Image formation apparatus with initial voltage polarity setting

An image formation apparatus includes an image carrier; a charge member; an exposure unit; a development member; a measurement unit that measures a stop time period when a rotation of the image carrier is being stopped, or a physical amount that varies as the stop time period increases; a setting unit that sets a polarity of an initial voltage to be applied to the development member, the polarity being determined based on the stop time period or the physical amount measured by the measurement unit; and a power source unit that applies the initial voltage with the polarity set by the setting unit to the development member, at rotation start time of the image carrier.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. 2015-016723 filed on Jan. 30, 2015, entitled “IMAGE FORMATION APPARATUS”, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosure relates to an electrophotographic image formation apparatus.

2. Description of Related Art

In electrophotographic image formation apparatuses, the surface voltage of a photosensitive drum is close to 0 volt immediately after a power supply is turned on or when a development unit is started up after a long standby state. When a normal development process is executed in that state, a negative voltage is applied to a development roller to negatively charge a developer on the development roller. In this process, a potential difference between the photosensitive drum and the development roller is generated, so that the negatively charged developer on the development roller is attracted to the photosensitive drum and consequently is wastefully consumed. To cope with this situation, for example, Japanese Patent Application Publication No. 10-105016 discloses a technology of preventing a developer from being attracted to a photosensitive drum by applying a positive voltage to a development roller until a region of a peripheral surface of the photosensitive drum, where the surface voltage of the photosensitive drum is close to 0 volt, finishes passing the development roller.

SUMMARY OF THE INVENTION

In an image formation apparatus disclosed in Japanese Patent Application Publication No. 10-105016, there is a case where an operation such as printing is ended and the photosensitive drum is temporarily stopped, and immediately after the temporal stop, an operation such as printing is started again. In such a case, the photosensitive drum sometimes starts to rotate before the surface voltage of the photosensitive drum is attenuated. If a positive voltage is applied to the development roller in that condition, a developer positively charged on the development roller is strongly attracted to the surface of the negatively charged photosensitive drum, and is consequently wastefully consumed.

An object of an embodiment of the invention is to provide an image formation apparatus capable of reducing the wasteful consumption of a developer.

An aspect of the invention is an image formation apparatus that includes: an image carrier including a peripheral surface with a photosensitive element; a charge member placed facing the peripheral surface and configured to charge the peripheral surface; an exposure unit that exposes a charged region of the peripheral surface charged by the charge member with light to form an electrostatic latent image; a development member placed facing the peripheral surface at a position downstream of the charge member in a rotation direction of the image carrier, and configured to develop the electrostatic latent image with a developer; a measurement unit that measures a stop time period when a rotation of the image carrier is being stopped, or a physical amount that varies as the stop time period increases; a setting unit that sets a polarity of an initial voltage to be applied to the development member, the polarity determined based on the stop time period or on the physical amount measured by the measurement unit; and a power source unit that applies the initial voltage with the polarity set by the setting unit to the development member, at rotation start time of the image carrier.

According to the aspect of the invention, the wasteful consumption of the developer can be reduced.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the invention are described in detail with reference to the drawings. The following explanation is merely one specific example of the invention, and the invention is not limited to the aspects below. Moreover, in the invention, placements, sizes, ratios of the sizes, and the like, of respective components are not limited to those illustrated in the drawings. The explanation is made in the following order.

An example in which the polarity of a development roller is set based on a stop time period.

2. Modification Examples

First modification example: another example in which the polarity of the development roller is set based on a stop time period.

Second modification example: an example in which the polarity of the development roller is set based on the temperature of a fixation unit.

Third modification example: another example in which the polarity of the development roller is set based on the temperature of the fixation unit.

Fourth modification example: an example in which the polarity of the development roller is set based on the surface voltage of a photosensitive drum.

Fifth modification example: an example in which the polarity of the development roller is set based on the environment in a housing.

Sixth modification example: various kinds of modification examples.

FIG. 1schematically illustrates a schematic configuration example of image formation apparatus1according to one embodiment of the invention. Image formation apparatus1is a printer that forms an image on medium PM using an electrophotographic system. Media PM are, for example, single-cut sheets. Image formation apparatus1is provided with paper feed unit10, conveyance unit20, image formation unit30, transfer unit40, fixation unit50, and delivery unit60. Paper feed unit10, conveyance unit20, image formation unit30, transfer unit40, fixation unit50, and delivery unit60are provided inside housing100.

In the description, a path along which medium PM is conveyed is called conveyance path PW. In conveyance path PW, “upstream in conveyance path PW” indicates a direction toward paper feed unit10from a given component or a position located closer to paper feed unit10than the given component is located. In conveyance path PW, “downstream of conveyance path PW” indicates a direction opposite to the direction toward paper feed unit10from a given component, or a position located farther from paper feed unit10than the given component is located. In conveyance path PW, conveyance direction F indicates a direction in which medium PM travels (in other words, a direction from the upstream of conveyance path PW toward the downstream of conveyance path PW).

(Configuration of Paper Feed Unit10)

Paper feed unit10is configured to supply media PM one by one in conveyance path PW. Paper feed unit10includes, for example, paper feed tray11and pickup roller12. Paper feed tray11contains media PM being stacked. Paper feed tray11is mounted to, for example, a lower portion of image formation apparatus1in an attachable and detachable manner. Pickup roller12supplies medium PM that is contained in paper feed tray11to conveyance unit20. Pickup roller12performs a rotation operation in a direction to allow medium PM to be fed out onto conveyance path PW under the control of controller101, which is described later inFIG. 3.

Conveyance unit20is configured to convey medium PM from paper feed unit10to transfer unit40along conveyance path PW while restricting any tilting of medium PM. Conveyance unit20is placed downstream of paper feed unit10in conveyance path PW. Conveyance unit20includes, for example, pairs of registration rollers21and22, and sensors23,24, and25.

Pair of registration rollers21is placed upstream of pair of registration rollers22in conveyance path PW, and specifically, is placed between paper feed tray11and pair of registration rollers22. Pair of registration rollers21performs a contact process on medium PM that is conveyed through conveyance path PW, and thereafter conveys medium. PM in conveyance direction F along conveyance path PW. The contact process indicates a process to bring a leading edge of medium PM conveyed from paper feed unit10into contact with a pair of registration rollers21which stop rotating. While the contact process is performed, no power of motor104(which is described later inFIG. 3) that is controlled by controller101is transmitted to pair of registration rollers21. In other words, pair of registration rollers21stops rotating while performing the contact process. Then, when conveying medium PM, pair of registration rollers21performs a rotation operation in a direction to convey medium PM in conveyance direction F under the control of controller101. Sensor23is placed upstream of pair of registration rollers21in conveyance path PW. Sensor23detects a position of medium PM so as to adjust a drive timing of pair of registration rollers21. Sensor23detects, for example, medium PM being conveyed along conveyance path PW.

Pair of registration rollers22is placed downstream of pair of registration rollers21in conveyance path PW, and is placed between sensor24and sensor25, for example. Pair of registration rollers22conveys medium PM conveyed through conveyance path PW along conveyance path PW in conveyance direction F. Pair of registration rollers22performs a rotation operation to convey medium PM in conveyance direction F under the control of controller101. Sensor24is placed upstream of pair of registration rollers22and sensor25, in conveyance path PW. Sensor24detects a position of medium PM so as to adjust a drive timing of pair of registration rollers22. Sensor24detects medium PM that is conveyed through conveyance path PW. Sensor25is placed downstream of sensor24in conveyance path PW. Sensor25detects a position of medium PM so as to adjust a timing for image formation in image formation unit30. Sensor25detects medium PM that is conveyed through conveyance path PW.

(Configuration of Image Formation Unit30)

FIG. 2schematically illustrates a schematic configuration example of image formation unit30. Image formation unit30is placed downstream of conveyance unit20in conveyance path PW. Image formation unit30is configured to form an image onto peripheral surface31A of photosensitive drum31, which is described later. Image formation unit30includes photosensitive drum31, charge roller32, light emitting diode (LED) head33, development roller34, supply roller35, cartridge36, regulation blade38, and cleaning blade39, for example, as illustrated inFIG. 2. Cartridge36is filled with developer37. Photosensitive drum31corresponds to a specific example of a “photosensitive drum” of the invention. Peripheral surface31A corresponds to a specific example of a “peripheral surface” of the invention. Charge roller32corresponds to a specific example of a “charge member” of the invention. LED head33corresponds to a specific example of an “exposure unit” of the invention. Development roller34corresponds to a specific example of a “development member” of the invention. Developer37corresponds to a specific example of “developer” of the invention.

Photosensitive drum31includes peripheral surface31A with a photosensitive element (for example, an organic photosensitive element), and is a cylindrical member that can support an electrostatic latent image on peripheral surface31A. Specifically, photosensitive drum31includes a conductive support, and a photoconductive layer that covers an outer circumference (surface) thereof. The conductive support is configured to include, for example, a metal pipe made of aluminum. The photoconductive layer includes a structure, for example, in which a charge generation layer and a charge transport layer are sequentially stacked. Photosensitive drum31performs a rotation operation to convey medium PM in conveyance direction F at a predetermined circumferential speed under the control of controller101.

Charge roller32is a member (charge member) that charges peripheral surface31A of photosensitive drum31. Charge roller32is placed so as to come into contact with peripheral surface31A of photosensitive drum31, and is placed facing peripheral surface31A at first point A. First point A corresponds to a point indicated as “A” inFIG. 2. Charge roller32includes, for example, a metal shaft made of stainless steel, and a semiconducting elastic layer (for example, a semiconducting epichlorohydrin rubber layer) that covers an outer circumference (surface) thereof. Charge roller32performs a rotation operation in a direction opposite to the direction of the rotation of photosensitive drum31by the transmission of a drive force from photosensitive drum31, for example.

LED head33is an exposure device that exposes charged region R1of peripheral surface31A that is charged by charge roller32to light to form electrostatic latent image R2in charged region R1of peripheral surface31A. Charged region R1corresponds to a specific example of a “charged region” of the invention. Electrostatic latent image R2corresponds to a specific example of an “electrostatic latent image” of the invention. Note that, charged region R1and electrostatic latent image R2are illustrated inFIG. 8, which is described later. LED head33is placed facing peripheral surface31A at second point B that is positioned downstream of first point A in the rotation direction of photosensitive drum31. Second point B corresponds to a point indicated as “B” inFIG. 2. LED head33includes a plurality of LED light emitters that are arranged in the width direction of photosensitive drum31. Each LED light emitter is configured to include, for example, a light source, such as a light-emitting diode, that emits irradiation light, and a lens array that forms an image with the irradiation light on the surface of photosensitive drum31.

Development roller34is a member that supports developer37on a surface thereof, and develops electrostatic latent image R2with developer37. Development roller34is placed so as to come into contact with peripheral surface31A of photosensitive drum31, and is placed facing peripheral surface31A at third point C that is positioned downstream of second point B in the rotation direction of photosensitive drum31. Third point C corresponds to a point indicated as “C” inFIG. 2. Development roller34includes, for example, a metal shaft made of stainless steel, and a semiconducting elastic layer (for example, a semiconducting urethane rubber layer) that covers an outer circumference (surface) thereof. Development roller34performs a rotation operation in the direction opposite to the direction of the rotation of photosensitive drum31at a predetermined circumferential speed by the transmission of a drive force from photosensitive drum31, for example.

Herein, for example, assume that photosensitive drum31has a diameter of 40 mm, and charge roller32and development roller34are placed at positions forming an angle of 120° with the center of photosensitive drum31as an axis. In this arrangement, distance L1from first point A to third point C is approximately 41.89 mm.

Supply roller35is a member (supply member) that supplies developer37to development roller34, and is placed so as to come into contact with a surface (peripheral surface) of development roller34. Supply roller35includes, for example, a metal shaft, and a foaming elastic layer (for example, a silicone rubber layer) that covers an outer circumference (surface) thereof. Supply roller35performs a rotation operation in the direction opposite to the direction of rotation of development roller34by the transmission of a drive force from development roller34, for example.

Cartridge36is a container in which developer37is contained. Regulation blade38regulates the layer thickness of developer37that is supported on the surface of development roller34. Developer37is, for example, a non-magnetic one-component developer. Regulation blade38is made of, for example, a stainless steel sheet, also known as a stainless use sheet SUS, being an acronym from the Japanese Industrial Standards. Cleaning blade39scrapes off developer37remaining on the surface of photosensitive drum31. Cleaning blade39is made of, for example, a flexible rubber material or plastic material.

(Configuration of Transfer Unit40)

Transfer unit40is configured to electrostatically transfer an image (developer image) that is formed on peripheral surface31A of photosensitive drum31onto medium PM that is conveyed from conveyance unit20. Transfer unit40is configured to include, for example, a transfer roller. The transfer roller is placed facing photosensitive drum31. The transfer roller is made of, for example, a foaming semiconducting elastic rubber material.

Fixation unit50is a member that applies heat and pressure to a developer image that is formed on medium PM, after passing transfer unit40, to fix the developer image onto medium PM. Fixation unit50is placed at the downstream side of transfer unit40in conveyance path PW. Fixation unit50is configured to include, for example, upper roller51and lower roller52.

Upper roller51and lower roller52are configured for each to include a heat source (fuser heater106described later inFIG. 3) that is a heater such as a halogen lamp in the inside thereof. Upper roller51and lower roller52function as heat rollers that apply heat to the developer image on medium PM. Upper roller51performs a rotation operation to convey medium. PM in conveyance direction F under the control of controller101. The heat sources in upper roller51and lower roller52are configured to respectively control the surface temperatures of upper roller51and lower roller52by being supplied with a bias voltage that is controlled by controller101. Lower roller52is placed facing upper roller51so as to allow a pressure contact portion to be formed with upper roller51, and functions as a pressurization roller that applies pressure to the developer image on medium PM. Lower roller52may preferably include a surface layer made of an elastic material.

(Configuration of Delivery Unit60)

Delivery unit60is configured to deliver medium PM on which a developer image is fixed by fixation unit50to the outside. Delivery unit60includes, for example, pairs of conveyance rollers61,62and63, and sensor64. Pairs of conveyance rollers61,62and63deliver medium PM to the outside through conveyance path PW, and cause delivered medium PM to be stacked in external stacker100A. Pairs of conveyance rollers61,62, and63perform rotation operations to convey medium PM in conveyance direction F under the control of controller101. Pairs of conveyance rollers61,62, and63further deliver medium PM facedown to the outside, for example.

Sensor64is placed upstream of pairs of conveyance rollers61,62, and63in conveyance path PW. Sensor64detects a position of medium PM so as to adjust the drive timings of pairs of conveyance rollers61,62, and63. Sensor64detects, for example, medium PM that is conveyed through conveyance path PW.

The following describes a part of the control mechanism of image formation apparatus1, with reference toFIG. 3in addition toFIG. 1.FIG. 3is a block diagram illustrating an example of a part of the control mechanism of image formation apparatus1.

As illustrated inFIG. 1andFIG. 3, image formation apparatus1includes, as the control mechanism, for example, controller101, I/O port102, drive circuits103, motors104, drive circuit105, fuser heater106, and storage unit107. Image formation apparatus1further includes, as the control mechanism, for example, measurement unit108, setting unit109, and power source unit110. Storage unit107corresponds to a specific example of a “storage unit” of the invention. Measurement unit108corresponds to a specific example of a “measurement unit” of the invention. Setting unit109corresponds to a specific example of a “setting unit” of the invention. Power source unit110corresponds to a specific example of a “power source unit” of the invention. Controller101, I/O port102, drive circuits103, motors104, drive circuit105, fuser heater106, storage unit107, measurement unit108, setting unit109, and power source unit110are connected to control line111, for example.

Controller101controls various kinds of controlled components in image formation apparatus1via control line111, for example. I/O port102outputs control signals for driving various kinds of motors104for driving to various kinds of drive circuits103under the control of controller101. I/O port102further outputs a control signal for driving fuser heater106to drive circuit105under the control of controller101. Drive circuits103perform pulse controls of motors104that rotate various kinds of drums or various kinds of rollers under the control of I/O port102. Drive circuit103for photosensitive drum31performs a pulse control of motor104that rotates photosensitive drum31.

Drive circuit105performs a pulse control of fuser heater106. Fuser heaters106are respectively provided inside upper roller51and lower roller52, and heat upper roller51and lower roller52. Fuser heaters106stop the heating of upper roller51and lower roller52in synchronization with a stop of the rotation of photosensitive drum31, or start the heating of upper roller51and lower roller52before a start of the rotation of photosensitive drum31. Fuser heater106is, for example, a heating heater such as a halogen lamp. Storage unit107stores therein a control program for operating image formation apparatus1. Storage unit107further stores therein development voltage table120(FIG. 5A) or development voltage function130(FIG. 5B), and threshold value Tth(FIG. 4). Development voltage table120corresponds to a specific example of a “table” of the invention. Development voltage function130corresponds to a specific example of a “function” of the invention. Threshold value Tthcorresponds to a specific example of a “first threshold value” of the invention.

Next, the following describes development voltage table120and development voltage function130. Waveform (A) inFIG. 4illustrates an example of a time-dependent change in surface voltage V31at first point A of photosensitive drum31. Waveform (B) inFIG. 4illustrates an example of a time-dependent change in charge voltage V32. Charge voltage V32is a voltage to be applied to charge roller32so as to charge a surface (surface layer portion) of charge roller32. The time is elapsed toward the right side on the horizontal axes in waveforms (A) and (B) inFIG. 4. The negative voltage becomes large toward the upper side on the longitudinal axis in waveform (A) inFIG. 4. InFIG. 4, T0represents the time when the rotation of photosensitive drum31is stopped (rotation stop time), and Toffrepresents a time period when the rotation of photosensitive drum31is being stopped, that is, a time period (stop time period) from rotation stop time T0to the time when the rotation of photosensitive drum31is started (rotation start time T1). Stop time period Toffcorresponds to a specific example of a “stop time period” of the invention. Rotation start time T1corresponds to a specific example of a “rotation start time” of the invention.

Simultaneously with the stop of the rotation of photosensitive drum31, charge voltage V32is cut off and becomes 0 volt. Surface voltage V31at first point A of photosensitive drum31is then attenuated with the elapse of time, and eventually becomes 0 volt or a voltage close to 0 volt. Development voltage table120and development voltage function130contain data obtained by the measurement or the prediction of such attenuation of surface voltage V31.

FIG. 5Aillustrates an example of development voltage table120.FIG. 5Billustrates an example of development voltage function130. Development voltage table120is a table in which development voltage V34is associated with stop time period Toff. Development voltage V34is a voltage to be applied to development roller34so as to make the surface (surface layer portion) of development roller34have a negative potential. In development voltage table120, development voltage V34varies to Va, Va, Vb, . . . , Vb as stop time period Toffvaries to t1, t2, t3, . . . , tn. Development voltage function130is a function in which development voltage V34is associated with stop time period Toff. In development voltage function130, development voltage V34varies to Va, Va, Vb, . . . , Vb as stop time period Toffvaries to t1, t2, t3, . . . , tn.

Herein, Va is a voltage higher than surface voltage V31and lower than 0 volt. Va is a voltage having a negative polarity, and includes negative polar data. Vb is a voltage higher than 0 volt. Vb is a voltage having a positive polarity, and includes positive polar data. Accordingly, development voltage table120is also a table in which the polarity of development voltage V34is associated with stop time period Toff. Moreover, development voltage function130is also a function in which the polarity of development voltage V34is associated with stop time period Toff.

Next, the following describes measurement unit108, setting unit109, and power source unit110.

Measurement unit108measures stop time period Toff. Measurement unit108is a counter that measures the time by seconds, for example. Measurement unit108starts the measurement of a time, for example, when detecting a control signal that is outputted from I/O port102, and is used to stop the driving of motor104connected to photosensitive drum31. Measurement unit108outputs the measured time period (stop time period Toff) to controller101. Measurement unit108stops the measurement of time, for example, when detecting a control signal that is outputted from I/O port102and is used to stop the driving of motor104connected to photosensitive drum31. Measurement unit108may be configured separately from controller101, or may be configured as one of the functions of controller101.

Setting unit109sets the polarity of initial voltage Vi to be applied to development roller34, based on stop time period Toffmeasured by measurement unit108. Initial voltage Vi corresponds to a specific example of an “initial voltage” of the invention. Setting unit109uses development voltage table120or development voltage function130that is read from storage unit107to obtain polar data corresponding to stop time period Toffmeasured by measurement unit108, and sets the obtained polar data as the polarity of initial voltage Vi. Moreover, setting unit109uses development voltage table120or development voltage function130that is read from storage unit107to obtain polar data and development voltage data corresponding to stop time period Toffas measured by measurement unit108, and sets the polarity and a voltage value of initial voltage Vi based on the obtained polar data and development voltage data.

Setting unit109sets the polarity of initial voltage Vi to be negative when stop time period Toffmeasured by measurement unit108is not more than threshold value Tth. Setting unit109sets the polarity of initial voltage Vi to be positive when stop time period Toffmeasured by measurement unit108is more than threshold value Tth. Threshold value Tthis stop time period Toffwhen the voltage of electrostatic latent image R2on photosensitive drum31is at a predetermined value (threshold value Vth). Threshold value Vthis an upper limit value of surface voltage V31of photosensitive drum31at which developer37is less likely to be attracted to photosensitive drum31even when the polarity of initial voltage Vi is positive. Further, threshold value Tthcorresponds to stop time period t2, for example, in development voltage table120and development voltage function130.

Setting unit109sets a voltage value higher than voltage V31_R1, which is described later inFIG. 6, as a value of initial voltage Vi, when setting the polarity of initial voltage Vi to be negative. Voltage V31_R1corresponds to a specific example of a “voltage value in the charged region” of the invention. When the polarity of initial voltage Vi is set to be negative, initial voltage Vi is preferably higher by at least 100 volts than voltage V31_R1by considering the instability of surface voltage V31. Herein, when a normal voltage (printing voltage Vp) to be applied to development roller34has a value of voltage higher than voltage V31_R1, which is described later, setting unit109may set initial voltage Vi to a value equal to a value of printing voltage Vp. Printing voltage Vp is a voltage to be applied to development roller34at printing start time T3and during printing. Printing voltage Vp corresponds to a specific example of a “voltage value to be applied to the development member during printing” of the invention.

Setting unit109outputs information related to set initial voltage Vi to power source unit110. When setting unit109sets the polarity of initial voltage Vi, setting unit109outputs information related to the set polarity of initial voltage Vi to power source unit110. When setting unit109sets the polarity and a voltage value of initial voltage Vi, setting unit109outputs information related to the set polarity and voltage value of initial voltage Vi to power source unit110. Further, when the value of negative voltage capable of being outputted as initial voltage Vi is limited to one value of negative voltage in power source unit110, setting unit109does not necessarily provide the one value of negative voltage to power source unit110. Accordingly, in such a case, setting unit109may set only the polarity of initial voltage Vi without setting a voltage value of initial voltage Vi, and output only information related to the set polarity of initial voltage Vi to power source unit110.

Power source unit110applies initial voltage Vi with the polarity set by setting unit109to development roller34at rotation start time T1of photosensitive drum31. Power source unit110applies initial voltage Vi with the polarity set by setting unit109to development roller34within a period (rotation initial period ΔTi) from rotation start time T1to transfer start time (printing start time T3) of a developer image formed by development by development roller34. Rotation initial period ΔTi corresponds to a specific example of a “period from rotation start time to the transfer start time of a developer image formed by development by the development member” of the invention. When the polarity of initial voltage Vi set by setting unit109is negative, power source unit110may apply, to development roller34, initial voltage Vi with a voltage value equal to a voltage value (printing voltage Vp) to be applied to development roller34during printing. Note that, printing start time T3and rotation initial period ΔTi are exemplified inFIG. 8, which is described later.

Power source unit110may apply initial voltage Vi to development roller34for a period that is only a part of rotation initial period ΔTi. Power source unit110may apply initial voltage Vi to development roller34only within a period, for example, from rotation start time T1to a time (initial voltage stop time T2) immediately before printing start time T3. Initial voltage stop time T2is the time when a portion of peripheral surface31A of photosensitive drum31located in first point A at rotation start time T1is moved to third point C with the rotation of photosensitive drum31. Further, initial voltage stop time T2is exemplified inFIG. 8, which is described later.

Next, the following describes an overview of an operation of image formation apparatus1. In image formation apparatus1, a developer image is formed on medium PM in the following manner. When a printing job is supplied to controller101via a communication channel from an image transfer apparatus connected to image formation apparatus1, controller101executes a printing process based on the printing job such that the respective members in image formation apparatus1perform the following operations.

Firstly, fuser heaters106start to heat upper roller51and lower roller52. When upper roller51and lower roller52reach a predetermined temperature, pickup roller12separates and takes out medium PM that is contained in paper feed tray11one by one from the upper-most part, and feeds out medium PM onto conveyance path PW. Next, pair of registration rollers21corrects the skew of medium PM by the contact process, and thereafter conveys medium PM to pair of registration rollers22. Subsequently, pair of registration rollers22(or, pairs of registration rollers21,22) conveys medium PM in conveyance direction F along conveyance path PW. At this time, sensor25detects medium PM while medium PM is passing a region opposed to sensor25. When sensor25detects medium PM, an operation of image formation unit30is started, medium PM is conveyed to transfer unit40, and a developer image formed in image formation unit30in the following manner is transferred onto medium PM. In this manner, an image is printed onto medium PM.

FIG. 6schematically illustrates the respective voltages of photosensitive drum31, development roller34, and supply roller35, and the transition of developer37, when a developer image is formed. In image formation unit30, a developer image is formed by the following electrophotographic process. Firstly, power source unit110applies a charge voltage V32to charge roller32to equally charge the surface (surface layer portion) of charge roller32, and along with the charge, a portion, within peripheral surface31A of photosensitive drum31, which is in contact with charge roller32is also charged to a predetermined voltage V31_R1(for example, −600 volts). Subsequently, LED head33emits irradiation light toward the region (charged region R1) within peripheral surface31A of photosensitive drum31, which is charged to voltage V31_R1to expose peripheral surface31A of photosensitive drum31, so that electrostatic latent image R2in accordance with a printing pattern defined by the above-mentioned printing job is formed on peripheral surface31A. At this time, voltage V31_R2at a portion, within peripheral surface31A of photosensitive drum31, which corresponds to electrostatic latent image R2becomes, for example, approximately 0 volt.

Meanwhile, power source unit110applies a supply voltage V35to supply roller35to cause the surface (surface layer portion) of supply roller35to have a predetermined voltage (for example, −400 volts). Similarly, power source unit110applies development voltage V34to development roller34to cause the surface (surface layer portion) of development roller34to have a predetermined voltage (for example, −300 volts). At this time, supply roller35and development roller34that come into contact with each other respectively rotate at predetermined circumferential speeds. This causes negatively charged developer37to be attracted to development roller34due to a potential difference between supply voltage V35and a development voltage V34. As a result, developer37is supplied from the surface of supply roller35to the surface of development roller34. Subsequently, developer37on development roller34is charged due to friction or the like of regulation blade38that comes into contact with development roller34. Herein, the thickness of developer37on development roller34is determined based on development voltage V34, supply voltage V35, a pressing pressure by regulation blade38, and the like. Moreover, development roller34and photosensitive drum31that come into contact with each other respectively rotate at predetermined circumferential speeds. This causes negatively charged developer37to be attracted to photosensitive drum31due to a potential difference between development voltage V34and voltage V31_R2at the portion, within peripheral surface31A of photosensitive drum31, which corresponds to electrostatic latent image R2. As a result, developer37is adhered onto electrostatic latent image R2on photosensitive drum31. Further, negatively charged developer37is not attracted to charged region R1because voltage V31_R1at the portion, within peripheral surface31A of photosensitive drum31, which corresponds to charged region R1is lower than development voltage V34.

Thereafter, a developer image on photosensitive drum31is transferred onto medium PM due to an electric field between photosensitive drum31and the transfer roller in transfer unit40. Further, cleaning blade39scrapes off and removes developer remaining on the surface of photosensitive drum31. Subsequently, fixation unit50applies heat and pressure to the developer image on medium PM to fix the developer image onto medium PM.

The following describes an operation of image formation apparatus1in detail. Hereinafter, specifically, an operation of image formation apparatus1when photosensitive drum31starts a rotation from a stop state is described in detail. Note that, hereinafter, it is assumed that measurement unit108is a counter that measures the time in units of one second, and measurement unit108is configured as one of the functions of controller101.

FIG. 7illustrates an example of a procedure of the operation of image formation apparatus1.FIGS. 8 and 9illustrate examples of various kinds of waveforms in image formation apparatus1. Waveforms (A) inFIGS. 8 and 9illustrate examples of the waveforms of surface voltage V31at third point C of photosensitive drum31. Waveforms (B) inFIGS. 8 and 9illustrate examples of the waveforms of drive voltage V104to be applied to motor104connected to photosensitive drum31. Waveforms (C) inFIGS. 8 and 9illustrate examples of the waveforms of charge voltage V32. Waveforms (D) inFIGS. 8 and 9illustrate examples of the waveforms of development voltage V34. Waveforms (E) inFIGS. 8 and 9illustrate examples of the waveforms of supply voltage V35. InFIGS. 8 and 9, ON (+) indicates that a voltage to be applied has a positive voltage value, whereas ON (−) indicates that a voltage to be applied has a negative voltage value. Moreover, inFIGS. 8 and 9, ΔTα indicates a time period (passage time period) necessary for a region from first point A to third point C in peripheral surface31A of photosensitive drum31to pass third point C with the rotation of photosensitive drum31.

Firstly, when controller101detects a power supply of image formation apparatus1being turned on, measurement unit108sets stop time period Toffat Tmax(Step S101). Tmaxis a value not less than threshold value Tth. Stop time period Toffis set at Tmaxbecause measurement unit108cannot measure the time while the power supply of image formation apparatus1is turned off. Moreover, stop time period Toffis set at Tmaxbecause an actual stop time period of photosensitive drum31is considered to be more than threshold value Tth. When measurement unit108measures stop time period Tofffrom rotation stop time T0while the power supply of image formation apparatus1is kept on, Step S101above is omitted.

Next, controller101determines whether the above-mentioned printing job is present (Step S102). If the above-mentioned printing job is not present, in other words, before controller101accepts the above-mentioned printing job, controller101determines whether stop time period Toffis less than Tmax(Step S103). As a result, if stop time period Toffis less than Tmaxand one second has passed from a previous count by a counter, controller101adds 1 to stop time period Toff(Step S104). On the other hand, if stop time period Toffis not less than Tmax, controller101returns the processing to Step S102. Meanwhile, if stop time period Toffis less than Tmaxand one second has not passed from the previous count by the counter, controller101returns the processing to Step S102without adding 1 to stop time period Toff.

If the power supply of image formation apparatus1is detected as being turned on, stop time period Toffis always equal to Tmaxirrespective of a timing when the above-mentioned printing job is inputted. On the other hand, if the power supply of image formation apparatus1is kept on and the counter measures stop time period Tofffrom rotation stop time T0, stop time period Toffmay be less than Tmaxor not less than Tmaxdepending on the timing when the above-mentioned printing job is inputted. Note that,FIG. 8illustrates examples of various kinds of waveforms (A) to (E) produced when stop time period Toffis not less than Tmax. Moreover,FIG. 9illustrates examples of various kinds of waveforms (A) to (E) produced when stop time period Toffis less than Tmax.

When the above-mentioned printing job is inputted and sensor25detects medium PM, controller101instructs I/O port102to output a control signal for driving motor104connected to photosensitive drum31. In response to the instruction, I/O port102outputs a control signal for driving motor104connected to photosensitive drum31to drive circuit103provided to photosensitive drum31under the control of controller101. This causes drive circuit103provided to photosensitive drum31to output drive voltage V104to motor104connected to photosensitive drum31. As a result, the rotation of motor104connected to photosensitive drum31is started (Step S105, T1). At this time, controller101instructs power source unit110to output charge voltage V32(for example, −800 volts) that charges charge roller32. In response to the instruction, power source unit110starts to apply charge voltage V32to charge roller32(Step S106, T1). This causes charge roller32to be negatively charged, for example, and first point A on peripheral surface31A of photosensitive drum31to have a negative voltage from charge roller32.

Controller101instructs setting unit109to compare stop time period Toffwith threshold value Tth. In response to the instruction, setting unit109starts to compare stop time period Toffwith threshold value Tth. Specifically, setting unit109determines whether stop time period Toffis more than threshold value Tth(Step S107). As a result, if stop time period Toffis more than threshold value Tth, controller101instructs power source unit110to output a positive voltage (initial voltage Vi) as development voltage V34at which development roller34is charged. In response to the instruction, power source unit110starts to apply a positive voltage (for example, an initial voltage Vi of +150 volts) to development roller34(Step S108, T1). This causes development voltage V34to become initial voltage Vi of +150 volts, for example, as illustrated inFIG. 10A. As a result, negatively charged developer37is attracted to the positive voltage of development roller34, so that developer37on development roller34does not move to peripheral surface31A of photosensitive drum31.

Moreover, as a result of the above-mentioned determination, if stop time period Toffis not more than threshold value Tth, controller101instructs power source unit110to output a negative voltage (initial voltage Vi) as development voltage V34at which development roller34is charged. In response to the instruction, power source unit110starts to apply a negative voltage (for example, an initial voltage Vi of −300 volts) to development roller34(Step S111). This causes development voltage V34to become initial voltage Vi of −300 volts, for example, as illustrated inFIG. 10B. As a result, negatively charged developer37is not attracted to charged region R1because voltage V31_R1at the portion, within peripheral surface31A of photosensitive drum31, which corresponds to charged region R1, becomes lower than development voltage V34.

After Step S108is executed, controller101determines whether photosensitive drum31rotates for passage time period ΔTα (Step S109). If photosensitive drum31rotates for passage time period ΔTα, controller101instructs power source unit110to stop the output of the positive voltage (initial voltage Vi). In response to the instruction, power source unit110stops the output of the positive voltage (initial voltage Vi) (Step S110, T2).

After Step S110is executed, or the application of a negative voltage (initial voltage Vi) to development roller34is executed because stop time period Toffis not more than threshold value Tth, controller101executes the following control. Firstly, at a predetermined timing, controller101instructs power source unit110to output a negative voltage (printing voltage Vp) as development voltage V34, and instructs power source unit110to output a negative voltage (for example, −400 volt) as supply voltage V35. In response to the instruction, power source unit110starts to apply a negative voltage (for example, a printing voltage Vp of −300 volts) to development roller34(Step S111, T3). This causes development roller34to have a negative voltage (for example, printing voltage Vp of −300 volts). Power source unit110further starts to apply a negative voltage (for example, supply voltage V35of −400 volts) to supply roller35(Step S111, T3). This causes supply roller35to have a negative voltage (for example, supply voltage V35of −400 volts). The timing when development voltage V34changes to printing voltage Vp and the timing when supply voltage V35is supplied to supply roller35are identical with each other, as illustrated inFIG. 8, for example.

Thereafter, controller101determines whether a stop request of printing is made (Step S112). Controller101repeatedly executes Step S112before the stop request of printing is made. If the stop request of printing is made, controller101instructs I/O port102to output a control signal for stopping the driving of motor104connected to photosensitive drum31. In response to the instruction, I/O port102outputs a control signal for stopping the driving of motor104connected to photosensitive drum31to drive circuit103provided to photosensitive drum31under the control of controller101. This causes drive circuit103provided to photosensitive drum31to stop the supply of drive voltage V104to motor104connected to photosensitive drum31. As a result, the rotation of motor104connected to photosensitive drum31is stopped (Step S113, T0).

At this time, controller101instructs power source unit110to stop the outputs of charge voltage V32, development voltage V34, and supply voltage V35. In response to the instruction, power source unit110stops the supply of charge voltage V32, development voltage V34, and supply voltage V35respectively to charge roller32, development roller34, and supply roller35(Step S114). Lastly, controller101sets stop time period Toffat 0 (Step S115).

Next, the following describes an effect of image formation apparatus1. Generally, an electrophotographic image formation apparatus is configured such that the surface voltage of a photosensitive drum is close to 0 volt immediately after a power supply is turned on or when a development unit is started up after a long standby state. When a normal development process is executed in that state, a negative voltage is applied to a development roller to negatively charge a developer on the development roller. At this time, a potential difference between the photosensitive drum and the development roller is generated, so that the negatively charged developer on the development roller is attracted to the photosensitive drum, and is consequently consumed wastefully. To cope with this situation, for example, it is conceivable to prevent a developer from being attracted to a photosensitive drum by applying a positive voltage to a development roller until a region of a peripheral surface of the photosensitive drum, where the surface voltage of the photosensitive drum is close to 0 volt, finishes passing the development roller.

FIG. 11illustrates examples of various kinds of waveforms in an image formation apparatus according to a comparative example.FIG. 12illustrates an example of the respective voltages (V31, V34, and V35) of photosensitive drum31, development roller34, and supply roller35, and the transition of developer37, when a developer image is formed. InFIG. 11, positive initial voltage Vi is applied to development roller34until a region of peripheral surface31A of photosensitive drum31, where surface voltage V31of photosensitive drum31is close to 0 volt, finishes passing development roller34.

Meanwhile, in a case where an operation such as printing is ended and the photosensitive drum31is temporarily stopped, and immediately after the temporal stop, the operation such as printing is started again, the photosensitive drum31starts to rotate before the surface voltage of the photosensitive drum31is attenuated, in some cases. In this case, as illustrated in waveform (D) ofFIG. 11, assume that a positive initial voltage Vi is applied to development roller34. At this time, for example, as illustrated inFIG. 12, developer37positively charged on development roller34is strongly attracted to the negatively charged surface of photosensitive drum31. This results in the wasteful consumption of developer37in the image formation apparatus according to the comparative example.

Meanwhile, in image formation apparatus1, the polarity of initial voltage Vi to be applied to development roller34is set based on stop time period Toffthat is a period when the rotation of photosensitive drum31is stopped. Specifically, the polarity corresponding to stop time period Toffmeasured by measurement unit108is set as the polarity of initial voltage Vi, using development voltage table120or development voltage function130that is read from storage unit107. If stop time period Toffmeasured by measurement unit108is not more than threshold value Tth, the polarity of initial voltage Vi is set to be negative. If stop time period Toffmeasured by measurement unit108is more than threshold value Tth, the polarity of initial voltage Vi is set to be positive. The polarity of initial voltage Vi is set in this manner in image formation apparatus1to prevent developer37on development roller34from being attracted to the surface of photosensitive drum31, as illustrated inFIG. 10AandFIG. 10B, for example. As a result, the wasteful consumption of developer37can be reduced.

The following describes modification examples of image formation apparatus1in the above-mentioned embodiment. Note that, hereinafter, the components common to those in the above-mentioned embodiment are assigned with the same reference numerals that are assigned in the above-mentioned embodiment. Moreover, explanations are made mainly to the components different from those in the above-mentioned embodiment, and explanations for the components common to those in the above-mentioned embodiment are omitted, as appropriate.

In a first modification example, polar table140and polar function150are respectively used instead of development voltage table120and development voltage function130. Storage unit107stores therein polar table140and polar function150instead of development voltage table120and development voltage function130. Polar table140corresponds to a specific example of a “table” of the invention. Polar function150corresponds to a specific example of a “function” of the invention.

FIG. 13Aillustrates an example of polar table140.FIG. 13Billustrates an example of polar function150. Polar table140is a table in which polarity P34of development voltage V34is associated with stop time period Toff. In polar table140, polarity P34of development voltage V34is minus (−) when stop time period Toffis t1and t2, and polarity P34of development voltage V34is plus (+) when stop time period Toffis t3to tn. Polar function150is a function in which polarity P34of development voltage V34is associated with stop time period Toff. In polar function150, polarity P34of development voltage V34is minus (−) when stop time period Toffis t1and t2, and polarity P34of development voltage V34is plus (+) when stop time period Toffis t3to tn.

In the first modification example, setting unit109sets the polarity of initial voltage Vi to be applied to development roller34, based on stop time period Toffmeasured by measurement unit108. Setting unit109uses polar table140or polar function150that is read from storage unit107to set polar data (P34) corresponding to stop time period Toffmeasured by measurement unit108, as the polarity of initial voltage Vi. Setting unit109sets the polarity of initial voltage Vi to be negative when stop time period Toffmeasured by measurement unit108is not more than threshold value Tth. Setting unit109sets the polarity of initial voltage Vi to be positive when stop time period Toffmeasured by measurement unit108is more than threshold value Tth. In the first modification example, threshold value Tthis t2.

In the first modification example, in image formation apparatus1, the polarity of initial voltage Vi to be applied to development roller34is set based on stop time period Toffthat is a period when the rotation of photosensitive drum31is stopped. Specifically, the polarity corresponding to stop time period Toffmeasured by measurement unit108is set as the polarity of initial voltage Vi using polar table140or polar function150that is read from storage unit107. If stop time period Toffmeasured by measurement unit108is not more than threshold value Tth, the polarity of initial voltage Vi is set to be negative. Meanwhile, if stop time period Toffmeasured by measurement unit108is more than threshold value Tth, the polarity of initial voltage Vi is set to be positive. The polarity of initial voltage Vi is set in this manner in image formation apparatus1to prevent developer37on development roller34from being attracted to the surface of photosensitive drum31, as illustrated inFIG. 10AandFIG. 10B, for example. As a result, the wasteful consumption of developer37can be reduced.

In a second modification example, development voltage table160and development voltage function170are respectively used instead of development voltage table120and development voltage function130. Storage unit107stores therein development voltage table160and development voltage function170instead of development voltage table120and development voltage function130. Storage unit107further stores therein threshold value Tfth. Development voltage table160corresponds to a specific example of a “table” of the invention. Development voltage function170corresponds to a specific example of a “function” of the invention. Threshold value Tfthcorresponds to a specific example of a “second threshold value” of the invention.

In the second modification example, measurement unit108measures temperature T60of fixation unit50that fixes developer37onto medium PM. Temperature T60of fixation unit50is a parameter that decreases as stop time period Toffincreases. Medium PM corresponds to a specific example of a “medium” of the invention. Temperature T60of fixation unit50corresponds to a specific example of a “physical amount” of the invention. Measurement unit108is, for example, a temperature sensor. Measurement unit108starts the measurement of temperature T60of upper roller51or lower roller52, for example, when detecting a control signal that is outputted from I/O port102and is used to stop the driving of motor104connected to photosensitive drum31. Measurement unit108outputs measured temperature T60to controller101. Measurement unit108further stops the measurement of temperature T60of upper roller51or lower roller52, for example, when detecting a control signal that is outputted from I/O port102and is used to start the driving of motor104connected to photosensitive drum31.

Waveform (A) inFIG. 14Aillustrates an example of time-dependent change in surface voltage V31at first point A of photosensitive drum31. Waveform (B) inFIG. 14Billustrates an example of time-dependent change in charge voltage V32. Note that, the horizontal axes in waveforms (A) and (B) inFIG. 14indicate temperature T60of fixation unit50that is correlated with the time, instead of the time. The temperature of fixation unit50become lower toward the right side on the horizontal axes in waveform (A) and (B) inFIG. 14. The negative voltage becomes large toward the upper side on the longitudinal axis in waveform (A) inFIG. 14A.

Simultaneously with the stop of the rotation of photosensitive drum31, charge voltage V32is cut off and becomes 0 volt. Surface voltage V31at first point A of photosensitive drum31is then attenuated with the elapse of time (with the decrease in temperature of fixation unit50), and eventually becomes 0 volt or a voltage close to 0 volt. Development voltage table160and development voltage function170contain data obtained by the measurement or the prediction of such attenuation of surface voltage V3.

Herein, Va is a voltage higher than surface voltage V31and lower than 0 volt. Va is a voltage having a negative polarity, and includes negative polar data. Vb is a voltage higher than 0 volt. Vb is a voltage having a positive polarity, and includes positive polar data. Accordingly, development voltage table160is also a table in which the polarity of development voltage V34is associated with temperature T60of fixation unit50. Moreover, development voltage function170is also a function in which the polarity of development voltage V34is associated with temperature T60of fixation unit50.

Setting unit109sets the polarity of initial voltage Vi to be applied to development roller34, based on temperature T60of fixation unit50measured by measurement unit108. Setting unit109uses development voltage table160or development voltage function170that is read from storage unit107to obtain polar data corresponding to temperature T60of fixation unit50measured by measurement unit108, and sets the obtained polar data as the polarity of initial voltage Vi. Moreover, setting unit109uses development voltage table160or development voltage function170that is read from storage unit107to obtain polar data and development voltage data corresponding to temperature T60of fixation unit50measured by measurement unit108, and sets the polarity and a voltage value of initial voltage Vi based on the obtained polar data and development voltage data.

Setting unit109sets the polarity of initial voltage Vi to be negative when temperature T60of fixation unit50measured by measurement unit108is not less than threshold value Tfth. Setting unit109sets the polarity of initial voltage Vi to be positive when temperature T60of fixation unit50measured by measurement unit108is less than threshold value Tfth. Threshold value Tfthis temperature T60of fixation unit50when the voltage of electrostatic latent image R2on photosensitive drum31is at a predetermined value (threshold value Vth). Note that, threshold value Tfthcorresponds to temperature Tf2, for example, in development voltage table160and development voltage function170.

In the second modification example, in image formation apparatus1, the polarity of initial voltage Vi to be applied to development roller34is set based on temperature T60of fixation unit50. Specifically, the polarity corresponding to temperature T60of fixation unit50measured by measurement unit108is set as the polarity of initial voltage Vi, using development voltage table160or development voltage function170that is read from storage unit107. If temperature T60of fixation unit50measured by measurement unit108is not less than threshold value Tfth, the polarity of initial voltage Vi is set to be negative. If temperature T60of fixation unit50measured by measurement unit108is less than threshold value Tfth, the polarity of initial voltage Vi is set to be positive. The polarity of initial voltage Vi is set in this manner in image formation apparatus1to prevent developer37on development roller34from being attracted to the surface of photosensitive drum31, as illustrated inFIG. 10AandFIG. 10B, for example. As a result, the wasteful consumption of developer37can be reduced.

In a third modification example, polar table180and polar function190are used instead of development voltage table120and development voltage function130, respectively. Storage unit107stores therein polar table180and polar function190instead of development voltage table120and development voltage function130. Storage unit107further stores therein threshold value Tfth. Polar table180corresponds to a specific example of a “table” of the invention. Polar function190corresponds to a specific example of a “function” of the invention. Threshold value Tfthof the invention is a specific example of a “second threshold value”.

FIG. 16Aillustrates an example of polar table180.FIG. 16Billustrates an example of polar function190. Polar table180is a table in which polarity P34of development voltage V34is associated with temperature T60of fixation unit50. In polar table180, polarity P34of development voltage V34is minus (−) when temperature T60of fixation unit50is Tf1and Tf2, and polarity P34of development voltage V34is plus (+) when temperature T60of fixation unit50is Tf3to Tfn. Polar function190is a function in which polarity P34of development voltage V34is associated with temperature T60of fixation unit50. In polar function190, polarity P34of development voltage V34is minus (−) when temperature T60of fixation unit50is Tf1and Tf2, and polarity P34of development voltage V34is plus (+) when temperature T60of fixation unit50is Tf3to Tfn.

In the third modification example, setting unit109sets the polarity of initial voltage Vi to be applied to development roller34, based on temperature T60of fixation unit50measured by measurement unit108. Setting unit109uses polar table180or polar function190that is read from storage unit107to set polar data (P34) corresponding to temperature T60of fixation unit50measured by measurement unit108, as the polarity of initial voltage Vi. Setting unit109sets the polarity of initial voltage Vi to be negative when temperature T60of fixation unit50measured by measurement unit108is not less than threshold value Tfth. Setting unit109sets the polarity of initial voltage Vi to be positive when temperature T60of fixation unit50measured by measurement unit108is less than threshold value Tfth. In the modification example, threshold value Tfthis Tf2.

In the third modification example, in image formation apparatus1, the polarity of initial voltage Vi to be applied to development roller34is set based on temperature T60of fixation unit50. Specifically, the polarity corresponding to temperature T60of fixation unit50measured by measurement unit108is set as the polarity of initial voltage Vi, using polar table180or polar function190that is read from storage unit107. If temperature T60of fixation unit50measured by measurement unit108is not less than threshold value Tfth, the polarity of initial voltage Vi is set to be negative. If temperature T60of fixation unit50measured by measurement unit108is less than threshold value Tfth, the polarity of initial voltage Vi is set to be positive. The polarity of initial voltage Vi is set in this manner in image formation apparatus1to prevent developer37on development roller34from being attracted to the surface of photosensitive drum31, as illustrated inFIG. 10AandFIG. 10B, for example. As a result, the wasteful consumption of developer37can be reduced.

In a fourth modification example, development voltage table210and development voltage function220are used instead of development voltage table120and development voltage function130, respectively. Storage unit107stores therein development voltage table210and development voltage function220instead of development voltage table120and development voltage function130. Development voltage table210corresponds to a specific example of a “table” of the invention. Development voltage function220corresponds to a specific example of a “function” of the invention.

In the fourth modification example, measurement unit108measures surface voltage V31of photosensitive drum31. Surface voltage V31of photosensitive drum31is a parameter that decreases as stop time period Toffincreases. Surface voltage V31of photosensitive drum31corresponds to a specific example of a “physical amount” of the invention. Measurement unit108is, for example, a voltmeter. Measurement unit108starts the measurement of surface voltage V31of photosensitive drum31, for example, when detecting a control signal that is outputted from I/O port102and is used to stop the driving of motor104connected to photosensitive drum31. Measurement unit108outputs measured surface voltage V31to controller101. Measurement unit108further stops the measurement of surface voltage V31of photosensitive drum31, for example, when detecting a control signal that is outputted from I/O port102and is used to start the driving of motor104connected to photosensitive drum31.

FIG. 17Aillustrates an example of development voltage table210.FIG. 17Billustrates an example of development voltage function220. Development voltage table210is a table in which development voltage V34is associated with surface voltage V31. In development voltage table210, development voltage V34varies to Va, Va, Vb, . . . , Vb as surface voltage V31of photosensitive drum31varies to V1, V2, V3, . . . , Vn. Development voltage function220is a function in which development voltage V34is associated with surface voltage V31. In development voltage function220, development voltage V34varies to Va, Va, Vb, . . . , Vb as surface voltage V31of photosensitive drum31varies to V1, V2, V3, . . . , Vn.

Herein, Va is a voltage higher than surface voltage V31and lower than 0 volt. Va is a voltage having a negative polarity, and includes negative polar data. Vb is a voltage higher than 0 volt. Vb is a voltage having a positive polarity, and includes positive polar data. Accordingly, development voltage table210is also a table in which the polarity of development voltage V34is associated with surface voltage V31of photosensitive drum31. Moreover, development voltage function220is also a function in which the polarity of development voltage V34is associated with surface voltage V31of photosensitive drum31.

Setting unit109sets the polarity of initial voltage Vi to be applied to development roller34, based on surface voltage V31of photosensitive drum31measured by measurement unit108. Setting unit109uses development voltage table210or development voltage function220that is read from storage unit107to obtain polar data corresponding to surface voltage V31of photosensitive drum31measured by measurement unit108, and sets the obtained polar data as the polarity of initial voltage Vi. Moreover, setting unit109uses development voltage table210or development voltage function220that is read from storage unit107to obtain polar data and development voltage data corresponding to surface voltage V31of photosensitive drum31measured by measurement unit108, and sets the polarity and a voltage value of initial voltage Vi based on the obtained polar data and development voltage data.

Setting unit109sets the polarity of initial voltage Vi to be negative when surface voltage V31of photosensitive drum31measured by measurement unit108is not less than threshold value Vth. Setting unit109sets the polarity of initial voltage Vi to be positive when surface voltage V31of photosensitive drum31measured by measurement unit108is less than threshold value Vth.

In the fourth modification example, in image formation apparatus1, the polarity of initial voltage Vi to be applied to development roller34is set based on surface voltage V31of photosensitive drum31. Specifically, the polarity corresponding to surface voltage V31of photosensitive drum31measured by measurement unit108is set as the polarity of initial voltage Vi, using development voltage table210or development voltage function220that is read from storage unit107. If surface voltage V31of photosensitive drum31measured by measurement unit108is not less than threshold value Vth, the polarity of initial voltage Vi is set to be negative. If surface voltage V31of photosensitive drum31measured by measurement unit108is less than threshold value Vth, the polarity of initial voltage Vi is set to be positive. The polarity of initial voltage Vi is set in this manner in image formation apparatus1to prevent developer37on development roller34from being attracted to the surface of photosensitive drum31, as illustrated inFIG. 10AandFIG. 10B, for example. As a result, the wasteful consumption of developer37can be reduced.

In the above-mentioned embodiment and the above-mentioned first to third modification examples, development voltage tables120and160, development voltage functions130and170, polar tables140and180, and polar functions150and190may be provided in consideration of the environment (temperature or humidity) inside housing100. In this case, measurement unit108is preferably configured to include a sensor that measures the temperature or the humidity.

The attenuation characteristic of surface voltage V31when the rotation of photosensitive drum31is stopped differs depending on the environment (temperature or humidity) inside housing100, in a strict sense. Therefore, development voltage tables120and160, development voltage functions130and170, polar tables140and180, and polar functions150and190preferably include parameters related to the environment (temperature or humidity) inside housing100.

The attenuation characteristic of surface voltage V31when the rotation of photosensitive drum31is stopped has such a tendency that the attenuation amount becomes large at high temperature and high humidity, whereas the attenuation amount becomes small at low temperature and low humidity. In other words, the attenuation time of surface voltage V31is short at high temperature and high humidity, and is long at low temperature and low humidity.

Therefore, controller101preferably adjusts the above-mentioned parameters such that the polarity and the voltage value of initial voltage Vi at high temperature and high humidity have values adapted to the attenuation characteristic when the attenuation amount is large. Controller101preferably adjusts, for example, threshold value Tthto a smaller value or threshold value Tfthto a higher value.

Moreover, controller101preferably adjusts the above-mentioned parameters such that the polarity and the voltage value of initial voltage Vi at low temperature and low humidity have values adapted to the attenuation characteristic when the attenuation amount is small. Controller101preferably adjusts, for example, threshold value Tthto a larger value or threshold value Tfthto a lower value, at low temperature and low humidity.

In the modification examples, development voltage tables120and160, development voltage functions130and170, polar tables140and180, and polar functions150and190are provided in consideration of the environment (temperature or humidity) inside housing100. This allows the polarity and the voltage value of initial voltage Vi to be set in accordance with the environment (temperature or humidity) inside housing100. As a result, even when the environment (temperature or humidity) inside housing100is changed, the wasteful consumption of developer37can be reduced.

The following describes various kinds of modification examples.

In the above-mentioned embodiment and the modification examples thereof, although an image is transferred by a direct method, an image may be transferred by an indirect method. Moreover, in the above-mentioned embodiment, a monochromatic image formation unit30is used. However, in the above-mentioned embodiment and the modification examples thereof, a multicolor image formation unit30may be used. Moreover, in the above-mentioned embodiment, LED head33is used. However, in the above-mentioned embodiment and the modification examples thereof, a laser element or the like may be used, instead of LED head33or together with LED head33.

A series of processes described in the above-mentioned embodiment and the modification examples thereof may be implemented by hardware (circuit) or may be implemented by software (program). If the above-mentioned series of processes is implemented by software, the software is configured to include a group of programs causing a computer to execute the functions. The programs may be incorporated in the above-mentioned computer in advance, or may be installed in the above-mentioned computer from a network or a recording medium, for example.

In the above-mentioned embodiment and the modification examples thereof, a mode carrying out the invention is described using an electrophotographic printer as an example. However, the invention is not limited to the application to a color device or a printer, but can be applied to a typical image formation apparatus that forms an image on a conveyed medium. The invention can be applied to, for example, monochrome copiers, color copiers, monochrome MFPs, color MFPs, or the like.

In the above-mentioned embodiment, as a specific example of the “image formation apparatus” in the invention, an image formation apparatus having a printing function is described. However, the invention is not limited to the image formation apparatus having a printing function, but can be applied to an image formation apparatus that functions as a multifunction peripheral having a scanning function or a facsimile function, for example.