JP4737145B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus having a function of detecting double feeding of a recording medium.

In the image forming apparatus, during a printing operation, a plurality of sheets (recording media) are transported in an overlapped state, which may affect the printing result. In order to cope with such a situation, a technique for detecting double feeding of sheets is known. For example, in Japanese Patent Application Laid-Open No. H10-228561, the magnitude of the transfer voltage applied between the photosensitive drum and the transfer roller when the toner image formed on the photosensitive drum is transferred onto the paper is detected and the reference value is obtained. Compare. When the paper enters between the photosensitive drum and the transfer roller in a state where the paper is double fed, the transfer voltage becomes larger than that during normal transfer. Therefore, if the detected transfer voltage is larger than the reference value, the paper It is determined that transmission has occurred.
Japanese Patent Laid-Open No. 11-174861

However, in the above configuration, when the user uses paper thicker than normal paper, the transfer voltage becomes larger than the reference value as in the case of double feed, and thus it is erroneously detected that double feed has occurred. There is a risk that.
The present invention has been completed based on the above-described circumstances, and an object thereof is to provide an image forming apparatus capable of accurately detecting double feeding of a recording medium.

As a means for achieving the above object, in the image forming apparatus according to the first invention, the recording medium enters the transfer position between the image carrier carrying the developer image and the image carrier. If a transfer unit that transfers the developer image on the image bearing member onto the recording medium by a voltage applied between the image carrier, the current applied to the transfer means during the transfer to a constant when detecting means for detecting the applied voltage to the said transfer means, said at least one of when the recording medium is to be detached from and the transfer position when entering into the transfer position, the detection value detected by the detection means Discriminating means that discriminates that double feeding of the recording medium has occurred when the recording medium changes in a plurality of stages, and a plurality of sets of the image carrier and the transfer means are provided along the conveyance path of the recording medium. , The detecting means The detection unit detects the detection value for each of the copying units, and the determination unit compares the detection values detected corresponding to the transfer units with a first reference value and a second reference value, respectively. The absolute value of at least one of the first reference value and the second reference value corresponding to the transfer unit on the downstream side of the path is larger than the absolute value corresponding to the transfer unit on the upstream side of the transport path. that has been.
According to a second aspect of the present invention, there is provided an image forming apparatus between an image carrier that carries a developer image and the image carrier when a recording medium enters a transfer position between the image carrier and the image carrier. Transfer means for transferring the developer image on the image carrier onto the recording medium by the applied voltage, and current applied to the transfer means when the voltage applied to the transfer means at the time of transfer is constant And at least one of when the recording medium enters the transfer position and when the recording medium moves away from the transfer position, and when the detection value detected by the detection means changes in a plurality of steps Discriminating means for discriminating that the double feed has occurred, a plurality of sets of the image carrier and the transfer means are provided along the transport path of the recording medium, and the detection means is the transfer means For each said detection The discriminating means compares the detected values detected corresponding to the transfer means with the first reference value and the second reference value, respectively, and transfers the transfer means downstream of the transport path. The absolute value of at least one of the first reference value and the second reference value corresponding to is smaller than the absolute value corresponding to the transfer means on the upstream side of the transport path.

When a plurality of recording media enter and leave the transfer position in a double feed state in which they are overlapped with each other, in general, there is a shift in the entry timing of each recording medium to the transfer position or the release timing from the transfer position. The impedance between the transfer means and the image carrier changes in a multistage manner. Based on this, according to the first and second inventions, the voltage / current applied at the time of transfer is detected, and when the detected value changes in a plurality of stages, it is determined that double feeding has occurred. Good detection can be performed.
In addition, when multiple recording media enter and leave the transfer position in a double feed state in which they overlap each other, the developer accumulates on the recording media and becomes thicker toward the downstream side of the transport path, and the image carrier Impedance between the body and the transfer means increases. Based on this, according to the first and second inventions, the absolute value of the reference value compared with the detection value for the downstream transfer unit is increased with respect to the absolute value for the upstream transfer unit. Therefore, it is possible to absorb the influence of the increase in impedance due to the developer thickness by increasing the current detection and decreasing the voltage detection. Accordingly, detection accuracy can be ensured.

The third invention is the first or second aspect, the determination means, after the previous danger detection value reaches the first reference value, reaches the second reference value after a predetermined period of time In this case, it is determined that double feeding has occurred.

According to the third invention, when the detection value reaches the first reference value and then reaches the second reference value after a lapse of a predetermined period, it is determined that the detection value has changed in a multistage manner. Thus, accurate detection can be performed with simple processing.

A fourth invention, in the third one of the invention from the first, before after danger detection value is changed one stage, when further stepwise change to the same side after a lapse of specified time period, the transfer means And a control unit that causes the transfer of the same developer image to be executed again, and when the change has been made stepwise before the lapse of the specified period, the transfer unit completes the transfer of the developer image by the transfer unit.

According to the fourth aspect of the invention, the period after the detection value changes by one step and then changes stepwise corresponds to the amount of positional deviation of the overlapping recording media. For this reason, if the deviation amount of the recording medium to be double fed is relatively large and the deviation of the transfer position may exceed the allowable range, transfer is performed again, and the deviation amount of the recording medium is relatively small. When the deviation is within an allowable range, the user can be conveniently provided by not performing the retransfer.

  When a plurality of recording media enter and leave the transfer position in a double feed state in which they are overlapped with each other, in general, there is a shift in the entry timing of each recording medium to the transfer position or the release timing from the transfer position. The impedance between the transfer means and the image carrier changes in a multistage manner. Based on this, voltage and current applied at the time of transfer are detected, and when the detected value changes in a plurality of stages, it is determined that double feeding has occurred, so that accurate detection can be performed.

Next, an embodiment of the present invention will be described with reference to FIGS.
(Entire printer configuration)
FIG. 1 is a side sectional view showing a schematic configuration of a printer 1 which is an example of an image forming apparatus of the present invention. In the following description, the right side in FIG.

  The printer 1 includes a main body casing 2, and a supply tray 4 on which paper 3 (an example of a recording medium) is loaded is provided below the main body casing 2. At the bottom of the supply tray 4, a paper pressing plate 4B that can be tilted to lift the front end side of the paper 3 by the bias of the spring 4A is provided. Further, a pickup roller 5 and a separation pad 6 that presses against the pickup roller 5 by urging of a spring (not shown) are provided above the front end of the supply tray 4. Further, a pair of paper dust removing rollers 7 is provided obliquely in front of the pickup roller 5, and a pair of registration rollers 8 is provided thereabove.

  The sheet 3 at the top of the supply tray 4 is pressed against the pickup roller 5 by the sheet pressing plate 4B, and when the sheet 3 is sandwiched between the pickup roller 5 and the separation pad 6 by the rotation of the pickup roller 5, one sheet at a time. Separated. The paper 3 is fed to the registration roller 8 after the paper dust adhering to the surface is removed by the paper dust removing roller 7. The registration roller 8 corrects the skew of the sheet 3 and then sends the sheet 3 onto the belt unit 11 of the image forming unit 10.

  The image forming unit 10 includes a belt unit 11, a scanner unit 19, a process unit 20, a fixing unit 31, and the like.

  The belt unit 11 has a configuration in which a belt 13 made of polycarbonate or the like is stretched between a pair of front and rear belt support rollers 12. Then, the belt support roller 12 on the rear side is rotationally driven, whereby the belt 13 circulates in the counterclockwise direction in the figure, and the paper 3 placed on the upper surface of the belt 13 is conveyed backward.

  Further, inside the belt 13, transfer rollers 14 (an example of a transfer unit) are provided at positions facing each photosensitive drum 28 of the process unit 20, which will be described later, across the belt 13. The transfer roller 14 is configured by covering a metal roller shaft with a conductive rubber material. Further, the transfer roller 14 is urged against the photosensitive drum 28, and the paper 3 conveyed on the belt 13 is sandwiched between the photosensitive drum 28. That is, the distance between the transfer roller 14 and the photosensitive drum 28 is configured to change according to the thickness of the paper 3.

  The scanner unit 19 irradiates the surface of the corresponding photosensitive drum 28 with laser light L for each color with laser light emitted from a laser light emitting unit (not shown).

  The process unit 20 includes a frame 21 that can be pulled out from the main casing 2 and four developing cartridges 22 (for example, black, yellow, magenta, and cyan in order from the upstream side) that can be attached to and detached from the frame 21. 22K, 22Y, 22M, and 22C). A photosensitive drum 28 and a charger 29 are provided below the frame 21 corresponding to each developing cartridge 22.

  Each developing cartridge 22 includes a toner storage chamber 23 in an upper portion of a box-shaped casing, and a supply roller 24, a developing roller 25, and a layer thickness regulating blade 26 on the lower side thereof. Each toner storage chamber 23 stores positively charged non-magnetic one-component toner (an example of a developer) of each color.

  The supply roller 24 is configured by coating a metal roller shaft with a conductive foam material, and the developing roller 25 is configured by coating the metal roller shaft with a conductive rubber material. Yes. The toner discharged from the toner storage chamber 23 is supplied to the developing roller 25 by the rotation of the supply roller 24, and is positively frictionally charged between the supply roller 24 and the developing roller 25. Further, as the developing roller 25 rotates, the toner supplied onto the developing roller 25 enters between the layer thickness regulating blade 26 and the developing roller 25, where it is further sufficiently frictionally charged to have a constant thickness. It is carried on the developing roller 25 as a thin layer.

  The photosensitive drum 28 (an example of an image carrier) includes a grounded metal drum main body, and the surface layer thereof is covered with a positively chargeable photosensitive layer made of polycarbonate or the like.

  The charger 29 is of a scorotron type, and is provided between the discharge wire 29A and the discharge wire 29A and the photosensitive drum 28, which are opposed to the photosensitive drum 28 with a predetermined distance therebetween. And a grid 29B for controlling the amount of discharge to the drum 28. In the charger 29, a high voltage is applied to the discharge wire 29A to cause corona discharge of the discharge wire 29A, thereby making the current flowing from the discharge wire 29A to the grid 29B constant, that is, by making the grid voltage constant, The surface of the drum 28 can be uniformly charged to a positive polarity.

  At the time of image formation, the photosensitive drum 28 is rotationally driven in the clockwise direction in the drawing, and accordingly, the surface of the photosensitive drum 28 is uniformly positively charged by the charger 29. The positively charged portion is exposed by high-speed scanning of laser light from the scanner unit 19, and an electrostatic latent image corresponding to the image to be formed on the paper 3 is formed on the surface of the photosensitive drum 28.

  Next, the electrostatic latent toner formed on the surface of the photosensitive drum 28 when the positively charged toner carried on the developing roller 25 contacts the photosensitive drum 28 by the rotation of the developing roller 25. Supplied to the image. As a result, the electrostatic latent image on the photosensitive drum 28 is visualized, and a toner image with toner attached only to the exposed portion is carried on the surface of the photosensitive drum 28.

  Thereafter, the toner image carried on the surface of each photosensitive drum 28 is transferred to the transfer roller 14 while the paper 3 conveyed by the belt 13 passes through each transfer position between the photosensitive drum 28 and the transfer roller 14. The images are sequentially transferred to the paper 3 by the applied negative transfer voltage. The sheet 3 having the toner image transferred thereon is then conveyed to the fixing device 31.

  The fixing device 31 includes a heating roller 31A having a heat source and a pressure roller 31B that presses the paper 3 toward the heating roller 31A, and heat-fixes the toner image transferred onto the paper 3 on the paper surface. Then, the paper 3 thermally fixed by the fixing device 31 is conveyed upward and discharged onto a discharge tray 32 provided on the upper surface of the main casing 2.

(Electrical configuration of printer)
FIG. 2 is a block diagram schematically showing the electrical configuration of the printer 1. As shown in FIG. 1, the printer 1 includes a CPU 40 (an example of a determination unit and a control unit), a ROM 41, a RAM 42, an NVRAM (nonvolatile memory) 43, and a network interface 44. These include an operation unit 45 and a display unit. 46, a main motor 47, a high voltage application circuit 48, a temperature / humidity monitor 49, the image forming unit 10 described above, and the like are connected.

  The ROM 41 stores a program for executing various operations of the printer 1 such as a print sequence process to be described later. The CPU 40 stores the processing result in the RAM 42 or the NVRAM 43 according to the program read from the ROM 41. While controlling each part. The network interface 44 is connected to an external computer or the like via a communication line (not shown), and can receive a print command or print data transmitted from the computer or the like.

  The operation unit 45 includes a plurality of buttons, and various input operations can be performed by the user. The display unit 46 includes a liquid crystal display, a lamp, and the like, and can display various setting screens, operation states, and the like. The main motor 47 is driven by the control of the CPU 40, and synchronously rotates the supply roller 24, the registration roller 8, the belt support roller 12, the transfer roller 14, the developing roller 25, the photosensitive drum 28, the heating roller 31A, and the like. .

  The high voltage application circuit 48 includes a developing voltage applied to the developing roller 25, a transfer voltage applied to the transfer roller 14, a charging voltage applied to the charging wire 29A of the charger 29, a grid voltage applied to the grid 29B, and the like. A high voltage is applied to each part. The temperature / humidity monitor 49 measures the ambient temperature and humidity.

(Transfer voltage application circuit)
FIG. 3 is a block diagram showing a main configuration of the transfer voltage application circuit 50 included in the high voltage application circuit 48. The high voltage application circuit 48 includes a total of four sets of transfer voltage application circuits 50 corresponding to the respective transfer rollers 14. The transfer voltage application circuit 50 applies a transfer voltage to the transfer roller 14 by constant current control of the CPU 40.

  The transfer voltage application circuit 50 (an example of a detection unit) includes a PWM signal smoothing circuit 51, a transformer drive circuit 52, a boosting / smoothing rectification circuit 53, and the like. The PWM signal smoothing circuit 51 receives a PWM (Pulse Width Modulation) signal S <b> 1 from the PWM port 40 </ b> A of the CPU 40, smooths it, and provides it to the transformer drive circuit 52. The transformer drive circuit 52 causes an oscillation current to flow through the primary winding 54A of the boosting / smoothing rectifier circuit 53 based on the received PWM signal S1.

  The step-up / smoothing rectifier circuit 53 includes a transformer 54, a diode 55, a smoothing capacitor 56, and the like. The transformer 54 includes a primary winding 54A, a secondary winding 54B, and an auxiliary winding 54C. One end of the secondary winding 54B is connected to an output end 50A of the transfer voltage application circuit 50 via a diode 55 and a resistor 57, and the output end 50A is connected to the roller shaft of the transfer roller 14. Further, the smoothing capacitor 56, the resistor 57, and the resistor 58 are respectively connected in parallel to the secondary winding 54B.

  Furthermore, the output terminal 50A is connected to the ground via resistors 60 and 61, and the connection point of the resistors 60 and 61 is connected to the A / D port 40B of the CPU 40. The other end of the secondary winding 54B (the end opposite to the diode 55) is grounded via a resistor 62, and the CPU 40 is connected to the A / D port 40C.

  With such a configuration, the oscillation current of the primary winding 54 </ b> A is boosted and rectified in the boosting / smoothing rectifier circuit 53 and applied to the roller shaft of the transfer roller 14 as the transfer voltage Vc. At this time, the detection signal P1 corresponding to the transfer voltage Vc applied to the transfer roller 14 is fed back to the A / D port 40B of the CPU 40, and the detection signal P2 corresponding to the current i1 flowing through the transfer roller 14 is A of the CPU 40. / D port 40C is fed back.

(Print sequence processing)
4 to 7 are flowcharts showing the flow of the print sequence process executed under the control of the CPU 40, and FIG. 8 is a flowchart showing the flow of the transfer high-pressure control process executed during the print sequence process.

  This print sequence process is executed when a print command is received from an external computer or the like, and performs processes such as control of the voltage applied by the high voltage application circuit 48 and detection of a double feed state. In parallel with this print sequence process, a print process such as image processing of print data is executed.

  When starting the print sequence process, the CPU 40 first sets each parameter to an initial value (S101). Here, four paper passing flags (TK, TY, TM, TC), four double feed flags (JK, JY, JM, JC), four time counters (TCNT1, TCNT2, TCNT3, TCNT4) and four PWMs Set the counters (PWM1, PWM2, PWM3, PWM4) to 0. The paper passing flag (TK, TY, TM, TC) is a flag indicating that the paper 3 has reached the transfer position of each color, and the double feed flag (JK, JY, JM, JC) is set at the transfer position of each color. This is a flag indicating that the sheet 3 may have arrived in the double feed state. The time counter (TCNT1, TCNT2, TCNT3, TCNT4) is a counter for counting the time from when the paper passing flag is turned on to when the double feed flag is turned on for each color. PWM2, PWM3, PWM4) are counters for determining the duty ratio of the PWM signal S1 applied to the transfer voltage application circuit 50 for each color.

  Subsequently, the CPU 40 serves as a reference for determining the value of the target current (IK, IY, IM, IC) that is the control target of the transfer voltage application circuit 50 for each color and the arrival of the sheet 3 at the transfer position of each color. Paper feed reference voltage (VKth1, VYth1, VMth1, VCth1, example of first reference value) and double feed reference voltage used as a reference for judging that the paper 3 has reached the transfer position of each color in a double feed state (VKth2, VYth2, VMth2, VCth2, an example of the second reference value) are set (S102). The ROM 41 stores a table showing the values of the target current, paper passing reference voltage, and double feed reference voltage corresponding to the measured values of the temperature / humidity monitor 49. The CPU 40 measures the temperature / humidity monitor 49. Set each value by referring to the table based on the value. Each paper feed reference voltage (VKth1, VYth1, VMth1, VCth1) and each double feed reference voltage (VKth2, VYth2, VMth2, VCth2) take different negative values and correspond to the upstream transfer position. The absolute value of the reference voltage corresponding to the downstream transfer position is larger than the absolute value of the reference voltage (ie, | VCth1 |> | VMth1 |> | VYth1 |> | VKth1 |, | VCth2 |> | VMth2 | > | VYth2 |> | VKth2 |).

  Next, the CPU 40 applies predetermined voltages to the discharge wires 29A and the grid 29B of the charger 29 and the developing roller 25 by the high voltage application circuit 48, and starts driving the main motor 47 (S103). Then, it waits for the timing for applying the transfer voltage (S104), and when the timing for applying the transfer voltage is reached (S104: Yes), the value of each parameter used for the transfer high voltage control processing is set (S104: Yes). S105). In the transfer high-voltage control process, parameters such as a paper passing flag T, a multifeed flag J, a target current I, a paper passing reference voltage Vth1, a multifeed reference voltage Vth2, a time counter TCNT, and a PWM counter PWM are used. The values of the corresponding black parameters (TK, JK, IK, VKth1, VKth2, TCNT1, and PWM1) are substituted respectively.

  Then, the transfer high-pressure control process corresponding to the black color is started (S106). In this transfer high-voltage control process, as will be described later, constant current control of the transfer voltage Vc and detection of the sheet passing / double feed state are performed. When the transfer high-pressure control process ends, among the parameters used in this process, the values of the paper passing flag T, double feed flag J, time counter TCNT, and PWM counter PWM correspond to the corresponding black color parameters (TK, JK). , TCNT1, PWM1) (S107).

  Subsequently, processing corresponding to S105 to S107 is executed in order for yellow, magenta, and cyan (S108 to S116). Then, after waiting for 0.5 ms (S117), if it is not the print end timing (S118: No), the process returns to S105. Thereby, the processing from S105 to S118 is repeated at intervals of 0.5 ms until the printing end timing.

Here, the operation of the transfer high-pressure control process will be described with reference to FIG. When starting the transfer high-voltage control process, the CPU 40 first sets the current value fed back by the detection signal P2 to Ic, and sets the voltage value fed back by the detection signal P1 to Vc (S201). Subsequently, it is checked whether or not the value of the measured current Ic is smaller than a value obtained by subtracting the predetermined amount ΔI from the target current I (S202). If the value is smaller (S202: Yes), the predetermined value ΔPWM is added to the PWM count PWM. (S203). Accordingly, duty ratio of the PWM signal S1 to be transmitted to the transfer voltage application circuit 50 is changed, as a result, the current flowing through the transfer roller 14 is increased.

  If the measured current Ic is not smaller than the target current I minus the predetermined amount ΔI (S202: No), is the current Ic greater than the target current I plus ΔI? If it is larger (S204: Yes), a predetermined value ΔPWM is subtracted from the PWM count PWM (S205). As a result, the duty ratio of the PWM signal S1 is changed, and the current flowing through the transfer roller 14 is reduced. When the current Ic is not larger than the value obtained by adding ΔI to the target current I, that is, when the current Ic is within the range of I ± ΔI (S204: No), the PWM count is not changed. The above-described processes of S202 to S205 are repeated at an interval of 0.5 ms during the printing sequence process, whereby constant current control is performed so that the current Ic flowing through the transfer roller 14 is within the range of the target current Ic + ΔI.

  Here, FIG. 9 is a graph showing an example of a change in the transfer voltage Vc when a pair of sheets 3 enter and leave one of the transfer positions in a double feed state. It is assumed that the pair of sheets 3 are the same size and overlap with each other with a predetermined amount of positional deviation in the transport direction. After the application of the transfer voltage Vc is started, in a state where the sheet 3 has not reached the transfer position, the transfer voltage Vc is substantially constant at V0. When the leading edge of the first sheet 3 enters the transfer position, the impedance between the photosensitive drum 28 and the transfer roller 14 increases, and the absolute value of the transfer voltage Vc becomes larger than before the sheet 3 reaches the transfer position. The value increases by one step to become the value of V1, and takes a substantially constant value until the second sheet 3 reaches the transfer position.

  Subsequently, when the leading edge of the second sheet 3 enters the transfer position, the two sheets 3 are sandwiched between the photosensitive drum 28 and the transfer roller 14, so that the impedance between the two sheets 28 and 14 is further increased. The absolute value of the transfer voltage Vc is further increased by one step from the time of entering the first sheet to become the value of V2, and thereafter takes a substantially constant value until the sheet 3 leaves the transfer position. As described above, when the pair of sheets 3 enters the transfer position in the double feed state, the transfer voltage Vc changes in two steps from the value before the entry of the sheet 3, and the graph has a two-step shape.

  Further, when the sheet 3 is detached from the transfer position, the second sheet 3 is separated after the first sheet 3 is separated, so that the value of the transfer voltage Vc is first 1 from V2. Change stepwise to V1, then change one step further to V0. As described above, even when the sheet 3 leaves the transfer position in the double feed state, the transfer voltage Vc changes in two steps, and the graph has a two-step shape. When one sheet 3 is normally conveyed, the value of the transfer voltage Vc is changed by one step from the value V0 before entering the transfer position of the sheet 3 to V1, and the sheet 3 is detached. Since it will return to V0 when you enter, it will change in one step both when entering and leaving. In addition, when three or more sheets 3 overlap each other and are displaced from each other and enter or leave the transfer position, the transfer voltage changes to a number corresponding to the number of sheets 3 on which the transfer voltage overlaps.

  In this transfer high-voltage control process, two threshold values of the sheet passing reference voltage Vth1 and the double feed reference voltage Vth2 are used to detect that the transfer voltage Vc changes in a plurality of stages. The sheet passing reference voltage Vth1 is larger than the value V0 of the transfer voltage Vc before entering the sheet 3 and smaller than the value V1 of the transfer voltage Vc when one normal sheet 3 enters the transfer position. Set to a value. The double feed reference voltage Vth2 has an absolute value greater than Vth1, is larger than the transfer voltage Vc value V1 when one normal sheet 3 enters the transfer position, and two normal sheets 3 are present. The value is set to be smaller than the value V2 of the transfer voltage Vc when entering the transfer position in an overlapping state.

  After completing the processing of S202 to S205, the CPU 40 determines whether or not the double feed flag J is 1 (S206). If the double feed flag J is 1 (S206: Yes), that is, this transfer voltage control. If the double feed flag J has already been set to 1 in the process, the transfer voltage control process is directly exited. If the double feed flag J is 0 (S206: No), it is determined whether the transfer voltage Vc is greater than the sheet passing reference voltage Vth1 (S207). If the transfer voltage Vc is not greater than the sheet passing reference voltage Vth1 (S207: No), that is, if it is determined that the sheet 3 has not reached the transfer position, the process is directly exited.

  If the transfer voltage Vc is greater than the paper feed reference voltage Vth1 (S207: Yes), it is determined whether the transfer voltage Vc is greater than the double feed reference voltage Vth2 (S208). When the transfer voltage Vc is not greater than the double feed reference voltage Vth2 (S208: No), that is, when it is determined that the normal sheet 3 has entered the transfer position, the sheet passing flag T And 1 is added to the time counter TCNT (S209), and this process is exited.

  If the transfer voltage Vc is greater than the double feed reference voltage Vth2 (S208: Yes), it is subsequently checked whether the sheet passing counter T is 1 (S210). If the sheet passing counter T is not 1 (S210: No), the transfer high-pressure control process is exited. Here, when the sheet passing counter T is not 1, the transfer voltage Vc reaches the double feed reference voltage Vth2 without detecting the state where the value of the transfer voltage Vc is within the range of the paper feed reference voltage Vth1 to the double feed reference voltage Vth2. Thus, it is highly possible that a thicker paper than the normal paper 3 was used. Therefore, in this case, some measures such as notifying the user that cardboard is being used may be performed. In this process, even when a plurality of normal sheets 3 are fed, it may not be detected if the amount of positional deviation between the sheets 3 is very small.

  If the sheet passing counter T is 1 (S210: Yes), it is at least 0.5 ms after the transfer voltage Vc is detected to be a value from the sheet passing reference voltage Vth1 to the double feed reference voltage Vth2. Since the transfer voltage Vc has reached the double feed reference voltage Vth2 after the elapse of time, it is estimated that the transfer voltage Vc has changed in multiple stages. For this reason, the double feed flag J indicating that there is a high possibility that the paper 3 has been double fed is set to 1 (S211), and the transfer high-pressure control process is exited.

  In the printing sequence processing of FIGS. 4 to 7, the processing from S105 to S118 is repeated, and when the print end timing is reached (S118: Yes), the values of the PWM counters PWM1 to PWM4 are set to 0. The application of the transfer voltage to each transfer roller 14 is terminated (S119). Subsequently, the voltage application to the discharge wires 29A and the grid 29B of the charger 29 and the developing roller 25 is finished, and the drive of the main motor 47 is stopped (S120).

  Subsequently, the CPU 40 sets the number (JK + JY + JM + JC) in which the multi-feed flag J is set to 1 in the transfer high-pressure control process for each color in the parameter JUSO (S121). When the value of the parameter JUSO is not 2 or more (S122: No), that is, when the multi-feed flag J of the four-color transfer high-pressure control processing is set to 1 is 0 or 1 color. The print sequence process ends. That is, when the multi-feed flag J is set to 1 only in the transfer high-pressure control process for one color, it is possible to suppress the occurrence of erroneous detection by determining that multi-feed has not occurred. If the value of the parameter JUSO is 2 or more (S122: Yes), an error message indicating that double feeding of the sheet 3 has occurred is displayed on the display unit 46 (S123).

  Subsequently, it is determined whether any of the time counters TCNT1 to TCNT4 has a value greater than 24 (specified period) (S124 to S127). Here, the values of the time counters TCNT1 to TCNT4 indicate the time from when the paper passing flag T is set to 1 to when the multifeed flag J is set to 1 for each color, that is, between the sheets 3 to be multifed. It corresponds to the amount of displacement. Therefore, if the value of any of the time counters TCNT1 to TCNT4 is not larger than 24 (S124: No, S125: No, S126: No, S127: No), even if there is a deviation in the transfer position of the toner image. Since it is considered to be small and within the allowable range, the print sequence processing is terminated as it is.

  Further, when any of the time counters TCNT1 to TCNT4 is larger than 24 (S124: Yes or S125: Yes or S126: Yes or S127: Yes), the deviation of the transfer position of the toner image may exceed the allowable range. Therefore, a display for allowing the user to select whether or not to perform reprinting is performed on the display unit 46 (S128). Therefore, when an instruction to perform reprinting is input from the operation unit 45 by the user (S129: Yes), the process returns to S101 and the print sequence process is repeated again. Thereby, printing based on the same print data is performed again. If an instruction not to perform reprinting is input from the operation unit 45 (S129: No), the print sequence process is terminated without executing reprinting.

(Effect of this embodiment)
When a plurality of sheets 3 enter or leave the transfer position in a double feed state in which the sheets 3 overlap each other, generally, there is a difference in the entry timing of each sheet 3 to the transfer position or the release timing from the transfer position. The impedance between the image carrier and the image carrier changes in a multistage manner. Based on this, in the present embodiment, the voltage / current applied at the time of transfer is detected, and when the detected value changes in a plurality of stages, it is determined that double feeding has occurred, so that accurate detection can be performed. it can. In particular, in the present embodiment, when thick paper thicker than the normal paper 3 is used, erroneous detection that it is in a double feed state is prevented.

  In addition, if the transfer voltage detection value Vc reaches the sheet passing reference voltage Vth1 and then reaches the double feed reference voltage Vth2 after a lapse of a predetermined period, it can be determined that the detection value has changed in multiple steps. Thus, accurate detection can be performed with simple processing.

  Further, at the time of transfer, generally, the toner is accumulated on the sheet 3 and becomes thicker toward the downstream side of the conveyance path, so that the impedance between the photosensitive drum 28 and the transfer roller 14 increases. Therefore, the absolute values of the sheet passing reference voltage and the double feed reference voltage compared with the transfer voltage Vc for the downstream transfer roller 14 correspond to the increase in impedance with respect to the same absolute value for the upstream transfer roller 14. (I.e., | VCth1 |> | VMth1 |> | VYth1 |> | VKth1 |, | VCth2 |> | VMth2 |> | VYth2 |> | VKth2 |) Can absorb the effect of increasing. Accordingly, detection accuracy can be ensured.

  Further, the period after the detected value changes by one step and then changes stepwise corresponds to the amount of positional deviation of the overlapping sheets 3. Therefore, if the misalignment amount of the recording medium to be fed is relatively large and the misalignment of the transfer position may exceed the allowable range, transfer is performed again, and the misalignment amount of the recording medium is relatively small. When the positional deviation falls within the allowable range, the user can be conveniently provided by not performing the retransfer.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention.
(1) In the above embodiment, the multi-feed state is detected when the recording medium enters the transfer position. However, according to the present invention, the transfer current / voltage is divided into a plurality of stages even when the recording medium is separated from the transfer position. The double feed state can be detected by detecting whether the change has occurred. Further, the accuracy can be further improved by determining the double feed state based on the detection results at both the time of entering and the time of leaving.

(2) As described above, when it is determined that double feeding has occurred when the second reference value is reached after a predetermined period has elapsed after the detection value by the detection means has reached the first reference value, When detection is performed when the medium enters the transfer position, the second reference value changes to a side corresponding to an increase in impedance between the transfer unit and the image carrier relative to the first reference value. It may be set to the value. When detection is performed when the recording medium leaves the transfer position, the second reference value may be a value that changes to the side corresponding to the decrease in impedance with respect to the first reference value. .

(3) In the above embodiment, the transfer voltage is controlled at constant current. However, the present invention is not limited to this, and according to the present invention, the power supplied to the transfer means is controlled by constant voltage control or other methods. May be.
(4) In the above embodiment, it is determined that the detection value has changed in a plurality of stages by comparing the detection value with a pair of reference values (threshold values). For example, based on the change rate of the detection value per unit time It may be determined by other methods such as determination.

(5) In the above embodiment, the reference value for determining the double feed state according to the temperature and humidity is shown. However, according to the present invention, for example, the type and thickness of paper (thin paper, normal paper) Paper, cardboard, etc.) may be set by the user, and the reference value may be determined based on the setting.
(6) In the above embodiment, an example in which the present invention is applied to a color image forming apparatus has been described. However, the present invention can also be applied to a monochrome image forming apparatus.

1 is a side sectional view showing a schematic configuration of a printer according to an embodiment of the present invention. Block diagram schematically showing the electrical configuration of the printer Block diagram showing the main configuration of the transfer voltage application circuit Flowchart showing the flow of print sequence processing Flowchart showing the flow of print sequence processing Flowchart showing the flow of print sequence processing Flowchart showing the flow of print sequence processing Flow chart showing the flow of the transfer high pressure control process Graph showing the change in transfer voltage when a pair of paper enters and leaves the transfer position in a double feed state

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 3 ... Paper (recording medium)
14: Transfer roller (transfer means)
28 ... Photosensitive drum (image carrier)
40 ... CPU (discriminating means, control means)
50. Transfer voltage application circuit (detection means)

Claims (4)

An image carrier for carrying a developer image;
Transfer means for transferring the developer image on the image bearing member by a voltage applied between the image carrier when the recording medium enters the transfer position onto the recording medium between the image bearing member When,
Detecting means for detecting the applied voltage to the transfer means when the current applied to said transfer means when the transfer constant,
In at least one of when the recording medium is to be detached from and the transfer position when entering into the transfer position, a detection value detected by the detection unit is double feeding of the recording medium when changing a plurality of steps occurs Discriminating means for discriminating;
Equipped with a,
A plurality of sets of the image carrier and the transfer unit are provided along a conveyance path of the recording medium,
The detection means detects the detection value for each of the transfer means,
The determination unit compares the detection values detected corresponding to the transfer units with a first reference value and a second reference value, respectively.
The absolute value of at least one of the first reference value and the second reference value corresponding to the transfer unit on the downstream side of the transport path is greater than the absolute value corresponding to the transfer unit on the upstream side of the transport path. the image forming apparatus you characterized in that is also larger. An image carrier for carrying a developer image;
Transfer means for transferring a developer image on the image carrier onto the recording medium by a voltage applied to the image carrier when the recording medium enters a transfer position between the image carrier and the image carrier. When,
A detection means for detecting a current applied to the transfer means when the voltage applied to the transfer means is constant at the time of transfer;
When at least one of when the recording medium enters the transfer position and when the recording medium leaves the transfer position, when the detection value detected by the detection means has changed in a plurality of stages, double recording of the recording medium has occurred. Discriminating means for discriminating;
With
A plurality of sets of the image carrier and the transfer unit are provided along a conveyance path of the recording medium,
The detection means detects the detection value for each of the transfer means,
The determination unit compares the detection values detected corresponding to the transfer units with a first reference value and a second reference value, respectively.
The absolute value of at least one of the first reference value and the second reference value corresponding to the transfer unit on the downstream side of the transport path is greater than the absolute value corresponding to the transfer unit on the upstream side of the transport path. An image forming apparatus characterized by being made smaller. The image forming apparatus according to claim 1 , wherein:
It said discriminating means, before after danger detection value reaches the first reference value, characterized by determining that double feed has occurred when it reaches the second reference value after a predetermined period of time Image forming apparatus . The image forming apparatus according to any one of claims 1 to 3 ,
Before After danger detection value is changed one stage, when further stepwise change to the same side after a lapse of specified time period, is executed again transcription of the same developer image by the transfer means, prior to the expiration prescribed period An image forming apparatus comprising: a control unit that terminates transfer of the developer image by the transfer unit when the transfer unit changes stepwise.

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