GB2392872A

GB2392872A - Operating ink jet valve during printing

Info

Publication number: GB2392872A
Application number: GB0216029A
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: GB0216029D0

Abstract

A method of operating an ink jet valve, the method comprising the steps of accelerating the valve to and from a closed position to an open position; maintaining the valve in the open position for a pre-determined period of time; and then accelerating the valve to the closed position, wherein the valve is decelerated as it approaches the open position. The valve may also be decelerated as it approaches the closed position. Furthermore, there is a method of operating an ink jet valve, the method comprising the steps of applying a first voltage to a coil to move a coil from a closed position to an open position; and applying a second voltage to a coil to maintain the coil in the open position for a pre-determined time period.

Description

- 1 DEVI CE AND METHOD

The present invention relates to a device, notably a high speed solenoid type valve, and a method of operating an 5 such a valve, preferably in an ink jet printer head.

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

2 - field ceases and the plunger returns under the bias of the

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 5 orifice. For convenience, the term drop on demand printer will be used to denote in general such types of ink jet printer. According to a first aspect of the present invention there 10 is provided a method of operating a solenoid valve, the method comprising the step of energising an electric coil to generate a magnetic field in order to reciprocally

drive a plunger within a tube, wherein the magnetic field

is controlled such that the speed of the plunger is 15 decreased as the plunger approaches at least one of its extremes of movement. The control of the magnetic field

may be achieved in a number of ways.

In a preferred embodiment, the magnetic field may be

20 controlled such that the speed of the plunger is decreased as the plunger approaches its closed position, in order to reduce the likelihood of impact damage. The magnetic field may be controlled such that the speed of the plunger

is decreased, the magnetic field resisting a force exerted

25 on the plunger by a return means.

The electrical coil can be energized using substantially triangular or substantially sinusoidal pulses.

30 DESCRIPTION OF THE DRAWINGS:

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 a solenoid 10 valve according to the present invention which is used within a drop on demand inkjet printer; Figure 3 shows a graphical depiction of current waveforms for use in the present invention; Figure 4 shows a first schematic depiction of a 15 preferred embodiment of printer apparatus that is operated according to the present invention; Figure 5 shows a second schematic depiction of a preferred embodiment of printer apparatus that is operated according to the present invention; and 20 Figure 6 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 30 ferromagnetic material (or any other magnetic material) and is received within the tube 30 so as to be able to

- 4 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 current generating a magnetic field within the tube, which

5 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 (not shown), such as a spring, that acts to return the 10 plunger to its initial position once the plunger has completed its full range of travel.

Conventionally, current is supplied to energise the coils as a simple square wave (or as a triangular wave having a 15 steep gradient) in order to provide a rapid acceleration of the plunger towards the closed end of the tube.

Similarly, once the plunger has reached its maximum travel within the tube, the current is reduced quickly in order to reduce the magnetic force acting on the plunger 20 quickly. This is advantageous as any magnetic force will oppose the force exerted upon the plunger by the return mechanism and thus the greater the magnetic force, the slower the return time of the plunger.

25 However, it has been established that in some high-speed applications for solenoid valves, such as their use within ink jet printers, and notably within 'drop on demand' ink jet printers, the increased rate at which magnetic forces are applied to and removed from the plunger are having a 30 deleterious effect upon the operation of the valve.

Figure 2 shows a schematic depiction of a solenoid valve 110 which is used within a drop on demand inkjet printer.

The plunger 120 is, in its closed position, received upon an end of a nozzle 150 so as to seal the nozzle. The tube 5 30 extends out at its open end to form a chamber 160 which comprises an inlet 170 through which ink can be supplied.

Energising the coils 140 through the application of an electrical current to the coils impels the plunger along the tube, towards the closed end of the tube. The 10 movement of the plunger unseals the end of the nozzle, allowing ink to flow into the nozzle. Once the magneto motive force (MMF) is removed from the plunger (through the removal of the current from the coils 140) the return mechanism (not shown) returns the plunger to its closed 15 position such that the plunger acts to seal the nozzle.

Some form of seal or baffle may be applied to the nozzle and/or the plunger in order to reduce the probability of ink entering the nozzle when the plunger is in its rest position. It is desirable for the return mechanism to return the plunger to its closed position quickly to avoid the nozzle being left open for too long: thus it is important to turn off the current pulse to the coils 40 as soon as possible.

25 The reciprocating motion of the plunger within the tube and the chamber is controlled so that a precisely controlled drop of ink will be propelled along the nozzle so as to be deposited upon a substrate. When used in applications such as ink jet printing, it is desirable to 30 operate solenoid valves at high speeds, for example above 4 kHz. Operation at such high speeds can cause problems

- 6 - due to the method by which the coils are energized.

The energising of the coils causes the plunger to undergo a rapid acceleration until its motion is impeded by the 5 end of the tube. Only the damping effect of the fluid within the tube and the force exerted by the return mechanism opposes the motion of the plunger caused by the energising of the coils.

10 The abrupt nature of the tube's motion has been observed to cause problems for ink jet printing. These problems include the formation of satellite droplets around the intended drops that are printed on the substrate. It is believed that the rapid acceleration of the plunger as it 15 moves away from its rest position is responsible for the formation of these satellite droplets and that the problem is exacerbated due to the limited fluid damping provided by the fluid within the tube. Furthermore it has been observed that if the force exerted upon the plunger by the 20 return means is too great and that the magnetic force applied to the plunger is minimal as the plunger returns to its rest position, then the impact of the plunger on the nozzle can cause damage to the structure of the plunger or the nozzle (or to any sealing means provide on 25 the nozzle or the plunger).

The possibility of producing such satellite droplets can be reduced by altering the method by which the plunger is impelled. Rather than using a square (or triangular) 30 current pulse to energise the coils as described above, it is possible to apply the current to the coils in a more

- 7 - gradual fashion. Similarly, if the manner in which the coils are deenergised is controlled appropriately, then the deceleration of the plunger will be less abrupt, which should serve to further reduce problems which are caused 5 by the impact of the plunger on the nozzle. It is believed that similar problems may occur when solenoid valves are operated at very high speeds in applications other than ink jet printing.

10 The current may be applied as a generally triangular pulse (which may or may not be symmetrical in the time domain), as a generally Gaussian pulse, a generally sinusoidal pulse or some other form of non-square pulse that reduces the initial acceleration and final deceleration of the 15 plunger. The exact nature of the solenoid valve and the rate at which it is being opened will determine whether or not the abrupt acceleration and deceleration of the plunger has a deleterious effect upon the operation of the solenoid valve and if so, what shape of pulse will be most 20 effective in reducing this effect.

Figure 3a shows a graphical indication of a typical triangular wave that is conventionally used to energise the coils. Figure 3b shows a graphical indication of a 25 triangular wave that is used according to the method of the present invention to energise the coils. It can be seen that in the first part of the waveform shown in Figure 3b the gradient of the wave is less than for the waveform of Figure 3a. This ensures that the plunger is 30 accelerated away from its rest position at a slower initial rate, reducing the possibility of forming

- 8 satellite droplets. It will also be noticed that in the latter part of the waveform there is a greater current so that the magnetic force exerted upon the plunger acts to damp the motion of the plunger, moderating the effect of 5 the return mechanism. It will be understood that the exact waveform will be dependent upon, amongst other factors, the nature and structure of the solenoid valve, the speed at which it is operated, the application in which it is used, etc., and that the waveform shown in 10 Figure 3b is purely exemplary.

Experimentation can be used to determine a suitable, or optimum, waveform or set of waveforms fort use with a particular application. One waveform that was found to be 15 of advantage is shown in Figure 3c. It has been found advantageous to initially open the valve by 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.

20 This reduces the possibility that the valve remains open for longer than is required to provide the 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 25 approximately 5V for the remainder of the time that the valve remains open.

During the high speed operation of solenoid valves in ink jet printing, the ink drops being deposited on a substrate 30 can be monitored using a CCD (charge coupled device) camera coupled to a computer control system to determine

- 9 - the number of problem satellite droplets that are being formed and the frequency with which they are being formed.

The collected data can be analysed by the computer, which can vary the current pulses accordingly to reduce the 5 number of satellite droplets being formed. The computer may select a current pulse from a range of pulses stored in memory, along with an indication of the likelihood of a given current pulse reducing the formation of satellite droplets. A balance must be struck between the demands of 10 high speed printing in order to operate with production line speeds and the quality of the printing and the computer control system should be able to strike a suitable balance between these two factors 15 In a preferred embodiment, the valve control means comprise a field programmable gate array (FPGA). FPGAs

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

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

Referring to Figure 4, the valves 610a, 610b,..., 610p are controlled by valve control means that comprise FPGA 616, electrically erasable programmable ROM (EEPROM) 617, RAM 25 618, programmable ROM (PROM) 619 and input/outputs 622, 624, 626. The FPGA 616 is connected to each of the valves 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 30 configuration data from PROM 619 and then follows the sequences that have been loaded from the PROM. The EEPROM

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 5 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 of the inputs. Input/output 622 is connected to the computer control system and input/output 624 can be used 10 to connect to a further valve control means (see below with reference to Figure 6). Input 626 provides a series of pulses that are used in co- ordinating the printing process. When the array of valves is printing onto a substrate, the substrate is normally moved underneath the 15 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 valves. 20 Figure 5 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 register 631 is used to write to and read from the EEPROM 617 and is also used when initialization data is read from 25 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 initiate a printing process. Second register 632 also writes print data to the RAM and is used to initialise the 30 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 applied to the print head. Fourth register 634 receives print data from the RAM and passes it to the fifth register 635, which uses the print data to operate the 5 valves 610.

A desired print image (which may include alphanumerical characters) is entered into the computer control system and this image is then converted into raster data that may 10 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 the print data can be supplied in the form of a raster comprising a 4 bit word for each valve, with the value of 15 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 is logically arranged in 16 rows, with each of the valves corresponding to a row. There are a plurality of columns, 20 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 supplies pulses to the FPGA.

25 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 order to generate the desired greyscale from a look- up table held in the first register. In theory, each valve 30 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 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, 5 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.

10 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 locations have been read then all the memory locations in 15 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 to the fourth register, which may perform further 20 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 calculates the number of shaft encoder pulses that are 25 equivalent to the valve times. The valves are then opened for a period of time equal to that number of shaft encoder pulses. As the valves 610 are electro-mechanical devices, their 30 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 required) then the matrix may be slanted so that the valves are closer together in one axis. The disadvantage 5 of this is that if no correction is made to the print rasters then the desired image will be printed out slanted. Such a correction may advantageously be provided using the 10 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 15 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 20 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.

In a further preferred embodiment, the valve control means 25 and valves described above with reference to Figure 4 will 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.

30 In such a case (see Figure 6), one of the boards 650a will be connected via serial input/output 622 to the computer

- 14 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 third board 650c via serial input/output 624, and so on.

5 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 O and transmits this address to the preceding board, which then assigns itself address 1.

10 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 the number of connected boards. The system will prefix 15 any communication with a board with the board's address.

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

The FPGA used in the preferred embodiment was a Xilinx 20 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 by blowing fuses to form logic elements, rather than 25 configurable through software.

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 30 applicant has found that the invention is of particular advantage when used with high speed solenoid valves that

- 15 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

- 16 CLAIMS

1. A method of operating an ink jet valve, the method comprising the steps of: 5 ta) accelerating the valve to from a closed position to an open position; (b) maintaining the valve in the open position for a predetermined period of time; and then (c) accelerating the valve to the closed position, 10 wherein during step (a) the valve is decelerated as it approaches the open position.

2. A method according to claim 1, wherein during step 15 (c), the valve is decelerated as it approaches the closed position.

3. A method according to claim 1 or claim 2, wherein during step (a) the acceleration of the valve has its 20 greatest value at substantially the midpoint between the closed position and the open position.

4. A method according to claim 1 or claim 2, wherein during step (a) the acceleration of the valve is increased 25 as the valve moves from the closed position prior to the deceleration of the valve as it approaches the open position.

5. A method of operating an ink jet valve, the method 30 comprising the steps of (a) applying a first voltage to a coil to move a

- 17 coil from a closed position to an open position; and (b) applying a second voltage to a coil to maintain the coil in the open position for a pre-determined time period.

6. A method according to claim 5, wherein the first voltage is greater than the second voltage.

7. A method according to claim 5 or claim 6, wherein the 10 first voltage is substantially 36V and is applied to the coil for substantially 80ps and the second voltage is substantially 5V and is applied to the coil for the remainder of the pre-determined time period.

15

8. An ink jet valve configure to perform the method of any preceding claim.

9. An ink jet printer comprising one or more ink jet valves according to claim 8.