GB2392871A

GB2392871A - Operating ink jet valves during printing

Info

Publication number: GB2392871A
Application number: GB0216025A
Authority: GB
Inventors: David Andrew Horsnell; Matthew Brian Tomlin; Ammar Lecheheb; Oliver John Prime; Michael James Fox; Christopher Michael Bates
Original assignee: Willett International Ltd
Current assignee: Willett International Ltd
Priority date: 2002-07-11
Filing date: 2002-07-11
Publication date: 2004-03-17
Also published as: GB0216025D0

Abstract

A method of operating an ink jet printer comprising one or more ink jet valves, comprising the steps of activating one or more of the ink jet valve or valves to generate a print matrix and analysing the print matrix. The method further comprising the step of altering the activation of one or more of the ink jet valve or valves in accordance with the print matrix analysis. Furthermore, an ink jet printer comprising one or more ink jet valves; imaging means for imaging the print matrix; and control means to control the activation of the one or more ink jet valves and to analyse the print matrix image.

Description

. - 1 - DEVI CE AND METHOD

The present invention relates to a device that can be used to control the operation of a print head and to a method 5 for controlling a print head under operation.

Ink jet printers are non-contact printers in which dots of ink are ejected from one or more nozzle orifices so as progressively to build up a printed image on a substrate 10 moved relative to the nozzle. One form of ink jet printer comprises a source of ink under pressure, typically a reservoir or bottle of ink which is pressurized to from 0.1 to 2 bar, notably about 1 bar. The pressure is created, for example, by pressurizing the air space above 15 the ink in the bottle or reservoir from which ink is fed to the nozzle orifice(s) in a print head through which it is ejected as a series of droplets onto the surface of the substrate. The flow of ink through the each nozzle orifice is controlled by a solenoid valve. Typically, 20 such a valve comprises an electromagnetic plunger journalled for axial movement within an axially extending electric coil. The distal end of the plunger is located within a valve head chamber through which ink flows from the reservoir to the nozzle orifice. When current is fed 25 through the coil, this generates a magnetic field which

acts on the plunger to move it axially and thus open, or shut, the inlet to nozzle orifice. Typically, the magnetic field acts to retract the plunger against the

bias of a coil spring to create a flow path between the 30 valve head chamber and the nozzle orifice. When the electric current no longer flows in the coil, the magnetic field ceases and the plunger returns under the bias of the

- 2 spring to seat against sealing ribs, lips or other means located at or around the inlet to a bore leading to the nozzle orifice to close the flow path to the nozzle orifice. For convenience, the term drop on demand printer 5 will be used to denote in general such types of ink jet printer. Conventional ink jet print heads have employed electro-

mechanical control and actuation systems that open the 10 valve for a predetermined period of time so that an ink drop can be ejected. The time for which the valve is held open determines the quantity of ink that is ejected from the valve and hence the size of the drop that will be formed on the substrate that is being printed upon.

15 Mechanical variations in the manufacture and assembly of the valves will lead to each valve ejecting a different volume of ink when the valve is held open for a pre-

determined period of time. Thus it is essential that all the valves used within a print head matrix are adjusted 20 such substantially equal volumes of ink are ejected when the valves are held open for the same period of time. This adjustment of the valves is time consuming and laborious as typically a manual adjustment must be made to each valve within the print head matrix.

A preferred embodiment of the invention and its operation under on-line software control will now be described by way of illustration only and with respect to the accompanying drawings, in which

Figure 1 shows a schematic depiction of a solenoid valve which is suitable for use with the method of the present invention; Figure 2 shows a schematic depiction of an apparatus 5 according to the present invention; Figure 3 shows a first schematic depiction of a preferred embodiment of printer apparatus that is operated according to the present invention; Figure 4 shows a second schematic depiction of a 10 preferred embodiment of printer apparatus that is operated according to the present invention; and Figure 5 shows a third schematic depiction of a preferred embodiment of printer apparatus that is operated according to the present invention; Figure 1 shows a schematic depiction of a solenoid valve 10 which is suitable for use with the method of the present invention. The valve 10 comprises plunger 20, tube 30 and coils 40. The plunger 20 comprises a 20 ferromagnetic material (or any other magnetic material) and is received within the tube 30 so as to be able to move freely along the axis of the tube. The plunger can be impelled, for example towards the open end of the tube, by the application of a current to the coils 40, the 25 current generating a magnetic field within the tube, which

causes a magneto motive force to act upon the plunger.

The timing and frequency of the current pulses applied to the coils can be controlled by computer (not shown). The solenoid valve additionally comprises a return mechanism 30 (not shown), such as a spring, that acts to return the plunger to its initial position once the plunger has completed its full range of travel.

- 4 In practice, a print head will comprise a matrix of such valves that are arranged in a square or rectangular arrangement. Figure 2 shows two exemplary valves 210a, 5 210b from such a print head matrix 220. Associated with each valve is valve control means 215a, 215b, each of the valve control means being in communication with a central computer system 230. The operation of each valve is controlled by the transmission of control pulses from the 10 central computer system 230 to each of the valve control means 215a, 215b. The valve control means are responsive to the central computer system such that the central computer system is able to vary the time that the valves are held open for. This controlled variation of the valve 15 enables ink drops of a desired size to be produced for depositing upon the substrate 250.

The print head can be calibrated upon manufacture and then at periodic intervals during its operation. The central 20 computer system instructs the print head to generate a predetermined matrix of drops. This test matrix is deposited on a test substrate and the printed image can be examined to determine the correlation of the printed image to the original test matrix. If the ratio of the size of 25 a printed pixel to the size of the respective pixel of the original test matrix is outside a threshold value then the respective valve control means can be instructed to change the time that the valve is to be opened for. If the printed pixel is too small then the valve open time will 30 be increased (either by the addition of more time or by multiplying the valve open time by a suitable constant).

Similarly, if the printed pixel is too large, then the

- 5 valve open time will be decreased accordingly. The threshold that is used to determine whether a printed pixel is too small or too large may be varied in accordance with the nature of the print substrate and/or 5 the application that the print head is being used for.

As variations in printed pixel size will depend upon mechanical variations within the valve, it is possible that a valve may operate satisfactorily for one size of 10 pixel or within a given range of valve operating rates.

Therefore, the calibration may need to be repeated across the range of pixel sizes and valve rates that will be used by the valve. The range of calibration factors that are required by each valve may be stored in a look-up table, 15 or it may be possible to determine one or more equations such that the relevant calibration factor can be calculated given the desired valve operation rate and pixel size.

20 In an alternative embodiment of the present invention, imaging means 240 may be additionally coupled to the computer control system and aligned so as to view the area of the substrate that the print head matrix prints upon.

When a test matrix is printed upon the substrate, the 25 image means is able to convert that image to an electrical signal that can be transmitted to the central computer system. The central computer system can, after any necessary image processing (digitizing, filtering, etc.), compare the printed image with the original test matrix 30 that is stored within the central computer system. The ratio of pixel sizes can be determined for each pixel and calibration factors calculated for each valve as required.

- 6 The central computer system can then communicate the calibration factors to the valve control means associated with the valves that require calibration.

5 The valve control means must be able to receive, interpret and execute signals that are received from the central computer system. It will be readily understood that the valve control means may be implemented such that each valve has a dedicated control means or alternatively that 10 a number of valves may be controlled by a single control means. In a preferred embodiment, the valve control means comprise a field programmable gate array (FPGA). FPGAs

15 comprise memory and logic elements that can be configured by the user to provide a desired functionality.

In the preferred embodiment, the FPGA, and associated devices, is used to control a linear array of 16 valves.

20 Referring to Figure 3, the valves 610a, 610b,, 610p are controlled by valve control means that comprise FPGA 616, electrically erasable programmable ROM (EEPROM) 617, RAM 618, programmable ROM (PROM) 619 and input/outputs 622, 624, 626. The FPGA 616 is connected to each of the valves 25 610a, 610b,, 610p, EEPROM 617, RAM 618 & PROM 619. All three input/outputs 622, 624, 626 interface with the FPGA.

When the FPGA is powered up, it loads its internal configuration data from PROM 619 and then follows the sequences that have been loaded from the PROM. The EEPROM 30 617 stores a range of data comprising a look-up table comprising data associated with each of the valves, data specific to the valve control means and FPGA, status

information, etc. The FPGA will load this data from the EEPROM and then initialise the RAM 618, by writing zero values into each memory location in RAM. The FPGA will then wait to receive print data or other commands from one 5 of the inputs. Input/output 622 is connected to the computer control system and input/output 624 can be used to connect to a further valve control means (see below with reference to Figure Z). Input 626 provides a series of pulses that are used in co-ordinating the printing 10 process. When the array of valves is printing onto a substrate, the substrate is normally moved underneath the valves. The series of pulses supplied to input 626 may be generated from an encoder applied to a shaft in the apparatus that is moving the substrate relative to the 15 valves.

Figure 4 shows a schematic depiction of a number of registers that are formed with the FPGA when the FPGA configuration data is loaded from PROM 619. The first 20 register 631 is used to write to and read from the EEPROM 617 and is also used when initialization data is read from the EEPROM. Second register 632 receives print data from the computer control system, such as the alphanumeric characters or bitmaps to be printed, or a signal to 25 initiate a printing process. Second register 632 also writes print data to the RAM and is used to initialize the RAM during the start-up phase. The third register receives configuration data from the computer control system such as data controlling the slant that may be 30 applied to the print head. Fourth register 634 receives print data from the RAM and passes it to the fifth

- 8 register 635, which uses the print data to operate the valves 610.

A desired print image (which may include alphanumerical 5 characters) is entered into the computer control system and this image is then converted into raster data that may be communicated with the valve control means. The valves 610 may be operated for different periods of time so as to provide the appearance of 16-level greyscale images. Thus 10 the print data can be supplied in the form of a raster comprising a 4 bit word for each valve, with the value of the 4-bit word determining the greyscale that is to be generated by the valves. The print data is received by the second register and written into the RAM 618. The RAM 15 is logically arranged in 16 rows, with each of the valves corresponding to a row. There are a plurality of columns, each of which corresponds to a time slot. Each raster scan also corresponds to a time slot and the time slot is determined by the frequency at which the shaft encoder 20 supplies pulses to the FPGA.

When print data is received at the FPGA the second register interprets the greyscale data for each valve, obtaining the time that each valve must be opened for in 25 order to generate the desired greyscale from a look-up table held in the first register. In theory, each valve should be held open for the same period of time in order to generate the dame greyscale, but mechanical variations in each valve will lead to each valve having slightly 30 different characteristics. Calibration factors that account for these differences are held in the look-up table. The valve times are then written into the RAM,

- 9 - using as many columns as are necessary to store all of the rasters. A write pointer is set to the first column of the data. Each memory location holds the grey scale value for the associated valve and time slot.

When the next shaft encoder pulse is received the RAM column indicated by the write pointer is read to see which of the 16 valves need to be operated, i.e. which memory locations have non-zero entries. Once the memory 10 locations have been read then all the memory locations in the column are overwritten with zero.

The identity of these valves, along with the time for which the valves are to be held open are then transmitted 15 to the fourth register, which may perform further operations on the valve times in order to correct for valve operation at high speed or a long time period between subsequent operations of the valve. The valve times are then passed to the fifth register which 20 calculates the number of shaft encoder pulses that are equivalent to the valve times. The valves are then opened for a period of time equal to that number of shaft encoder pulses. 25 As the valves 610 are electro-mechanical devices, their size provides a limitation to the print resolution that can be obtained. Typically, each valve may be provided at an offset of 4mm from the adjacent valve(s). If a greater resolution (i.e. smaller pixel separation is 30 required) then the matrix may be slanted so that the valves are closer together in one axis. The disadvantage of this is that if no correction is made to the print

- 10 rasters then the desired image will be printed out slanted. Such a correction may advantageously be provided using the 5 RAM to provide a slant to the print raster data. Once the greyscale data has been translated into valve open times, rather than writing the valve data into a vertical column, the write data can be offset across a number of columns.

For example, if the desired slant angle is 45 then the 10 valve open time for the first valve should be written into the column indicated by the write pointer, the valve open time for the second valve should be written into the next column along from the column indicated by the write pointer, and so on, such that the valve open time is 15 written into the RAM at the desired slant angle.

Typically the 16-level greyscale can be provided using valve open times between approximately 80ps and 250ps. It has been found advantageous to initially open the valve by 20 providing a first voltage for a first period of time and to provide a second voltage, that is lower than the first voltage, for a further period of time in order to hold the valve open. This reduces the possibility that the valve remains open for longer than is required to provide the 25 desired greyscale, leading to decreased printing performance. It has been found particularly advantageous to apply a 36V pulse for approximately 80ps and a second pulse of approximately 5V for the remainder of the time that the valve remains open.

In a further preferred embodiment, the valve control means and valves described above with reference to Figure 3 will

- 11 be co-located upon a single circuit board 650. A number of circuit boards can then be connected in serial and physically located in a vertical array so that the valves can deposit a two-dimensional matrix on a print substrate.

5 In such a case (see Figure 5), one of the boards 650a will be connected via serial input/output 622 to the computer control system 230 and to the second board via serial input/output 624. The second board 650b will be connected to the first board via serial input/output 622 and to the 10 third board 650c via serial input/output 624, and so on.

The last board in the serial chain can detect its position as its serial input/output 624 will have no connection.

On power up the last board in the serial chain assigns itself address 0 and transmits this address to the 15 preceding board, which then assigns itself address 1.

This process continues, with the address value being incremented until each board has an assigned address. The first board 650a will then report its address to the computer control system such that the system is aware of 20 the number of connected boards. The system will prefix any communication with a board with the board's address.

Preferably 16 boards are connected together to provide a 16 x 16 printing matrix.

25 The FPGA used in the preferred embodiment was a Xilinx Spartan II XC2S100 which was preferred as its configuration was determined by the data loaded from the PROM in start up. Such an FPGA may be replaced by a cheaper device in which the FPGA is hardwired, for example 30 by blowing fuses to form logic elements, rather than configurable through software.

- 12 It will be understood that the present invention is suitable for use with any type of solenoid valve and in any application in which solenoid valves are used. The applicant has found that the invention is of particular 5 advantage when used with high speed solenoid valves that find advantage in drop on demand ink jet printers.

Specifically, the invention is of advantage when used with the high speed solenoid valve described in our copending application GB 0203439.5.

Claims

- 13 CLAIMS

1. A method of operating an ink jet printer, the printer 5 comprising one or more ink jet valves, the method comprising the steps of: (a) activating one or more of the ink jet valve or valves to generate a print matrix; and (b) analysing the print matrix.

2. A method of operating an ink jet printer according to claim 1, the method further comprising the step of (c) altering the activation of one or more of the ink jet valve or valves in accordance with the print 15 matrix analysis.

3. A method of operating an ink jet printer according to claim 2, wherein the activation time of one or more of the ink jet valve or valves is increased.

4. A method of operating an ink jet printer according to claim 2 or claim 3, wherein the activation time of one or more of the ink jet valve or valves is decreased.

25 5. A method of operating an ink jet printer according to any preceding claim in which steps (a), (b) and (c) are repeated for a range of different valve actuation frequencies. 30 6. A method of operating an ink jet printer according to any preceding claim in which steps (a), (b) and (c) are repeated for a range of different print sizes.

- 14 7. A method according to any preceding claim in which the one or more ink jet valves are activated in response to receiving a control signal from a control means.

5 8. A method according to claim 8, wherein the control means comprises a data byte.

9. A method according to claim 7 or claim 8, wherein the value of the received control signal corresponds to the 10 valve activation time.

10. An ink jet printer for performing any of the methods of claims 1 to 9, the printer comprising one or more ink jet valves; 15 imaging means for imaging the print matrix; and control means to control the activation of the one or more ink jet valves and to analyse the print matrix image.