a photoreceptor (15); a developing roller (4) for forming a layer of developer (D) on a surface thereof to supply toner to the photoreceptor at a developing position; bias supply means (7) for applying bias voltage between the developing roller and ground; a conductive line (5,8,R1) for electrically connecting the developer layer with ground; and first detector means (10) for detecting toner density of the developer layer (D) based on an amount of electric current flowing through the conductive line; characterised by voltage control means (7,8;D1) for preventing surface voltage of the developer layer from falling below a predetermined level, despite the electric current flowing into the conductive line, at least during a developing operation. An image forming apparatus as defined in claim 1 wherein the voltage control means includes: switch means (8) provided in a specified position on the conductive line for connecting and disconnecting the developer layer to and from ground; second detector means (10a) for detecting whether the developing operation is currently executed; and control means for driving the switch means to disconnect the developer layer from ground in the case where the second detector means detects that the developing operation is currently executed. An image forming apparatus as defined in claim 2 further comprising charger means (16) for charging the photoreceptor (15) at a charging position wherein the second detector means includes: charge signal generator means for detecting start and end of the driving of the charger means to generate a charge start signal and a charge end signal; and third detector means for detecting start and end of the developing operation based on a predetermined period required for a specified position of the photoreceptor to move from the charging position to the developing position, the charge start signal and the charge end signal. An image forming apparatus as defined in claim 1 where the voltage control means includes constant voltage generator means (7,D1) provided in a specified position on the conductive line for applying constant voltage between the developer layer (D) and ground. An image forming apparatus as defined in claim 4 wherein the constant voltage generator means comprises a voltage-regulator diode (D1). An image forming apparatus as defined in any one of the preceding claims wherein the conductive line has a connecting member (5) in contact with the developer layer (D) and the connecting member serves as a regulating member for regulating the thickness of the developer layer formed on the surface of the developing roller (4).

Inventors: AKASHI MASAKUTSU, HIROBE JUNICHI, KOBAYASHI MASAHIKI, SUGAYA TSUTOMU, AKASHI, MASAKUTSU, HIROBE, JUNICHI, KOBAYASHI, MASAHIKI, SUGAYA, TSUTOMU
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO. LTD.
Application year: 1992
Main classification: G03G15
IPC codes: G03G15