JP2008044742A - Ink jet printer and method of eliminating static electricity from print medium for ink jet printer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively remove charges on the surface of a print medium even when the frequency of polarity change or the potential of an applied voltage for charging a conveyor belt is varied. <P>SOLUTION: When a voltage applied to a print medium charging roller 16 is zero, the potential on the surface of the print medium 2 in contact with the print medium charging roller 16 on the opposite side of the conveyor belt 1 is detected from the partial voltage of the voltage generated in the print medium charging roller 16. A print medium AC charging pattern signal (applied voltage) to the print medium charging roller 16 is so controlled that the detected potential on the surface of the print medium 2 on the opposite side of the conveyor belt 1 can be suppressed. Consequently, the charges on the surface of the print medium 2 can be effectively removed even when the frequency Lt of the polarity change or the potential Vbp of the conveyor belt AC charging pattern signal (applied voltage) for charging the conveyor belt is varied. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention conveys a printing medium such as printing paper or an OHP sheet, and, for example, discharges fine particles (dots) of a plurality of colors of liquid ink onto the printing medium to draw predetermined characters and images. The present invention relates to a printer, and is particularly suitable for transporting a print medium attracted to the surface of a transport belt by electrostatic force.

Such an inkjet printer is generally inexpensive and can easily obtain a high-quality color printed matter. Accordingly, along with the widespread use of personal computers and digital cameras, it has become widespread not only in offices but also in general users.
In such an ink jet printer, generally, a moving body called a carriage or the like, which is integrally provided with an ink cartridge and an ink discharge head (generally called an ink jet head), runs on a print medium in a direction intersecting the transport direction. By driving the actuator while reciprocating, fine particles (ink droplets) of liquid ink are ejected from the nozzles formed on the ink jet head, thereby forming minute ink dots on the print medium, thereby forming predetermined characters and A desired printed matter is created by drawing an image. The carriage is equipped with four color (yellow, magenta, cyan) ink cartridges including black (black) and an inkjet head for each color, so that not only monochrome printing but also full-color printing combining each color is easy. (Furthermore, 6 colors, 7 colors, or 8 colors in which light cyan, light magenta, etc. are added to these colors are also put into practical use).

  In addition, in an ink jet printer that performs printing while reciprocating the ink jet head on the carriage in the direction intersecting the print medium transport direction (width direction of the print medium), the entire page is printed neatly. Therefore, it is necessary to reciprocate the inkjet head several tens of times to 100 times or more. On the other hand, in an ink jet printer of a type in which a long ink jet head (not necessarily integral) having the same dimensions as the width of the print medium is disposed and the carriage is not used, the ink jet head is moved in the width direction of the printing paper. There is no need, and so-called one-pass printing is possible, so that high-speed printing is possible. The former inkjet printer is generally referred to as a “multi-pass printer”, and the latter inkjet printer is generally referred to as a “line head printer”.

  By the way, in this type of line head type ink jet printer, a configuration in which a print medium is adsorbed and conveyed on a conveyance belt for conveyance is often used. In order to efficiently attract the print medium to the conveyor belt, the surface of the conveyor belt is alternately charged to a reverse polarity potential, and the electrostatic force is equivalent between the adjacent reverse polarity potential of the conveyor belt and the print medium. It is desirable to configure a circuit to adsorb the print medium. Generally, a charging roller is brought into contact with the surface of the conveyance belt, and a so-called alternating voltage is used in which the voltage applied to the charging roller is alternately reversed in polarity. As a result, the surface of the transport belt is charged to a potential having a polarity opposite to that of the transport belt.

In such electrostatic force adsorption to the conveyance belt of the print medium, dielectric polarization occurs in the print medium, and charges having a polarity opposite to the charge potential of the conveyance belt are collected on the back side of the conveyance belt side of the print medium. Charges of the opposite polarity are collected on the surface side opposite to the belt. That is, the surface of the print medium is also charged. However, if the surface of the print medium is charged, ink droplets ejected from the inkjet head may be unable to be ejected to the target position due to the influence of the electric field. Therefore, in the ink jet printer described in Patent Document 1 below, a pressure roller that applies a voltage having a polarity opposite to the charging potential of the conveying belt charged at a constant period, as a slow-charging means for removing the charge on the surface of the printing medium. And a conductive brush. Specifically, a pressure roller or a conductive brush, which is a slow-charging means, is disposed on the downstream side of the conveying belt charging roller by being separated by a conveying belt moving distance corresponding to 1.5 times the conveying belt charging cycle. The same alternating voltage signal is supplied (the partial voltage is supplied to the slow voltage means to reduce the potential). In other words, the voltage applied to the print medium by the pressure roller and the conductive brush is also reverse in polarity at a constant cycle.
JP 2005-324877 A

  By the way, the resistance value of the print medium differs depending on the temperature and humidity as well as the type of the print medium. When the resistance value of the print medium is different, the adsorption force of the print medium is different even if the conveyance belt is in the same charging state. Specifically, if the resistance value of the print medium is large, the amount of charge per unit time accumulated between the print medium and the conveyance belt (gap) is small, so the adsorption force is weak, and the resistance value of the print medium is small. Adsorption power increases. Therefore, in order to stabilize the adsorption force of the print medium due to the electrostatic force on the conveyance belt, the polarity change period and potential of the applied voltage must be changed according to the type, temperature, humidity, and the like of the print medium. However, in the prior art described in the above-mentioned Patent Document 1, the grading means including a pressure roller and a conductive brush is fixed at a specific position, and the applied voltage (partial pressure) to the conveying belt charging roller is controlled by the grading means. Therefore, the charge on the surface of the print medium cannot be effectively removed by changing the polarity change period or potential of the applied voltage.

  The present invention has been made paying attention to the above problems, and even when the polarity change period and potential of the applied voltage for charging the conveyor belt are changed, the charge on the surface of the print medium is effectively reduced. An object of the present invention is to provide an ink jet printer that can be removed.

  [Invention 1] In order to solve the above-described problem, an ink jet printer according to Invention 1 has a charging roller in contact with the surface of the conveyance belt, and the surface of the conveyance belt is alternately reversed in the moving direction of the conveyance belt. In an inkjet printer that is charged with an electric potential, adsorbs and conveys a print medium to the surface of the charged conveyance belt with electrostatic force, and ejects ink droplets from the inkjet head onto the print medium, and at least the print medium is conveyed A surface potential detecting means for detecting the potential of the surface opposite to the belt; a print medium charging roller contacting the surface opposite to the print medium conveying belt; and a print medium conveying belt detected by the surface potential detecting means. And a printing medium surface potential control means for controlling the voltage applied to the printing medium charging roller so that the potential on the surface opposite to the printing medium is suppressed, The surface potential detecting means detects the potential of the surface of the print medium opposite to the conveyor belt from the voltage generated by the print medium charging roller when it is in contact with the surface of the print medium opposite to the conveyor belt. It is characterized by.

  According to the ink jet printer according to the first aspect of the invention, the charging roller is brought into contact with the surface of the conveyance belt, and the surface of the conveyance belt is alternately charged to a potential having a polarity opposite to the moving direction of the conveyance belt. When the print medium is adsorbed to the surface of the transport belt by electrostatic force and transported, and ink droplets are ejected from the inkjet head onto the print medium for printing, at least the potential on the surface opposite to the transport belt of the print medium is detected. In addition, since the voltage applied to the print medium charging roller is controlled so that the detected potential of the surface opposite to the conveyance belt of the print medium is suppressed, the polarity change of the applied voltage for charging the conveyance belt is suppressed. Even when the period or potential is changed, the charge on the surface of the print medium can be effectively removed. In addition, since the configuration is such that the potential of the surface opposite to the conveyance belt of the printing medium is detected from the voltage generated by the printing medium charging roller when it is in contact with the surface opposite to the conveyance belt of the printing medium. The potential sensor becomes unnecessary, and the configuration can be simplified, the apparatus can be downsized, and the cost can be reduced accordingly.

  [Invention 2] The ink jet printer of Invention 2 is the ink jet printer of Invention 1, wherein a resistor is interposed between the output of the print medium surface potential control means and the print medium charging roller, and the print medium charging is performed. A voltage dividing circuit is configured by interposing a series resistor between the roller and the ground, and the surface potential detecting means has a voltage applied to the print medium charging roller of zero and is opposite to the conveyance belt of the print medium. It is characterized in that the surface potential of the printing medium in contact with the printing medium charging roller is detected from the output of the voltage dividing circuit of the voltage generated by the printing medium charging roller when in contact with the surface. is there.

  According to the ink jet printer according to the second aspect of the present invention, the voltage applied to the print medium charging roller is zero and the partial pressure of the voltage generated by the print medium charging roller is in contact with the surface of the print medium opposite to the conveying belt. Since the surface potential of the print medium in contact with the print medium charging roller is detected from the output of the circuit, the surface potential of the print medium can be detected appropriately, and the applied voltage for charging the conveyor belt can be detected. Even when the potential is changed, the potential of the voltage applied to the print medium charging roller can be set easily and appropriately.

[Invention 3] Further, in the inkjet printer of Invention 3, in the inkjet printer of Invention 1 or 2, the surface potential detection means detects the potential of the surface opposite to the conveyance belt of the print medium, at least the type of the print medium This is performed when a change is made.
According to the ink jet printer according to the third aspect of the present invention, the detection of the potential on the surface of the print medium opposite to the conveyance belt is performed at least when the type of the print medium is changed. Even when the applied voltage potential for charging the conveyor belt is changed, the surface potential of the print medium can be detected appropriately, and the applied voltage potential to the print medium charging roller can be set easily and appropriately. be able to.

  [Invention 4] In addition, in the printing medium grading method of the ink jet printer according to Invention 4, the surface of the conveyance belt is alternately reversed in the moving direction of the conveyance belt by contacting the charging roller with the surface of the conveyance belt. In an inkjet printer that is charged with an electric potential, adsorbs and conveys a print medium to the surface of the charged conveyance belt with electrostatic force, and ejects ink droplets from the inkjet head onto the print medium, and at least the print medium is conveyed The potential of the surface opposite to the belt is detected from the voltage generated by the print medium charging roller contacting the surface opposite to the conveyance belt of the print medium, and the detected surface of the surface opposite to the conveyance belt of the print medium is detected. The voltage applied to the print medium charging roller is controlled so that the potential is suppressed.

  According to the printing medium grading method of the ink jet printer according to the fourth aspect of the present invention, the charging roller is brought into contact with the surface of the transport belt, and the surface of the transport belt is alternately set to a potential having a reverse polarity in the moving direction of the transport belt. At least on the opposite side of the print medium conveyance belt when charging, the print medium is adsorbed and conveyed by electrostatic force to the surface of the charged conveyance belt, and ink droplets are ejected from the inkjet head onto the print medium. The voltage applied to the print medium charging roller is controlled so that the detected potential on the surface opposite to the conveyance belt of the print medium is suppressed, so that the charge for the conveyance belt is charged. Even when the polarity change period and potential of the applied voltage are changed, the charge on the surface of the print medium can be effectively removed. In addition, since the configuration is such that the potential of the surface opposite to the conveyance belt of the printing medium is detected from the voltage generated by the printing medium charging roller when it is in contact with the surface opposite to the conveyance belt of the printing medium. The potential sensor becomes unnecessary, and the configuration can be simplified, the apparatus can be downsized, and the cost can be reduced accordingly.

Next, an embodiment of an inkjet printer according to the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an ink jet printer according to the present embodiment. Reference numeral 1 in the figure denotes an endless conveying belt for conveying a printing medium 2 such as printing paper. As described above, the print medium 2 is a medium / high resistance sheet-like member such as printing paper or an OHP sheet. Further, the transport belt 1 has a surface (outer peripheral surface) made of an insulating resin such as PET, polyimide, or fluorine resin, and a back surface (inner peripheral surface) of low to medium resistance (1 × 10 ¹⁰ Ω / □ or less). This is a so-called two-layer belt made of the above resin. The conveyor belt 1 is wound around a driving roller 3 disposed at the left end in the figure, a driven roller 4 disposed at the right end in the figure, and a tension roller 5 disposed below the central part thereof. It has been turned. The drive roller 3 is rotationally driven in the direction of the arrow in the figure by a drive source (not shown), and the print medium 2 is sucked from the right side of the figure in a state where the print medium 2 is adsorbed to the conveyance belt 1 charged by the charging roller described later. Transport to the left, that is, in the direction of the arrow. The driven roller 4 is grounded so as to apply a voltage with the conveying belt 1 being sandwiched between the driven roller 4 and a contact portion of a charging roller described later. The tension roller 5 is urged downward by a spring (not shown), thereby applying tension to the transport belt 1.

  A conveying belt charging roller 7 as a charging unit is in contact with the conveying belt 1 so as to face the driven roller 4. As will be described later, the conveying belt charging roller 7 has a conveying belt charging control unit 8. To a predetermined period, an alternating voltage signal (hereinafter also referred to as a conveyor belt AC charging pattern signal) is applied. The conveyor belt charging controller 8 sets the cycle and potential of the conveyor belt AC charging pattern signal to the conveyor belt charging roller 7 in accordance with a command from the system controller 6. The arrangement of the conveying belt charging roller 7 corresponds to the position immediately before the feeding position of the printing medium 2. Therefore, when a voltage signal having a potential reversed to a reverse polarity in a predetermined cycle, that is, an alternating voltage signal is applied to the transport belt charging roller 7, the surface of the transport belt 1 is alternately charged to a reverse potential along the transport direction ( Stripe charging), and a dielectric polarization is generated in the print medium 2 by the respective charges, and the charge of the print medium 2 due to the dielectric polarization, the charge of the surface of the transport belt 1, and the charge of opposite polarity on the surface of the adjacent transport belt 1 And the electric charge of the printing medium 2 is formed, and electrostatic force is generated, and the printing medium 2 is attracted to the surface of the conveyance belt 1.

A paper pressing roller 9 is disposed above the driven roller 4. The paper pressing roller 9 is urged downward by a spring (not shown) and has a function of pressing the print medium 2 against the conveying belt 1 on the driven roller 4. As described above, when the print medium 2 is mounted on the outer peripheral surface of the charged transport belt 1 and the print medium 2 is pressed against the transport belt 1 by the paper pressing roller 9, the print medium 2 is moved to the outer peripheral surface of the transport belt 1 by electrostatic force. To be adsorbed. A feed roller 13 for feeding the printing medium 2 of the paper feeding unit and a guide 15 for supporting the fed printing medium are disposed on the upstream side of the paper pressing roller 9 in the conveyance direction of the printing medium 2.
Further, an encoder sensor 12 for detecting the positions of the conveyance belt 1 and the print medium 2 from a linear scale attached to the conveyance belt 1 is provided on the downstream side of the paper pressing roller 9 in the conveyance direction of the print medium 2. A print medium detection sensor 10 that detects the front end position of the print medium 2 in the transport direction is provided on the downstream side in the transport direction.

  An inkjet head 11 is provided on the downstream side of the print medium detection sensor 10 in the conveyance direction of the print medium 2. The ink-jet head 11 is arranged so as to be shifted in the transport direction of the printing medium 2 for each of the four colors of yellow (Y), magenta (M), cyan (C), and black (K). Ink is supplied to each inkjet head 11 from an ink tank of each color (not shown) via an ink supply tube. Each inkjet head 11 is formed with a plurality of nozzles in a direction intersecting with the conveyance direction of the print medium 2, and by ejecting a necessary amount of ink droplets from these nozzles to a necessary location at the same time, the print medium 2. A minute ink dot is formed and output on the top. By performing this for each color, it is possible to perform so-called one-pass printing by passing the print medium 2 adsorbed on the conveyor belt 1 once. That is, the area where the inkjet head 11 is disposed corresponds to the print area.

  As a method for discharging and outputting ink from each nozzle of the ink jet head, there are an electrostatic method, a piezo method, a film boiling ink jet method, and the like. In the electrostatic system, when a drive signal is given to the electrostatic gap, which is an actuator, the diaphragm in the cavity is displaced, causing a pressure change in the cavity, and ink drops are ejected from the nozzle by the pressure change. It is. In the piezo method, when a drive signal is given to a piezo element that is an actuator, the diaphragm in the cavity is displaced to cause a pressure change in the cavity, and ink droplets are ejected from the nozzle by the pressure change. . In the film boiling ink jet method, there is a minute heater in the cavity, the ink is instantaneously heated to 300 ° C. or more, the ink becomes a film boiling state, bubbles are generated, and ink droplets are ejected from the nozzle by the pressure change. That's it. Any ink output method can be applied to the present invention.

  A specific configuration of the ink jet head of the present embodiment will be described with reference to FIG. The ink jet head 11 uses a piezoelectric actuator, and includes a diaphragm 21, a piezoelectric actuator 22 that displaces the diaphragm 21, and ink inside that is a liquid inside. And a nozzle (24) communicating with the cavity (23) and ejecting ink as droplets by increasing / decreasing the pressure in the cavity (23).

  More specifically, the inkjet head 11 includes a nozzle substrate 25 on which nozzles 24 are formed, a cavity substrate 26, a vibration plate 21, and a laminated piezoelectric actuator 22 in which a plurality of piezoelectric elements 27 are laminated. Yes. The cavity substrate 26 is formed in a predetermined shape as shown in the figure, whereby a cavity 23 and a reservoir 28 communicating with the cavity 23 are formed. The reservoir 28 is connected via an ink supply tube 29 to an ink cartridge 33 that is an ink reservoir. The piezoelectric actuator 22 includes comb-shaped electrodes 31 and 32 arranged opposite to each other, and piezoelectric elements 27 arranged alternately with the comb teeth of the electrodes 31 and 32. The piezoelectric actuator 22 is joined to the vibration plate 21 at one end side through the intermediate layer 30.

  In the piezoelectric actuator 22 having such a configuration, the drive pulse from the drive pulse source applied between the first electrode 31 and the second electrode 32 expands and contracts in the vertical direction as indicated by arrows in FIG. The mode to use is used. Therefore, in the piezoelectric actuator 22, for example, when a drive pulse as shown in FIG. 2 is applied, the diaphragm 21 is displaced, the pressure in the cavity 23 changes, and an ink droplet is ejected from the nozzle 24. It has become. Specifically, the volume of the cavity 23 is enlarged to draw ink, and then the volume of the cavity 23 is reduced to eject ink droplets.

  Returning to FIG. 1, a print medium charging roller 16 is provided between the print medium detection sensor 10 and the encoder sensor 12 so as to come into contact with the surface of the print medium 2 attracted to the conveyance belt 1 on the side opposite to the conveyance belt 1. A grounding roller 17 is provided on the opposite side of the print medium charging roller 16 with the conveying belt 1 interposed therebetween. As will be described later, an alternating voltage signal (hereinafter also referred to as a print medium AC charge pattern signal) having a predetermined period is applied to the print medium charging roller 16 from the print medium charge control unit 18. The print medium charge control unit 18 sets the cycle and potential of the print medium AC charging pattern signal to the print medium charging roller 16 in accordance with a command from the system control unit 6. In the present embodiment, the printing medium charging roller 16 is in contact with the surface of the printing medium 2 and the potential of the printing medium AC charging pattern signal to the printing medium charging roller 16 is zero. The surface potential of the print medium 2 is detected from the generated voltage. That is, the print medium charging roller 16 serves as a print medium surface potential sensor.

  For example, as shown in FIG. 3, the system control unit 6 controls the printing apparatus, the paper feeding apparatus, and the like based on the print data input from the host computer 60 such as a personal computer or a digital camera. A printing process is performed. An input interface unit 61 that receives print data input from the host computer 60, a control unit 62 configured by, for example, a microcomputer that executes print processing based on the print data input from the input interface unit 61, A carriage motor driver 63 for driving and controlling the feed roller motor 41, a driving roller motor driver 64 for driving and controlling the driving roller motor 51 of the conveying belt 1, a head driver 65 for driving and controlling the inkjet head 11, and the drivers 63 and 64 , 65 output signals are converted into control signals used by the external feed roller motor 41, drive roller motor 51, and inkjet head 11, and output, or command signals from the control unit 62 are transferred to the conveyor belt charging control unit 8 or printing. Control to the medium charging control unit 18 The control unit 62 inputs the surface potential of the print medium 2 detected by the print medium charging control unit 18 and the output signals of the encoder sensor 12, the print medium detection sensor 10, and the surface potential sensor 14. And an interface 67 for converting into a signal.

  The control unit 62 temporarily stores a CPU (Central Processing Unit) 62a that executes various processes such as a print process, and print data input through the input interface 61 or various data when the print data print process is executed. A ROM (Read-Only ROM) comprising a RAM (Random Access Memory) 62c that temporarily stores an application program such as print processing or the like, and a non-volatile semiconductor memory that stores a control program executed by the CPU 62a Memory) 62d. When the control unit 62 obtains print data (image data) from the host computer 60 via the interface unit 61, the CPU 62a performs a predetermined process on the print data and ejects ink droplets from any nozzle. Alternatively, print data indicating how many ink droplets are to be ejected is output, and control signals are output to the drivers 63 to 65 based on the print data and input data from various sensors. When the control signals are output from the drivers 63 to 65, these are converted into drive signals by the interface unit 67, and the actuators 22, feed roller motors 41, drive roller motors 51 corresponding to the plurality of nozzles 24 of the inkjet head 11 are provided. The conveyance belt charging control unit 8 and the printing medium charging control unit 18 are operated to execute printing processing on the printing medium. Each component in the control unit 62 is electrically connected through a bus (not shown).

  Next, the conveying belt charging waveform generation circuit 34 constructed in the conveying belt charging control unit 8 will be described with reference to FIG. The transport belt charging waveform generation circuit 34 generates a transport belt AC charging pattern signal of an alternating voltage signal that alternately has a reverse polarity potential for the transport belt charging roller 7 that charges the transport belt 1. When the transport belt AC charging pattern signal amplitude instruction value Vbp for instructing the amplitude value of the transport belt AC charging pattern signal is input from the system controller 6 as digital data, it is converted into an analog value by the D / A converter, and the buffer A direct current voltage value is output from B1 to the resistor R11. On the other hand, when a conveyor belt AC charging pattern signal cycle instruction pulse Lbp (with a cycle of Lt) that determines the charging cycle length of the conveyor belt AC charging pattern signal is input to the base of the transistor Tr1, a predetermined amplitude voltage A pulse corresponding to the value is output from the collector of the transistor Tr1. The capacitor C1 extracts the AC component of this pulse and grounds it through the resistor R12. Therefore, the capacitor C1 is input to the amplifier AMP1 as a plus / minus voltage change centered on the ground GND (= 0 V), and is amplified. A charging pattern signal Sbp is applied to the conveying belt charging roller 7.

  Next, the printing medium charging waveform generation circuit 36, the printing medium surface potential detection circuit 35, and the printing medium charging waveform output circuit 37 constructed in the printing medium charging control unit 18 will be described with reference to FIG. First, the printing medium charging waveform generation circuit 36 generates a printing medium AC charging pattern signal of an alternating voltage signal having a reverse polarity potential alternately for the printing medium charging roller 16 that charges the printing medium 1. When the print medium AC charging pattern signal amplitude instruction value Vpp for instructing the amplitude value of the printing medium AC charging pattern signal is input as digital data from the system control unit 6 described above, it is converted into an analog value by the D / A converter, The DC voltage value is output from the buffer B2 to the resistor R21. On the other hand, a printing medium AC charging pattern signal cycle instruction pulse Lpp (the cycle is the same Lt as the conveying belt AC charging pattern signal) that determines the charging cycle length of the printing medium AC charging pattern signal based on the pulse width is input to the base of the transistor Tr2. Then, a pulse corresponding to a predetermined amplitude voltage value is output from the collector of the transistor Tr2. Since the capacitor C2 extracts the AC component of this pulse and grounds it via the resistor Rs2, it is input to the amplifier AMP2 as a plus / minus voltage change centered on the ground GND (= 0V), and is amplified. A charging pattern signal Spp is applied to the printing medium charging roller 16.

  The print medium charging waveform output circuit 37 is provided in parallel with the print medium charging waveform generation circuit 36. The print medium charging waveform output circuit 37 functions as output start timing detection means for the print medium AC charging pattern signal. The output of the printing medium charging waveform generation circuit 36 described above is output to the printing medium charging roller 16 via the resistor Rs3. When there is no output of the printing medium charging waveform generation circuit 36 (the applied voltage is zero), The surface potential of the charged print medium 2 also acts on the print medium charging roller 16. Therefore, the surface potential of the printing medium 2 is grounded through two series resistors Rs1 + Rs2, and the partial pressure of the resistance Rs1 on the surface potential side of the printing medium 2 is inverted and amplified by the inverting amplifier AMPs1, and then compared by the comparator CMPs1. The pulse Pcomp is output. The rising or falling edge of the output pulse Pcomp is latched by a latch circuit, and a latch signal Lch is output. The output timing of the latch signal Lch is the output start timing of the print medium AC charging pattern signal, that is, the output start timing of the printing medium AC charging pattern signal cycle instruction pulse Lpp. The latch circuit is reset by a reset signal rst. Note that the resistance value of the series resistance Rs1 + Rs2 needs to be a large value of, for example, several tens of MΩ or more so that the charging potential of the conveyor belt 1 is not consumed through the printing medium 2. The print medium AC charging pattern signal from the print medium charging waveform generation circuit 36 is applied to the print medium charging roller 16 with a value divided by the series resistance Rs1 + Rs2 and the resistance Rs3. If the resistance value is set to a large value of 10 times or more of the resistance value of the resistor Rs3, the voltage does not decrease so much. Of course, the print medium AC charging pattern signal amplitude instruction value Vbp itself may be a value that allows for a voltage drop due to the series resistance Rs1 + Rs2.

  The print medium surface potential detection circuit 35 is provided in parallel with the print medium charging waveform output circuit 37. The print medium surface potential detection circuit 35 converts the surface potential of the print medium 2 acting on the print medium charging roller 16 into digital data and outputs it to the system control unit 6 as described above. When there is no output from the print medium charge waveform generation circuit 36 (the applied voltage is zero), the print medium is divided by the series resistance Rs1 + Rs2 of the print medium charge waveform output circuit 37 and inverted and amplified by the inverting amplifier AMPs1. 2 is rectified by a rectifier, smoothed by a filter, converted to a DC value by a buffer B3, and then digitized by an A / D converter, which is a print medium AC charging pattern signal amplitude indication value. It is output to the system control unit 6 as Vpp. It should be noted that the system controller 6 prints the print medium AC charge pattern signal (print medium AC charge pattern) until the print medium surface potential detection circuit 35 detects the charge pattern signal amplitude instruction value Vpp, that is, the surface potential of the print medium 2. Although the signal amplitude instruction value Vpp and the print medium AC charging pattern signal cycle instruction pulse Lpp) are not output, the print medium AC charging pattern signal is internally output when the latch signal Lch is output from the print medium charging waveform output circuit 37 described above. Is set to the conveyance belt AC charging pattern signal amplitude instruction value Vbp, and is corrected when the charging pattern signal amplitude instruction value Vpp is detected.

  Next, an equivalent circuit of electrostatic force for attracting the print medium 2 to the transport belt 1 will be described. First, the insulating layer on the surface side of the conveyor belt 1 is regarded as a capacitor, and in the circuit from the conveyor belt charging roller 7 to the driven roller 4 that is grounded, the charge of the conveyor belt charging roller 7 is insulated from the surface of the conveyor belt 1. The capacitor Cb of the layer is charged. When the print medium 2 is mounted on the transport belt 1 charged with this charge, a charge having a potential opposite to that of the surface of the transport belt 1 is generated by dielectric separation on the back surface of the print medium 2. Assuming that a minute gap between the front surface of the conveyance belt 1 and the back surface of the printing medium 2 is a capacitor Cg that develops an adsorption force due to electrostatic force, printing is performed between adjacent charged capacitors Cb of opposite polarity on the conveyance belt 1. An equivalent circuit is formed by the resistance Rp of the medium 2, the capacitor Cg in the gap, and the resistance Ra of the low to medium resistance layer on the back side of the transport belt 1, and a potential difference Vg is generated between the transport belt 1 and the print medium 2. This potential difference Vg becomes an electrostatic force that attracts the print medium 2 to the conveyance belt 1. This electrostatic force = adsorptive force F (t) is given by the following equation (1). Q is the charge charged in the gap capacitor Cg, E is the electric field between the electrodes of the gap capacitor Cg, and d is the gap distance.

  The process in which the potential difference Vg is generated is a transient phenomenon in which a capacitor Cg is formed through the resistance Rp of the printing medium 2 and the surface of the conveying belt 1 and the back surface of the printing medium 2 are used as electrodes and the gap between them is used as a dielectric layer. And finally reaches the charge potential Vb of the conveyor belt 1. The transient potential difference Vg (t) during charging is expressed by the following two equations. Vb is the charging potential of the conveyor belt, C is the combined capacity of the equivalent circuit, and R is the combined resistance of the equivalent circuit.

  FIG. 6 shows the attractive force F (t) due to the transient potential difference Vg (t) that changes with the passage of time. The combined capacitance C and combined resistance R of the equivalent circuit of the transient potential difference Vg (t) are related to the resistance Rp of the print medium 2 and the capacitor (capacitance) Cg of the gap, for example, the type of the print medium 2 Varies with temperature and humidity. Therefore, in order to obtain a stable adsorption force of the printing medium 2 to the conveying belt 1, that is, a transient potential difference Vg (t), the charging state of the conveying belt 1 according to the type, temperature, and humidity of the printing medium 2, specifically It is necessary to change the polarity change period and potential of the applied voltage.

  Further, the surface charge q of the print medium when the conveying belt 1 is charged in a striped manner with a predetermined period Lt is expressed by the following three equations. In the equation, Ey is an electric field acting in the thickness direction of the printing medium 2, Ex is an electric field acting in the lateral direction of the printing medium 2, ρv is a volume resistivity of the printing medium, ρs is a surface resistivity of the printing medium, and λ is a stripe shape. This is the period length of charging.

  According to these three equations, when the resistance value of the print medium 2 is large, the charge per unit time for moving through the above-described equivalent circuit is small, so the surface potential of the print medium 2 is large. On the other hand, when the resistance value of the printing medium 2 is small, the surface potential of the printing medium 2 becomes small because the charge per unit time that moves through the equivalent circuit increases. FIG. 7a shows the surface potential of the conveyor belt 1 and the print medium 2 when the humidity is 20%, and FIG. 7b shows the surface potential of the conveyor belt 1 and the print medium 2 when the humidity is 60%. When the humidity is high, the resistance value of the print medium 2 is small, so that the surface potential of the print medium 2 is small, which is suitable for the three types of evaluation.

  Next, calculation processing for setting and outputting the cycle Lt and the potential Vbp of the transport belt AC charging pattern signal and the output start timing and cycle of the printing medium AC charging pattern signal based on such a principle is shown in FIGS. 9 shows. This calculation process is performed during continuous printing from immediately before the actual printing operation. First, in step S1, the voltage applied to the printing medium charging roller 16 is set to 0 V and the reset signal rst is set to the Low level.

Next, the process proceeds to step S2, and the conveyance belt charging conditions such as the type, temperature and humidity of the printing medium are read.
Next, the process proceeds to step S3, and a transport belt AC charging pattern signal (cycle Lt, potential Vbp) to the transport belt charging roller 7 is set based on the transport belt charging condition read in step S2.

Next, the process proceeds to step S4, and the conveyance belt AC charging pattern signal set in step S3 is output.
Next, the process proceeds to step S5, and timer counting is started. This timer count is incremented by the moving time of the conveying belt 1 from the conveying belt charging roller 7 to the printing medium charging roller 16.

Next, the process proceeds to step S6, where it is determined whether the timer has been counted up. If the timer has been counted up, the process proceeds to step S37, and if not, the process waits.
In step S7, the voltage generated by the printing medium charging roller 16 is taken into the printing medium charging waveform output circuit 37, and the charging polarity is detected from the output pulse Pcomp of the comparator COMPs1 of the printing medium charging waveform output circuit 37.

In step S8, the reset signal rst is set to Hi level, and the latch circuit of the print medium charging waveform output circuit 37 is reset.
In step S9, it is determined whether or not the falling edge of the output pulse Pcomp (charging polarity) of the comparator COMPs1 of the print medium charging waveform output circuit 37 is detected, and the falling edge of the output pulse Pcomp is determined. When it detects, it transfers to step S10, and when that is not right, it waits.

In step S10, the latch signal Lch is output.
Next, the process proceeds to step S11, and the output start timing of the print medium AC charging pattern signal (cycle is Lt) to the print medium charging roller 16 is set. Specifically, the phase difference with the conveying belt AC charging pattern signal is adjusted.
Next, the process proceeds to step S12, and conveyance of the print medium is started.

In step S13, the print medium detection sensor 10 detects passage of the front end of the print medium 2.
In step S14, the surface potential digital data Vpp when the voltage applied to the print medium charging roller 16 is zero is sampled by the print medium charging roller 16 and the print medium surface potential detecting circuit 35.

Next, the process proceeds to step S15, and it is determined whether or not the sampling of the surface potential digital data Vpp of the print medium 2 is completed using, for example, whether or not the falling of the charging polarity of the print medium 2 is detected twice. If the sampling of the surface potential digital data Vpp of the printing medium 2 is completed, the process proceeds to step S16, and if not, the process waits.
In step S15, the surface potential digital data Vpp is stored, and the potential of the print medium AC charging pattern signal to the print medium charging roller 16 is changed and set to the surface potential digital data Vpp.

In step S17, the print medium AC charging pattern signal is output to the print medium charging roller 16.
Next, the process proceeds to step S18 to determine whether or not the type of the print medium 2 has been changed. If the type of the print medium 2 has been changed, the process proceeds to step S21. If not, the process proceeds to step S19. Transition.

In step S19, it is determined whether or not printing has been completed for a preset number of sheets, for example, 10 sheets. If printing has been completed for a set number of sheets, the process proceeds to step S21. Then, the process proceeds to step S20.
In step S20, it is determined whether or not printing is finished. If printing is finished, the process returns to the main program. If not, the process proceeds to step S18.
In step S21, the voltage applied to the print medium charging roller 16 is set to zero, and then the process proceeds to step S1.

  According to this arithmetic processing, each time the leading end of the print medium 2 passes, the surface potential is detected by the print medium charging roller 16 and the print medium surface potential detection circuit 35, and the surface potential is the same in magnitude and reverse. By outputting the print medium AC charging pattern signal having a polarity potential at the same cycle Lt as the cycle Lt of the conveying belt AC charging pattern signal, the surface potential of the printing medium 2 can be reliably suppressed. The reason why the latch signal Lch is output at the falling edge of the output pulse Pcomp is that the polarity of the voltage generated by the print medium charging roller 16 is inverted by the inverting amplifier AMPs1, and this inverted polarity is the print polarity. Since the actual charging polarity of the medium 2 is output, in order to output a printing medium AC charging pattern signal having a potential opposite to that of the medium 2, the positive edge of the output pulse Pcomp is changed at the falling edge of the output pulse Pcomp. This is because a potential needs to be applied.

Next, the operation of the arithmetic processing and the transport belt charging waveform generation circuit 34, the printing medium charging waveform generation circuit 36, the printing medium charging waveform output circuit 35, and the printing medium surface potential detection circuit 37 will be described with reference to the timing chart of FIG. explain.
In this timing chart, a conveying belt AC charging pattern signal having a cycle of Lt and a potential Vbp is output, and the conveying belt 1 is striped charged. The print medium 2 starts from a state in which the print medium 2 has not been conveyed to the position of the print medium charging roller 16 although the print medium 16 is in contact therewith. Therefore, the output pulse Pcomp of the comparator of the print medium charging waveform output circuit 35, the output of the inverting amplifier AMPs1 of the print medium surface potential detection circuit 37, and the output of the buffer B3 are all charged with the charge polarity and charge potential of the surface of the transport belt 1. The output of the buffer B3 of the print medium surface potential detection circuit 37 is a value proportional to the potential Vbp of the conveying belt AC charging pattern signal. Also, the output of the print medium detection sensor 10 remains at the low level.

From this state, when the reset signal rst is set to the Hi level at time t ₀₁ , the latch signal Lch is output at time t ₀₂ when the output pulse Pcomp of the comparator of the print medium charging waveform output circuit 35 falls next. After time t ₀₂ , in the system control unit 6, the transport belt AC charging pattern signal potential Vbp and the printing medium AC charging pattern signal with the period Lt are processed, but the surface potential digital data Vpp of the printing medium 2 is still detected. Therefore, the print medium AC charging pattern signal is not output to the print medium charging roller 16.

Next, at time t ₀₃ , the leading end of the print medium 2 in the transport direction passes through the print medium charging roller 16, but the print medium detection sensor 10 is on the downstream side of the print medium charging roller 16. The surface potential digital data Vpp is not sampled. Next, at time t ₀₄ , the leading end of the print medium 2 in the transport direction passes through the print medium detection sensor 10, and sampling of the surface potential digital data Vpp of the print medium 2 is started from time t ₀₄ . The sampling of the surface potential digital data Vpp is completed at time t _{05 when the} trailing edge of the charging polarity of the output pulse Pcomp of the comparator of the printing medium charging waveform output circuit 35 is detected twice, and at the same time, charging of the printing medium is performed. A print medium AC charging pattern signal having a potential Vpp and a cycle Lt is output toward the roller 16.

  As described above, according to the ink jet printer of this embodiment, the conveyance belt charging roller 7 is brought into contact with the surface of the conveyance belt 1, and the surface of the conveyance belt 1 is alternately reversed in the moving direction of the conveyance belt 1. At least a printing medium is charged when charged with an electric potential, conveyed by adsorbing and conveying the print medium 2 to the surface of the charged conveyance belt 1 by electrostatic force, and ejecting ink droplets from the inkjet head 11 onto the print medium 2. 2 is detected, and the printing medium AC charging to the printing medium charging roller 16 is suppressed so that the detected potential of the surface of the printing medium 2 opposite to the conveying belt 1 is suppressed. Since the pattern signal (applied voltage) is controlled, the polarity change period Lt and the potential Vbp of the conveying belt AC charging pattern signal (applied voltage) for charging the conveying belt. Charge on the surface of the print medium 2, even when changing can be effectively removed. And detecting the potential of the surface of the printing medium 2 opposite to the conveying belt 1 from the voltage generated by the printing medium charging roller 16 when it is in contact with the surface of the printing medium 2 opposite to the conveying belt 1. This eliminates the need for a separate surface potential sensor, thereby simplifying the configuration, reducing the size of the device, and reducing the cost.

  Further, the voltage generated by the printing medium charging roller 16 when the printing medium AC charging pattern signal (applied voltage) to the printing medium charging roller 16 is zero and is in contact with the surface of the printing medium 2 opposite to the conveying belt 1. Since the surface potential Vpp of the printing medium 2 that is in contact with the printing medium charging roller 16 is detected from the output of the voltage dividing circuit, the surface potential Vpp of the printing medium 2 can be appropriately detected and transported. Even when the potential Vbp of the conveying belt AC charging pattern signal (applied voltage) for belt charging is changed, the potential Vpp of the printing medium AC charging pattern signal (applied voltage) to the printing medium charging roller 16 is easily and appropriately set. Can be set.

  In addition, since the detection of the potential Vpp on the surface of the print medium 2 opposite to the conveyance belt 1 is performed at least when the type of the print medium 2 is changed, charging of the conveyance belt according to the type of the print medium 2 is performed. Even when the potential Vbp of the conveying belt AC charging pattern signal (applied voltage) for the printing is changed, the surface potential Vpp of the printing medium 2 can be appropriately detected, and the printing medium AC to the printing medium charging roller 16 can be detected. The potential Vpp of the charging pattern signal (applied voltage) can be set easily and appropriately.

  In the above embodiment, only an example in which the ink jet printer of the present invention is applied to a so-called line head type ink jet printer has been described in detail. However, the ink jet printer of the present invention can be applied to all types of ink jet including multi-pass type. Applicable to printers.

1 is a front view illustrating a schematic configuration of an embodiment of an inkjet printer according to the present invention. It is a longitudinal cross-sectional view which shows an example of the inkjet head of the inkjet printer of FIG. It is a block diagram of the system control part of FIG. FIG. 2 is a block diagram of a conveyance belt charging waveform generation circuit constructed in the conveyance belt charging control unit of FIG. 1. FIG. 2 is a block diagram of a print medium charge waveform generation circuit, a print medium surface potential detection circuit, and a print medium charge waveform output circuit constructed in the print medium charge control unit of FIG. 1. It is explanatory drawing of the electrostatic attraction force to the conveyance belt of a printing medium. It is explanatory drawing of the surface potential of a printing medium in case humidity differs. 2 is a flowchart illustrating a calculation process for setting and outputting an applied voltage to a conveyance belt and an applied voltage to a print medium performed in the control unit of FIG. 1. 2 is a flowchart illustrating a calculation process for setting and outputting an applied voltage to a conveyance belt and an applied voltage to a print medium performed in the control unit of FIG. 1. 10 is a timing chart showing the operation of the arithmetic processing of FIGS. 8 and 9.

Explanation of symbols

  1 is a conveyance belt, 2 is a printing medium, 3 is a driving roller, 4 is a driven roller, 5 is a tension roller, 6 is a system control unit, 7 is a conveyance belt charging roller, 8 is a conveyance belt charging control unit, and 9 is a paper presser Roller, 10 is a print medium detection sensor, 11 is an inkjet head, 12 is an encoder sensor, 13 is a feed roller, 15 is a guide, 16 is a print medium charging roller, 17 is a ground roller, 18 is a print medium charging control unit, and 34 is Conveying belt charging waveform generation circuit, 35 is a printing medium surface potential detection circuit, 36 is a printing medium charging waveform generation circuit, and 37 is a printing medium charging waveform output circuit.

Claims (4)

  A charging roller is brought into contact with the surface of the conveyor belt, and the surface of the conveyor belt is alternately charged to a potential having a polarity opposite to the moving direction of the conveyor belt, and the print medium is electrostatically applied to the surface of the charged conveyor belt. A surface potential detecting means for detecting a potential of a surface of the printing medium opposite to the conveying belt; and a printing medium in an ink jet printer for performing printing by ejecting ink droplets from an ink jet head onto the printing medium A printing medium charging roller that is in contact with the surface opposite to the conveying belt of the printing medium, and the printing medium charging roller so that the potential on the surface opposite to the conveying belt of the printing medium detected by the surface potential detecting means is suppressed. Printing medium surface potential control means for controlling the applied voltage of the printing medium, and the surface potential detection means is in contact with the surface of the printing medium opposite to the conveying belt. The printing medium charging roller ink jet printer, characterized by detecting the potential of the conveyor belt and the opposite surface of the print medium from the generated voltage of the time.   A resistor is interposed between the output of the printing medium surface potential control means and the printing medium charging roller, and a voltage dividing circuit is configured by interposing a series resistor between the printing medium charging roller and the ground, The surface potential detecting means outputs a voltage dividing circuit for a voltage generated by the printing medium charging roller when the voltage applied to the printing medium charging roller is zero and is in contact with the surface of the printing medium opposite to the conveying belt. 2. The ink jet printer according to claim 1, wherein a surface potential of the print medium in contact with the print medium charging roller is detected.   3. The ink jet printer according to claim 1, wherein the surface potential detection unit detects the potential of the surface opposite to the conveyance belt of the print medium at least when the type of the print medium is changed. .   A charging roller is brought into contact with the surface of the conveyor belt, and the surface of the conveyor belt is alternately charged to a potential having a polarity opposite to the moving direction of the conveyor belt, and the print medium is electrostatically applied to the surface of the charged conveyor belt. In an ink jet printer that sucks and conveys ink and ejects ink droplets from the ink jet head to the print medium, at least the surface potential on the opposite side of the print medium conveyance belt is at least opposite to the print medium conveyance belt. The voltage applied to the print medium charging roller is controlled so as to suppress the potential of the surface opposite to the conveyance belt of the detected print medium. An ink jet printer printing medium grading method.

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