Image forming apparatus

An image forming apparatus includes: a carrier, a power unit, a switch, and a controller. The carrier is configured to carry a recording medium. The power unit is configured to provide drive power to the carrier. The switch is configured to switch an operation state of the carrier between an ON state and an OFF state of a transmission of the drive power from the power unit to the carrier. The controller is configured to provide a control power to control the switch such that the control power is reduced after switching from the OFF state to the ON state.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2008-071995 filed Mar. 19, 2008. The entire content of this priority application is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an image forming apparatus, and more particularly, to the control of a solenoid switch employed therein.

BACKGROUND

A known art discloses an image forming apparatus comprising a solenoid switch for transmitting and cutting off a driving force sent from a driving source to a registration roller. In the art, a solenoid switch is turning on/off so that a paper sheet being delivered once stops at a registration roller and is held at its edge before being delivered to a transfer position at a prescribed timing.

However, the control of a solenoid switch in the art is performed merely by turning on/off the solenoid switch, and therefore, might generate mechanical noises when turning off or heat in the solenoid switch caused by larger electrical consumption used for keeping it turned on for a long time.

SUMMARY

In order to attain at least one of the above and other objectives, a technology is provided for implementing a more power-saving drive of a switch such as a solenoid switch.

This invention provides an image forming apparatus. The image forming apparatus includes: a carrier, a power unit, a switch, and a controller. The carrier is configured to carry a recording medium. The power unit is configured to provide a drive power to the carrier. The switch is configured to switch an operation state of the carrier by switching between an on state and an off state of a transmission of the drive power from the power unit to the carrier. The controller is configured to provide a control power to control the switch such that the control power is reduced after a switching from the off state to the on state.

In the image forming apparatus of the present invention, the controller is configured to provide a control power to control the switch such that the control power is reduced after a switching from the off state to the on state. This enables a power saving of the control power required for controlling the switch because the power for holding the status of the switch requires lower power than the power for switching the on-off status of the switch.

DETAILED DESCRIPTION

An image forming apparatus according to some aspects of the invention is explained hereafter via reference toFIGS. 1 to 8.

1. General Configuration of Printer

Firstly, the general configuration of a printer is described in reference toFIGS. 1 to 3.

FIG. 1is a sectional side view showing the general configuration of a color printer1as one example of an image forming apparatus of the present invention.FIG. 2is a configuration diagram schematically showing the configuration according to each moving path of a paper sheet at single-side printing, double-side printing, and manual printing of the color printer1. Additionally, in the following description, the right side inFIGS. 1 and 2is the front of the color printer1. In addition, an image forming apparatus is not limited to a color printer, and may be, for example, such as a black and white printer or a so-called MFP comprising such as a copying function.

The color printer1comprises a body casing2, and provided in the bottom thereof is a paper tray4in which a paper sheet3(one example of a recording medium) is loaded. On the upper side of the front edge of the paper tray4, a pick-up roller (one example of a carrier)5, a paper feed roller7, an auxiliary paper feed roller7a, a paper end sensor S1, and a solenoid switch for the pick-up roller (one example of a switch)61are provided.

Along with the rotation of the pick-up roller5, a paper sheet3loaded on the uppermost inside of the paper tray4is picked up, and then delivered to a registration roller6(one example of a carrier) by the paper feed roller7and the auxiliary paper feed roller7a. The paper end sensor S1detects the end of the paper sheet3, that is to be delivered to the side of the registration roller6. Also, the solenoid switch for pick-up roller (hereinafter referred to simply as “pick-up solenoid”)61turns on/off such as the rotational power to be transmitted from a motor (one example of a power unit) M to the pick-up roller5via a power transmitting mechanism (not shown).

A paper feed opening (one example of an insertion opening)8for manually paper-feeding the paper sheet3is provided in the front surface of the body casing2, and provided in the vicinity of the paper feed opening8is a manual insertion sensor (one example of a second detector) S2. The paper sheet3discharged from the paper feed opening8is detected by the manual insertion sensor S2, and then similarly carried to the registration roller6.

In addition, a before-registration sensor (one example of a first detector) S3is provided between the registration roller6and the manual insertion sensor S2, in short, in the front of the upstream side in the paper carrying direction of the registration roller6, and detects the paper sheet3carried to the front of the registration roller6. The registration roller6corrects the skew of the paper sheet3, and then delivers the same onto a belt13at a prescribed timing. In addition, an after-registration sensor S4is provided right behind the registration roller6in the downstream side, and detects the paper sheet3that has passed through the registration roller6.

Moreover, a solenoid switch for registration roller (one example of a carrier)62for turning on/off the rotation of the registration roller6is provided in the vicinity of the registration roller6. The solenoid switch for registration roller (hereinafter referred to simply as “registration solenoid”)62, as shown inFIG. 3, turns on/off the rotational power transmitted from a motor M to the registration roller6via a power transmitting mechanism74, a planetary gear75, and a registration roller gear6a. Here,FIG. 3is an explanation view schematically showing a motion of the registration solenoid62at the time of ON/OFF.

More particularly, in the OFF state of the registration solenoid62, an iron core72in a clutch71is not sucked into the registration solenoid62, and therefore the clutch71is pushed toward a direction A shown inFIG. 3by a restoring force F of a spring73and then coupled with tooth of the planetary gear75. In this time, a rotational power from the motor M is transmitted to the registration roller gear6a, thereby rotating the registration roller6. On the other hand, in the ON state of the registration solenoid62, the iron core72in the clutch71is moved into a direction B shown inFIG. 3against the restoring force F of the spring73, and therefore, the clutch71is released from the joint with the tooth of the planetary gear75, withdrawing from the planetary gear75. In this time, a rotational power from the motor M is not transmitted to the registration roller gear6a, and the registration roller6does not therefore rotate.

Next, returning now toFIG. 1, a printing unit10can include such as a belt unit11, a processing unit20, and a fixing device28. The belt unit11has a structure including a belt13stretched between a pair of front and rear support rollers12, and a drive of the belt13carries the paper sheet3placed on the belt13toward the rear side. In addition, inside of the belt13, a transferring roller14is provided in each position opposed to each photoreceptor drum26in the processing unit20, with the belt13positioned therebetween.

The processing unit20includes, for example, developing cartridges22(22Y,22M,22C, and22K) corresponding to colors (yellow, magenta, cyan, and black). A scanning unit17is independently provided in each developing cartridge22. The scanning unit17applies a laser light L emitted from a laser emitting unit (not shown) to the surface of each photoreceptor drum26corresponding to each color (respectively,26Y,26M,26C, and26K). This exposes the surface of the photoreceptor drum26based on printing data.

In each developing cartridge22, such as a toner storing chamber23for storing each color toner as a developer, a feed roller24, a developing roller25, a photoreceptor drum26, and a scorotron-type charger27are respectively provided.

The toner released from the toner-storing chamber23is fed to the developing roller25by the rotation of the feed roller24, and here, thereby being positively and triboelectrically charged between the feed roller24and the developing roller25. With the rotation, the surface of the photoreceptor drum26is, first of all, uniformly and positively charged by the charger27, and then exposed by the laser light L emitted from the scanning unit17, thereby forming an electrostatic latent image corresponding to an image ought to be formed on the paper sheet3. Next, the rotation of the developing roller25supplies the toner on the developing roller25to the surface of the photoreceptor drum26, thereby developing the electrostatic latent image as a visible image. After that, the toner image held on the surface of the photoreceptor drum26is then transferred onto the paper sheet3by a transferring bias voltage applied to the transferring roller14during the passage of the paper sheet3between the photoreceptor drum26and the transferring roller14.

The paper sheet3after the transfer is then delivered to the fixing device28by the belt unit11, and here, the transferred toner image is heat-fixed onto the paper sheet3. In this way, the paper sheet3under goes a printing processing conducted by the printing unit10based on the printing data. The heat-fixed paper sheet3is then discharged onto a catch tray29provided in the upper surface of the body casing2by means of a paper-discharging roller30(one example of a carrier), that is rotationally driven by the motor M controlled by the later-described CPU120(one example of a controller) In this case, the paper sheet3is discharged on the catch tray29in a face down manner (in a state where the surface of the paper sheet3on which a toner image has been formed is on the lower side). In addition, a solenoid switch for paper-discharging roller (one example of a switch)63is provided in the vicinity of the paper-discharging roller30, and to turn on/off the solenoid switch for paper-discharging roller (hereinafter referred to simply as “paper-discharging roller solenoid”)63switches between positive rotation and inverse rotation of the paper-discharging roller30.

Additionally, a re-carrying path51for double-side printing is provided in the lower part of the body casing2and positioned between the belt13and the paper tray4. The re-carrying path51extends longitudinally in the body casing2, and provided therein for carrying the paper sheet3to two positions on the path is a relay roller53rotationally driven by the motor M, that is controlled by the CPU120. This re-carrying path51has a front end joining in the upstream side of the registration roller6with a carrying path55in a U shape extending from the above-mentioned paper feed roller7, while having a rear end joining with a carrying path57extending from the belt13and the fixing device28.

Moreover, a discharging sensor S5is provided in the front of a discharging port63of the body casing2. In double-side printing, the paper sheet3having a toner image heat-fixed thereonto is therefore carried to the paper feeding side through the re-carrying path51by the reverse rotation of the paper-discharging roller30right after passing through the discharging sensor S5. A toner image is then formed on the back surface of the paper sheet3by the printing unit10, and the paper sheet3is discharged onto the catch tray29, with the back surface side in a face down manner (meaning the front surface side in which a toner image has been firstly formed is in a face up manner). Accordingly, in the color printer1, the above-mentioned configuration enables double-side printing for printing both surfaces of the paper sheet3, as well as single-side printing for printing only one surface of the paper sheet3delivered from the paper tray4.

2. Electrical Configuration

Next, as referring toFIG. 4, the electrical configuration of a laser printer1according to the present invention is described.

As shown inFIG. 4, the laser printer1comprises a circuit substrate100, which includes such as a CPU120, RAM121, ROM122, and a timer123. Such as a motor M, a rear end sensor S1, a manual insertion sensor S2, a before-registration sensor S3, an after-registration sensor S4, a paper-discharging sensor S5, a pick-up solenoid61, a registration solenoid62, and a paper-discharging roller solenoid63are connected with the circuit substrate100. These are all connected to the CPU120.

In addition, an interface129is provided in the circuit substrate100and enables communication with higher-level devices (for example, such as a personal computer not shown). On receiving printing data as well as printing instructions through the interface120, the CPU controls the entire color printer1to form a desired image on the paper sheet3.

The CPU120is, for example, composed of an ASIC (Application Specific Integrated Circuit) and controls such as the carrying timing of the paper sheet3, in short, the starting timing of movements for operating each of the solenoid switches61to63. In addition, such as a program storing area as well as a paper carrying data storing area are provided inside of the memory122, while various controls of the CPU120, for example, a processing program for conducting a processing according to the later-described Pulse Width Modulation (PWM) control of a solenoid switch are stored inside of the program storing area.

3. PWM Control of Solenoid Switch

Next, examples of the PWM control of the CPU120against solenoid switches are described in reference toFIGS. 5 to 8. The CPU120conducts the following PWM control of a solenoid switch in accordance with a prescribed processing program as mentioned above. And also, in the following description, the time with no particular explanation about when it is from a reference time is decided beforehand in such as experiments, and counted with, for example, a prescribed timer in the timer unit123from a prescribed reference time.

3-1. Control Example 1 of Registration Solenoid

Firstly, in Aspect 1, one control example of the registration solenoid62is described as referring toFIG. 5.

When a user inserts the paper sheet3from the manual paper feed opening8in the color printer1, and when the paper sheet3is detected by the manual insertion sensor S2at a time t0in FIG.5, the manual insertion sensor S2provides a detection signal on a low logical level to the CPU120. According to the detection signal, the CPU120then provides a PWM signal of 100% duty ratio to the registration solenoid62, thereby turning on (ON) the registration solenoid62. Then, as shown inFIG. 3, the iron core72of the clutch71is moved into the registration solenoid62such that the clutch is withdrawn from the planetary gear75.

Additionally, the duty ratio for turning on the registration solenoid62is not limited to 100%, and may be, for example, 90% or 95%. In short, any duty ratio of the PWM signal which can withdraw the clutch71from the planetary gear75by turning on the registration solenoid62may be possible.

Next, at a time t1when a predetermined period of time has passed from the time t0, the CPU120reduces the duty ratio of the PWM signal from 100% to 50%. Here, the reason for reducing the duty ratio of the PWM signal after the start of the registration solenoid62is as follows.

In short, at the time of start of the registration solenoid62, energy larger than that at the start is required for withdrawing the clutch71from coupling with the planetary gear75. This is the reason why energy for holding the clutch71in a withdrawn state after the withdrawal from the planetary gear75can be smaller than that for withdrawing the clutch71.

Therefore, in a case where the duty ratio of the PWM signal (after the start of the registration solenoid62) is reduced, the driving energy can be saved, compared with the case where the duty ratio of the PWM signal is set constantly at 100% even after the start of the registration solenoid62.

In addition, the reduced duty ratio may be at least a value that can hold the clutch71in a state withdrawn from the planetary gear75, and not limited to 50%. For example, the reduced duty ratio may be 40% or 65%, and may only have to be set in accordance with the energy required for holding. In either case, compared with a case for holding at 100% duty ratio, the driving energy can be saved.

Next, when the before-registration sensor S3detects the paper sheet3at a time t2, a detection signal on a low logical level is provided to the CPU120. When the paper sheet3moves further, the paper sheet3abuts on the registration roller6around a time t3. Next, at a time t4when a predetermined period of time has passed from the time t3, the CPU120starts driving the motor M. And then, at a time t5as a paper sheet drawing timing when a predetermined period of time has passed from the time t4, the CPU120reduces the duty ratio of the PWM signal from 50% to 0%, thereby turning off (OFF) the registration solenoid62. Then, as shown inFIG. 3, the iron core72of the clutch71is moved from the registration solenoid62by means of a restoring force F of the spring73, so that the clutch71couples with the planetary gear75. Here, the rotational power of the motor M is transmitted to the registration roller6through the power transmitting mechanism74, the planetary gear75, and the registration roller gear6to rotate the registration roller6, thereby moving the paper sheet3down into the registration roller6.

And then, at a time t6for stopping drawing the paper sheet3, the CPU120provides the PWM signal having a 100% duty ratio to the registration solenoid62, thereby turning on the registration solenoid62. Then, the iron core72of the clutch71is sucked into the registration solenoid62such that the clutch is again withdrawn from the planetary gear75. The transmission of the rotational power of the motor M to the registration roller6is then cut off to stop the rotation of the registration roller6, so that the drawing of the paper sheet3by the registration roller6is stopped, and an edge of the paper sheet3is held between the rollers.

Next, at a time t7when a predetermined period of time has passed from the time t6, the CPU120stops driving the motor M. And then, the CPU120, similar to the time t1, reduces the duty ratio of the PWM signal from 100% to, for example, 50% at a time8when a predetermined period of time has passed from the time t7for stopping the motor, and therefore, the registration solenoid62holds the clutch1in a state withdrawn from the planetary gear75.

Then, at a time t9when a predetermined period of time has passed from the time t8, the CPU120, as same as the time5, reduces the duty ratio of the PWM signal from 50% to 0%, thereby turning off (OFF) the registration solenoid62. Here, the iron core72of the clutch71is moved from the registration solenoid62by means of a restoring force F of the spring73, so that the clutch71couples with the planetary gear75. However, the registration roller6does not rotate since the motor M at this moment is stopped. In the above manner, a printing preparation is completed.

In Aspect 1, as mentioned above, the CPU120, after turning the registration solenoid62from OFF state to ON state, controls the duty ratio of the PWM signal (for example, 50%) as a driving signal of the registration solenoid62so as to be less than the value at starting (for example, 100%). This is why the driving energy for the registration solenoid62is saved, while at the same time, the heat generated from the registration solenoid is controlled.

In addition, the registration solenoid62is employed in Aspect 1 as an example of a solenoid switch, but not limited to this. The control shown in Aspect 1 for reducing the duty ratio of the PWM signal after starting a solenoid may be applied also to the pick-up solenoid61and the paper-discharging roller solenoid63, both as a solenoid switch.

3-2. Control Example 2 of Registration Solenoid

Next, in Aspect 2, another control example of the registration solenoid62is described as referring now toFIG. 6. Additionally, the times t0to t9inFIG. 6indicate the same times as those in Aspect 1. Therefore, a description repeating that in Aspect 1 is omitted, thereby describing only the elements different from those in Aspect 1.

In Aspect 2, as shown inFIG. 6, the duty ratio of the PWM signal for holding the clutch71(for example, 50%) after turning the registration solenoid62from OFF state to ON state is not set as a constant value, but changed according to conditions such as a disturbance. In short, in Aspect 2, the CPU120increases the duty ratio of the PWM signal at least after the before-registration sensor S3detected the paper sheet3.

In particular, when the before-registration sensor S3detects the paper sheet3at the time t2, the CPU120gradually increases the duty ratio of the PWM signal from 50%, in order to increase, for example, up to 80% in a period between the time t2and a time t2a. Preferably, the duty ratio is increased at every predetermined period of time by a prescribed value.

Then, in a period of time τ1from the time t2ato a time t3a, the CPU120sets the duty ratio of the PWM signal constantly at 80%. In addition, this period of time τ1includes at least the time t3when the paper sheet3abuts on the registration roller6. The reason is as below.

In short, when the paper sheet3abuts on the registration roller6, the vibration thereof is transmitted to the registration solenoid62since being positioned in the vicinity of the registration roller6, and the registration solenoid62might release its hold of the clutch71. When the hold of the clutch71is released, the clutch71is pushed out from the registration solenoid62by the restoring force F of the spring73, thereby coupling with the planetary gear75. When the motor M is driven at the time t4with the clutch71coupled with the planetary gear75, the registration roller6starts rotating and draws down the paper sheet3, with the result of drawing more of the paper sheets3than a prescribed amount. This causes a trouble in the later described printing processing. In Aspect 2, during the period of time τ1including at least the time t3when the paper sheet3abuts on the registration roller6, the duty ratio of the PWM signal is therefore increased from 50% to 80%, so that the clutch71is surely held even with vibration caused by the impact on the paper sheet3at the time of abutting on the registration roller6. Additionally, the increased duty ratio of the PWM signal is not limited to 80%, and may be at least the value that can surely hold the clutch71, possibly, for example, 75% or 85%. The period of time τ1is counted with, for example, a prescribed timer in the timer unit123.

Next, in a period between the time t3aand a time t3b, the duty ratio of the PWM signal is reduced preferably at every predetermined period of time by a prescribed value, in order to be reduced to, for example, 50%. Then, from the time t4that is subsequent to the time t3b, the same processing as in Aspect 1 is conducted.

As mentioned above, in Aspect 2, the CPU120increases the duty ratio of the PWM signal after the before-registration sensor S3detected the paper sheet3. The registration solenoid62can therefore preferably maintain the hold of the clutch71in a state withdrawn from the planetary gear75, even with a disturbance such as vibration caused by the impact on the paper sheet3at the time of abutting on the registration roller6.

3-3. Control Example 3 of Registration Solenoid

Next, in Aspect 3, another control example of the registration solenoid62is described as referring now toFIG. 7. Additionally, the times t0to t9inFIG. 7indicate the same times as those in Aspect 1. Therefore, a description repeating that in Aspect 1 is omitted, thereby describing only the elements different from those in Aspect 1.

In Aspect 3, as shown inFIG. 7, the duty ratio (for example, 50%) of the PWM signal (for holding the clutch71after the registration solenoid62is turned from OFF state to ON state) is not set as a constant value but changed according to conditions such as a disturbance. On the other hand, the duty ratio is gradually reduced when the registration solenoid62is turned from ON state to OFF state. Preferably, the duty ratio is reduced at every predetermined period of time by a prescribed value. And also, the CPU120gradually reduces the value of the duty ratio when the paper sheet3is held between the registration rollers6, at the time of warming-up. At that time, the duty ratio is preferably reduced at every predetermined period of time by a prescribed value.

In particular, when the before-registration sensor S3detects the paper sheet3at the time t2, the CPU120gradually increases the duty ratio of the PWM signal from 50% up to, for example, 80% in a period between the time t2and the time t2a. Preferably, the duty ratio is increased at every predetermined period of time by a prescribed value.

Then, unlike Aspect 2, during a period between the time t2ato a time t4a, which is subsequent to the time t4for starting the motor M, the CPU120maintains the duty ratio of the PWM signal constantly at 80%. Here, in a period until the time t4a, the CPU120sets the duty ratio of the PWM signal constantly at 80%, for the purpose of restraining the hold of clutch71to be released by the registration solenoid62due to, for example, the vibration caused by the start of the motor M at the time t4. In other words, though increasing the power consumption, by extending the period for setting the duty ratio of the PWM signal constantly at 80% (compared with Aspect 1), thus allows the influence caused by disturbances to be further restrained.

Next, in a period between a time t5and the time t4afor turning the registration solenoid62from ON state to OFF state, the duty ratio is gradually reduced from 80% to, for example, 30%. The clutch71can therefore be released from the registration solenoid62, with the restoring force of the spring3reduced. Consequently, mechanical noises generated when the clutch71is coupled with the planetary gear75can be reduced.

Also, in a period between the time t8and the time t9, the CPU120reduces the duty ratio of a state where the paper sheet3is held between the registration roller6gradually from, for example, 100% to 50%. At that time, the duty ratio is preferably reduced at every predetermined period of time by a prescribed value. In general, at the time of warming-up of the color printer1, the registration solenoid62has to be generally turned ON until the end of warming-up so that the paper sheet3may not be carried to the section subject to the warming-up in the downstream than the registration roller6in the carrying direction, however, the control as mentioned above allows the power consumption to be largely saved. When turning OFF the registration solenoid62at the time t9, the mechanical noises generated when the clutch71couples with the planetary gear75can also be reduced.

As mentioned above, in Aspect 3, the CPU120increases the duty ratio of the PWM signal at least after the before-registration sensor S3detected the paper sheet3. Therefore, the registration solenoid62can preferably maintain the hold of the clutch71in a state withdrawn from the planetary gear75, even with a disturbance such as vibrations caused by the impact on the paper sheet3at the time of abutting on the registration roller6or by the start of the motor M. Moreover, the mechanical noises generated when the registration solenoid62is turned OFF can be reduced, and at the same time, the power consumption of the registration solenoid can be saved.

3-4. Control Example 1 of Paper-Discharging Roller Solenoid

Next, in Aspect 4, one control example of the paper-discharging roller solenoid63is described as referring now toFIG. 8.

In order to perform printing processing of the color printer1, it is assumed that the paper sheet3was carried to the downstream (in the carrying direction by) the registration roller6, and here, at a time t10inFIG. 8, the after-registration sensor S4detected no-paper of the paper sheet3. Next, at a time t11, it is assumed that a paper-discharging sensor S5detected the paper sheet3, and the paper sheet3was then carried to the discharging port31by the paper-discharging roller30. At a time t12, it is also assumed that the paper-discharging sensor S5detected no-paper of the paper sheet3.

The CPU120then provides the PWM signal having 100% duty ratio to the paper-discharging roller solenoid63at a time t13after a lapse of a predetermined period of time τ2from the time t12, so as to turn on the paper-discharging roller solenoid63. Then, the direction of the rotational power from the motor M to the paper-discharging roller30is reversed by the power transmitting mechanism (not shown), thereby inversely rotating the paper-discharging roller30. The carrying direction of the paper sheet3is therefore reversed, from the discharging port31back to the side of the re-carrying path51. Additionally, the paper-discharging sensor S5is configured to detect no-paper of the paper sheet3even when the paper sheet3(with its carrying direction reversed) has passed.

Next, at a time t14when a predetermined period of time has passed from the time13, the CPU120reduces the duty ratio of the PWM signal from 100% to, for example, 50%. As mentioned, the reason for reducing the duty ratio of the PWM signal from 100% to 50% in a ON state of the paper-discharging roller solenoid63is because, similar to the registration solenoid62, when starting the paper-discharging roller solenoid63, an energy larger than the one after the start is required, however, after the start, the energy for holding the clutch of the paper-discharging roller solenoid63in a withdrawn state can be smaller than the one for withdrawing. Consequently, compared with a case where the duty ratio of the PWM signal is set constantly at 100% even after the start of the paper-discharging roller solenoid63, the driving energy can be saved.

Then, after a predetermined period of time τ3has passed from the time t13, and from a time t15before the reversed paper sheet3entering into the re-carrying path51that joins with the carrying path57, the CPU120gradually increases the duty ratio of the PWM signal from 50%. At that time, the duty ratio is preferably increased at every predetermined period of time by a prescribed value. Then, at a time t16when the reversed paper sheet3enters into the re-carrying path51that joins with the carrying path57, and if the duty ratio of the PWM signal reaches, for example, 80%, the CPU120gradually reduces the duty ratio of the PWM signal from 80% to, for example, 50%. At that time, the duty ratio is preferably reduced at every predetermined period of time by a prescribed value. The duty ratio of the PWM signal is then kept at 50%, and, at a time t17when a predetermined period of time τ4has passed from the time t15, the paper-discharging roller solenoid63is turned off. Here, the predetermined period of times τ2, τ3, and τ4are respectively counted with, for example, a corresponding prescribed timer in the timer unit123. In a period of time between the time t10and the time t17, the motor M is driven.

As mentioned, the reason for increasing the duty ratio of the PWM signal from 50% to 80% in a ON state of the paper-discharging roller solenoid63is because, when the reversed paper sheet3enters into the re-carrying path51joining with the carrying path57, the paper sheet3might contact a driving mechanism such as a carrying roller (not shown) provided in the re-carrying path51, or enter into the section of a large curvature from the section of a small curvature within the re-carrying path51. Here, when the vibration caused by the impact provided to the paper sheet3is transmitted to the paper-discharging roller30that is still holding the paper sheet3, the vibration is then transmitted to the paper-discharging roller solenoid63since being positioned in the vicinity of the paper-discharging roller30. This might cause the paper-discharging roller solenoid63to release its hold of the clutch. Releasing the clutch held by the paper-discharging roller solenoid63causes the paper-discharging roller30to positively rotate, thereby disturbing the printing processing.

In Aspect 4, the duty ratio of the PWM signal is therefore increased from 50% to 80%, so that the clutch is surely held by the paper-discharging roller solenoid63even with vibration caused by the impact on the paper sheet3at the time of entering into the re-carrying path51joining with the carrying path57. This enables the clutch to be surely held by the paper-discharging roller solenoid63even with disturbances such as vibration caused by the impact on the paper sheet3at the time of entering into the re-carrying path51joining with the carrying path57.

Other Aspects

The present invention is not limited to the aspects described in the above with reference to the accompanying figures, and, for example, the following can also be included in the technical scope of the present invention.

(1) In the above aspects, the CPU120may increase the duty ratio (the magnitude of driving signal) of the PWM signal of the solenoid switch at a high environmental temperature to be larger than that of the solenoid switch at a low environmental temperature. For example, the on-duty ratio is set at 90% when the environmental temperature is 10 degrees C., while being set at 100% when the environmental temperature is 30 degrees C.

This is because, in general, the resistance value of solenoid of a solenoid switch increases along with a rise of environmental temperature. And thus, the solenoid current relative to the same duty ratio decreases along with the suction force of the solenoid. The purpose is therefore to obtain a prescribed suction force of the solenoid without depending on the environmental temperature. In this case, even when the duty ratio is increased depending on a rise in the environmental temperature, the solenoid current does not increase, and thus, the solenoid can be controlled effectively according to the environmental temperature.

Additionally, in this case, the image forming apparatus is preferably provided with a temperature detecting member for detecting the environmental temperature to control the duty ratio of the PWM signal based on the detection result of the temperature detecting member. Alternatively, a solenoid current detecting member may be provided in order to control the duty ratio of the PWM signal based on a detection result thereof.

(2) In the above aspects, the example in which the CPU120turns on the registration solenoid62according to the detection of the paper sheet3by the manual insertion sensor S2at the time t0is shown, but is not limited to this. For example, the registration solenoid62may be turned on before the paper sheet3is detected by the manual insertion sensor S2. At that time, the CPU120preferably increases the duty ratio (the magnitude of driving signal) of the PWM signal at the time of detection of the paper sheet3by the manual insertion sensor (one example of a first detector) S2to be greater than the duty ratio at the time of detection of the paper sheet3by the manual insertion sensor (one example of a second detector) S2. In this case, even when the paper sheet3is inserted from the paper feed opening8by an user, the electricity consumption can be controlled, while at the same time allowing control of the influence on motions of such as a registration solenoid arising out of disturbances such as vibration caused by the insertion of the paper sheet3into the paper feed opening8.

(3) In the above aspects, examples are shown in which an image forming apparatus according to the present invention is applied to a color printer, however, an image forming apparatus according to the present invention can also be applied to a black-and-white printer. In such case, the duty ratio of the PWM signal of a registration solenoid (one example of a switch) is preferred to be greater than that for a color printer.

The reason for the above is, in a case of a black and white printer, an image forming unit for forming images on the paper sheet3(recording medium) is smaller compared with the one in a color printer, and thus, the registration solenoid for shifting its operation state easily gets high in temperature due to a shorter distance between a registration roller and a fixing unit compared with a color printer. Along the increasing temperature, the solenoid resistance increases easily.

(4) The configuration to gradually increase or decrease the duty ratio of the PWM signal is not limited to increasing or decreasing at every predetermined period of time by a prescribed value, in short, changing the duty ratio of the PWM signal in a constant gradient. A change gradient (changing amount) of the duty ratio of the PWM signal may be changed in accordance with time divisions.

(5) In the above aspects, the example in which the PWM signal is a driving signal of a solenoid switch (one example of a switch) while the magnitude of the driving signal is the duty ratio of the PWM signal is shown, but is not limited to this. For example, the driving signal may be a voltage signal of an alternating current, and the magnitude of the driving signal may be a voltage value thereof.