GB2557169A - Improvements in or relating to continuous inkjet printers - Google Patents

Abstract

A method of calibrating a zero voltage level in a continuous inkjet printer comprises: setting a voltage of a charge electrode at a point expected to be zero; measuring a potential difference on a charge electrode; comparing with earth and minimizing the difference between the measured voltage and ground. The printer preferably compares voltages between prints A, E and phase tests C.

Description

(71) Applicant(s):

Domino UK Limited

Trafalgar Way, Bar Hill, CAMBRIDGE,

Cambridgeshire, CB23 8TU, United Kingdom (72) Inventor(s):

Christopher Adrian Chapman Simon Brierley Daniel John Lee (74) Agent and/or Address for Service:

Ipca Consulting Limited

Northpoint House, 52 High Street, KNAPHILL, Surrey, GU21 2PY, United Kingdom (51) INT CL:

B41J 2/02 (2006.01) B41J 2/08 (2006.01)

B41J2/12 (2006.01) (56) Documents Cited:

US 4408211 A US 20130342597 A1

US 20130249982 A1 (58) Field of Search:

INT CL B41J

Other: EPODOC WPI Patent Fulltext (54) Title of the Invention: Improvements in or relating to continuous inkjet printers Abstract Title: Zero calibration for a continuous inkjet printer (57) A method of calibrating a zero voltage level in a continuous inkjet printer comprises: setting a voltage of a charge electrode at a point expected to be zero; measuring a potential difference on a charge electrode; comparing with earth and minimizing the difference between the measured voltage and ground. The printer preferably compares voltages between prints A, E and phase tests C.

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IMPROVEMENTS IN OR RELATING TO CONTINUOUS INKJET PRINTERS

Field of the Invention

This invention relates to continuous inkjet (‘CIJ’) printers and, in particular, to a method for ensuring correct drop charging in such printers.

Background to the Invention

CIJ printers are widely used to print identification codes and other image data on products. Typically a CIJ printer includes a printer housing and a printhead connected to the housing via a conduit so that the printhead can be positioned adjacent to a production line or conveyor along which products to be coded are transported. In use ink is pressurised in the housing and then passed, via an ink feed line in the conduit, to the printhead. At the printhead the pressurised ink is passed through a nozzle to form an inkjet. A vibration or perturbation is applied to the inkjet causing the jet to break into a stream of droplets.

The printer includes a charge electrode to charge selected droplets, and an electrostatic facility to deflect the charged droplets away from their original trajectory and onto a substrate. By controlling the amount of charge that is placed on droplets, the trajectories of those droplets can be controlled to form a printed image.

A CIJ printer is so termed because the printer forms a continuous stream of droplets irrespective of whether or not any particular droplet is to be used to print. The printer selects the drops to be used for printing by using the charge electrode to apply a charge to those drops, unprinted drops being allowed to continue, on the same trajectory as they were jetted from the nozzle, into a catcher or gutter. The unprinted drops collected in the gutter are returned from the printhead to the printer housing via a gutter line included in the same conduit as contains the pressurised ink feed line feeding ink to the printhead. Ink, together with entrained air, is generally returned to the printer housing under vacuum, the vacuum being generated by a pump in the gutter line.

It follows from the description above that is important that printed droplets are charged correctly so that they are placed correctly on the substrate. It is also important that any unprinted drops carry no charge, or very close to no charge, so that they are properly collected by the ink catcher/gutter for return to the ink recirculation system.

If the unprinted droplets carry a small amount of charge, they will be deflected by the electrostatic field. At a certain level the unprinted droplets can strike the edge of the gutter causing the droplet to shatter into a mist of finely charged particles. The ink mist will be attracted to the components in close proximity to the gutter, and to the gutter itself, soiling the printhead and ultimately causing failure or at least requiring the printhead to be cleaned.

Many electrical circuits, including those controlling the charging or non-charging of ink droplets, suffer from a drift in performance with changing environmental conditions. In particular temperature changes are well known to affect the values of capacitors and resistors and to cause voltages or currents to change slightly from their designed values. Normally such drifts in component values with temperature are not problematic for the appearance of a print in a CIJ printer as the printer control has a degree of tolerance to the absolute charges on a sequence of droplets as long as they move together, which is the case when such drift occurs. However a drift in the value of the zero level of the charge electrode can be catastrophic for the reasons outlined above.

One method used to minimize environmental drift is to use high tolerance and expensive components in the charging circuit however is undesirable in terms of cost, undertaken.

It is an object of the present invention to provide a method to resolve or at least go some way to addressing the aforementioned drawback; or to at least provide a novel and useful alternative.

Summary of the Invention

Accordingly, in one aspect, the invention provides a method of calibrating a zero voltage level in a continuous inkjet printer configured to generate a stream of ink droplets and having a charge electrode to impart charges to selected droplets, said method being characterised in that it includes the steps of setting a voltage of the charge electrode at a point expected to give a zero output; measuring a potential difference of the charge electrode relative to earth; and modifying the set point so that the difference between the charge electrode potential and earth level is minimised.

Preferably said printer is configured to undertake phase measurements between prints, said method being performed between a phase measurement and a print.

Preferably said method further including analysing a message to be printed to determine if space exits to perform said method while printing said message.

Preferably said printer includes a central processing unit including a clock; and a field programmable gate array (FPGA) receiving a signal from said clock, said method being controlled using said FPGA.

Many variations in the way the present invention can be performed will present themselves to those skilled in the art. The description which follows is intended as an illustration only of one means of performing the invention and the lack of description of variants or equivalents should not be regarded as limiting. Wherever possible, a description of a specific element should be deemed to include any and all equivalents thereof whether in existence now or in the future.

Brief Description of the Drawings

One embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1: shows a schematic view of a typical CIJ printhead;

Figure 2: shows a charge and comparison circuit for performing a calibration according to the invention; and

Figure 3: shows a timing diagram between prints using a CIJ printer.

Detailed Description of Working Embodiment

A typical CIJ printhead 10 is shown in Figure 1. Ink is fed under pressure into a chamber 12 of droplet generator 11, from which it emerges as a jet 13 from a small outlet nozzle 14. The droplet generator further includes an electro-mechanical transducer such as a piezoelectric transducer assembly shown schematically at 15. The assembly 15 may be located within the chamber 12 or externally thereof.

Downstream of the nozzle 14 are located a charge electrode or pair of charge electrodes 16 and a phase detection electrode or pair of phase detection electrodes 17 that are arranged so as to be either side of and close to the jet 13. Typically, the two electrodes or sets of electrodes are placed within 500 microns of the jet and, for simplicity and accuracy of alignment, are preferably embodied in a single assembly.

The printhead 10 further comprises a pair of charged deflector plates 18 configured to generate a static electric field there-between; and a catcher or gutter 19 to collect unprinted drops.

In operation a sinusoidal electrical drive signal, commonly referred to as the modulation signal, is applied to the electro mechanical transducer 15. The frequency of the sine wave is chosen to match the nozzle size and jetting speed as defined by the physics of Rayleigh instability, to cause the jet 13 to break into a stream of droplets. For example, a frequency of around 80kHz applied to ink jetted through a 60 micron nozzle at 20m/s should lead to the formation of droplets from the jet 13.

In normal operation, when modulation is applied, droplets form within the charge electrode 16. Those droplets that are to be printed are charged by applying a square electrical pulse to the charge electrode, which is the full width of the period of the modulation signal. The charged droplets fly past the phase detection electrodes 17 and are deflected by the electric field held between the charged deflector plates 18. Charges are chosen on successive drops to form a character subject to a minimum charge value being necessary to deflect a drop past the gutter or catcher 19.

The drop charging voltage needs to be a very wide ranging signal as printing droplets are charged typically in the range of 40V to 250V whilst non-printing droplets that are used to perform charging tests in between prints, known as phase tests, are charged with a potential of opposite sign to that used for printing. The charge controller therefore needs to provide a range of voltages from, say, -14V to 250V ( or -250V to 14V depending on polarity) and this is typically controlled by a digital to analogue converter (DAC). The DAC may be incorporated in a field programmable gate array (FPGA), or provided as a stand-alone device.

Referring now to Figure 2, the charging system is preferably managed by an FPGA 20 that interacts with a microcomputer central processing unit (CPU) 21 running a control program. The CPU feeds drop charging data, or image characteristics, to the FPGA and the FPGA outputs logical values to the DAC 22 in time with the CPU clock input. Logical values are used to map the range of voltages required at a predetermined resolution. For example with an 8-bit DAC, a value of 0 will represent the lowest available potential e.g. -14V, whilst a value of 256 will represent a potential of +250V and the DAC is typically mapped linearly between these values. The DAC 22 converts the logical values into a series of square pulses, which are amplified by a circuit 23 (represented as Gain A) and passed to the charge electrode 16 to charge the droplets of ink. These types of low noise high voltage amplification circuits are well known (see for example ‘The Art of Electronics 2^nd Edition,

Horowitz and Hill p256) and will not be discussed in detail herein suffice to say that such circuits are a combination of transistors and operational amplifiers to provide gain and a number of resistors to set gain levels and capacitors to smooth signals and reduce noise.

It is well known that the level of gain provided by such circuits can drift as the values of the resistors and capacitors change with temperature and the invention therefore proposes that the level of potential fed to the charge electrode is monitored via a second amplification circuit 24 (Gain B). A potential divider 25 is used to scale down the size of the signal and this is further reduced by configuring the Gain B circuit 24 to give fractional (i.e. less-than-unitary) gain. The resulting monitoring signal is fed into an analogue to digital converter ADC 26, which may be a stand-alone device or incorporated inside the FPGA 20, and the digital output fed to FPGA 20.

The calibration is performed when the potential of the charge electrode 16 is zero, i.e. when the logical value fed into the DAC 22 is set to the level expected to be 0. The calibration can therefore take place in between prints and when the printer is not performing phase test measurements.

In the worst case for performing such a test, the printer is printing at a high speed, and therefore has a high droplet utilisation and there are few periods where the charge electrode voltage is zero for long enough to allow calibration to occur. The printer is designed so that it typically performs at least one phase measurement between prints and, as explained above, control of the charge electrode and hence the charging of the drops for printing and phase tests is managed by an FPGA so that the time between prints is minimised whilst still allowing for a phase test to be performed.

Accordingly, if an FPGA controls all aspects of charge control, the initiation of a phase test period can be considered deterministic when based on the next available droplet once the last printed droplet has passed the sensor 17. The time period in which the FPGA is waiting for the droplets to pass the sensor therefore presents an opportunity to check the zero charge level on the charge electrode.

Figure 3 illustrates a typical charging scheme as a function of time for a CIJ printer printing codes 5 drops high. In section A, the last stroke of a printed message is shown, printing vertically 3 drops, followed by a non-printed drop and a further printed drop in the top position. In section B, the printer waits a pre-determined time for the highly charged printing droplets to pass the sensor 17. The wait time may be pre-determined by experiment generally for a range of inks and pressure conditions and stored in a look up table, or by the printer by using the time taken between charging a phasing drop and detecting it at the last phase test. In all cases the time period is measured in whole cycles of the drive frequency for drop production so that a whole number of drops are formed. During this wait time the FPGA is able to perform a zero calibration, without compromising the availability of the printer to provide a print.

As soon as the print is finished and section B begins, the FPGA 20 issues a digital value representative of zero onto the charge electrode . The DAC 22 outputs a signal and this is amplified by the amplification stage 23. A fraction of the signal is monitored by a potential divider 25 and this is further reduced by a less than unit gain amplification stage 26. This signal fed into ADC 26, the output of which is fed into the FPGA 20.

The output from the analogue to digital converter is only monitored during the zero calibration process. It is expected that the maximum deviation at the stage of the potential divider 25 is +/- 3 V and this is further profiled by amplification stage 26 so that it becomes a value between 0 and IV. The full resolution of the ADC, which is the same resolution as the charge DAC is mapped across 0 - IV compared with the charge DAC where is mapped over the range -14V to 250V. The level of ADC 26 is read by the FPGA to give a charge delta from zero; if it is reading +ve it will decrease the zero point on the charge DAC 22, if it is reading at zero then no change is made and if it is reading -ve then the charge DAC 22 setting for 0V is increased. As the ADC 26 has a much higher resolution than the charge DAC 22 this gives a good basis for control. Typically the charge DAC and ADC are at least 16-bit devices and thus give more than sufficient resolution for the adjustment of the charge stage to zero, providing approximately 256 steps per volt.

It will be clear to those skilled in the art, that it is likely at the lower resolution of the charge DAC 22 that it is not possible for the charge level at the demand DAC value to be absolutely zero, but some small delta either side of it. The control system is therefore biased so that this small deviation always makes the charge value either zero or slightly negative (i.e. opposite to the potential used to charge droplets), so that drops are deflected into the ink gutter 19 rather than towards the edge of the gutter.

Returning to Figure 3, once the pre-determined time has passed, the FPGA will start charging phase test drops as shown in section C. Once all phasing drops in the packet are charged there is another wait illustrated as section D, which is equal to the predetermined time from section B or until the sensor has detected that the drops have passed the sensor. Section D affords another point in time where the printer can carry out a calibration if required, without changing the availability of the printer for printing.

As the FPGA controls printing as well as phasing, it is possible for the system to analyse the printed message and use periods where spaces are printed within the message. For example, the commonly printed message BEST BEFORE contains a space between the T and the B, the system will analyse the message to see if enough drops would be unused during the printing of the message and allowing for the space to perform the calibration test and if required will do so.

The zero calibration procedure described above is performed periodically and preferably every 10s. However, it is realised that drift is most likely to occur during the first minutes after powering on the system as the printer warms with usage. The system may therefore be configured to perform the measurement more frequently after the printer is first powered up, until the demand DAC value for printing has stabilised.

Claims (4)

Claims

1. A method to calibrate a zero voltage level in a continuous inkjet printer configured to generate a stream of ink droplets and having a charge electrode to impart charges to selected droplets, said method being characterised in that it includes the steps of setting a voltage of the charge electrode at a point expected to give a zero output; measuring a potential difference of the charge electrode relative to earth; and modifying the set point so that the difference between the charge electrode potential and earth level is minimised.

2. A method as claimed in claim 1 wherein said printer is configured to undertake phase measurements between prints, said method being performed between a phase measurement and a print.

3. A method as claimed in claim 1 further including analysing a message to be printed to determine if space exits to perform said method while printing said message.

4. A method as claimed in any one of the preceding claim wherein said printer includes a central processing unit including a clock; and a field programmable gate array (FPGA) receiving a signal from said clock, said method being controlled using said FPGA.

Intellectual

Property

Office

Application No: GB1617505.1 Examiner: Dr David Palmer