GB2461014A

GB2461014A - Detection of deflection field collapse in a continuous inkjet printer

Info

Publication number: GB2461014A
Application number: GB0802348A
Authority: GB
Inventors: Jonathan Morgan; Andrew Robert Lester; James Edward Holmes
Original assignee: Domino Printing Sciences PLC
Current assignee: Domino Printing Sciences PLC
Priority date: 2008-02-08
Filing date: 2008-02-08
Publication date: 2009-12-23
Anticipated expiration: 2028-02-08
Also published as: GB0802348D0; GB2461014B

Abstract

A number of methods of detecting the collapse of the high-voltage field used in a continuous inkjet printer to effect deflection of charged ink drops are described. The methods are based on processing of signals derived from one or more charge detectors which may be also be used for detecting phase and/or ink drop velocity measurement. At least one ink drop charge detector is positioned within deflection field. The charge detector may be used to monitor the phasing of the ink drops. The collapse may be identified when the timing of a phasing signal is not within a time period in which the phasing signal is expected. A second charge detector may be used to further discriminate between phasing signals. A field collapse event may be identified if two detectors register signals simultaneously and printer logic can be programmed to discriminate between a phase signal which would be separate in time and a breakdown signal even signal which would be simultaneous. Alternatively a charge detector associated with a velocity sensor is used to identify signals indicative of field collapse (ie signals which are approximately an order of magnitude larger than expected). Alternatively a charge detector connected to an amplifier with a sufficiently low gain may be used to identify catastrophic field collapse.

Description

IMPRO VEMENTS IN OR RELA TING TO CONTINUOUS INKJET

PRINTERS

Field of the Invention

This invention relates to continuous inkjet (CIJ) printers and, in particular, to a method and/or apparatus to enhance the reliable operation thereof

Background to the Invention

Continuous ink jet printing involves the formation of electrically charged drops from a jet of ink, and the subsequent deflection of the charged drops by an electric field to produce an image on a print medium. In a typical embodiment of a single-jet printer of this type, electrically conducting ink is forced through a nozzle by applying pressure to the ink. This forms ajet of ink, the velocity of which requires careful control. This control is often achieved by control of the constituency of the ink in conjunction with controlling the pressure. Pressure control, in turn, is often achieved by varying the speed of the pump which produces the flow, using feedback from a pressure transducer. Pressure control is also achieved using feedback from a velocity measurement device.

A controlled sequence of ink drops, each with identical drop volumes and with constant separation between adjacent drops, can then be formed by modulating the jet to give active and controlled drive to the natural process of jet break up. This is usually achieved by modulating the ink pressure in a sinusoidal way, at fixed frequency and amplitude, or by modulating the ink velocity relative to the nozzle. A range of options and techniques are known to introduce pressure modulation, velocity modulation, or a combination of both, so that uniform drop sequences are obtained.

Charge is induced on individual drops through capacitive coupling. Desired levels of charge are induced on drops by applying a voltage to charge electrodes, positioned adjacent to the nozzle, at the time the drop separates from the jet. Care must be taken to time the charge pulses so that they have a fixed relationship with the time that the droplet breaks from the ink stream. In normal operation variations in jet velocity or pressure caused by control loops, viscosity drift, or temperature fluctuations, cause the exact moment when the droplet breaks from the stream to vary. In order to charge a droplet in synchronization with the moment the droplet breaks from the stream, the printer establishes a phase relationship between the droplet break-off time and the modulating drive frequency.

The phase relationship is determined by applying charging pulses with different phase relationships to the modulation signal, and measuring the amount of charge imparted to the drops. The charge on the drops is established by capacitively coupling to the drops, and measuring the signal.

The device used to perform this function is referred to as a phase sensor. The phase relationship that imparts the maximum amount of charge to the drops is chosen. The measurement and control of the phase relationship is a key element to ensuring the reliable operation of a CIJ printer and methods for achieving this during the printing operation are well known to those skilled in the art.

After charging, drops travel through a constant electric field whose field lines are substantially perpendicular to the jet. Charged drops are deflected by an amount that approximately scales with the charge on the drops. This electric field, or deflection field', is formed by applying a high potential difference between two surfaces between which the drops pass.

Uncharged drops are collected by a gutter for ink re-flow and re-use.

Another significant factor in the reliable operation of a CIJ printer is ensuring that the high potential deflection field remains at a constant level. Any event concerning the deflection field that causes catastrophic field collapse, such as the shorting of a high voltage (HV) plate to a lower potential by a conducting path, will result in the misplacement of drops. In the worst cases a significant amount of ink can be deposited on the substrate in an uncontrolled way.

Further, if the field collapse arises as a result of break-down of air saturated by ink vapour, then the spark formed could present a fire hazard as many of the inks are volatile.

Due to the undesirable consequences of catastrophic field collapse, most CIJ printers are equipped with a sensor that detects such events and, when tripped, causes the printer to be shutdown.

A usual method for sensing deflection field collapse is to employ a capacitive sensor in the wire that connects the high voltage deflection plates with the high voltage power supply. It is usual to form a capacitor using the wire itself and making additions to the sleeve; or by connecting a suitably rated capacitor between the high voltage conductor and suitable detection/conditioning circuitry. These methods are problematic in that the capacitors experience the full high voltage (typically up to 8kv). This means that a reliable device becomes quite bulky and needs the use of well-controlled materials, so that the sensor itself does not breakdown and become a source of the catastrophic

field collapse that it is attempting to sense.

It is an object of this invention to provide a method and/or apparatus which addresses the problems set forth above; or which at offers a novel and useful alternative.

Summary of the Invention

Accordingly, in one aspect, the invention provides a method of determining deflection field collapse in a continuous inkjet printer, said printer having an ink droplet charge detector positioned within said deflection field, said method being characterized in that it includes processing a signal derived from said drop charge detector to discriminate between a charge on an ink

droplet and collapse of said deflection field.

Preferably said printer includes a droplet charging electrode and a further ink droplet charge detector positioned further from said charging electrode than the first-mentioned ink droplet charge detector, said ink droplet charge detector and said further ink droplet charge detector combining to provide an ink droplet velocity detector, said method comprising processing a signal derived from said further ink droplet charge detector to determine collapse of

said deflection field.

Preferably said method further includes conditioning signals derived from said ink droplet charge detector and said further ink droplet charge detector at a position closely adjacent to said detectors.

In one embodiment a signal from one of said ink droplet charge detectors is amplified and compared to a threshold wherein a signal above said threshold is taken as indicating deflection field collapse. Preferably the signal derived from said further ink droplet charge detector is used for the comparison.

In an alternative embodiment the signal from one of said ink droplet charge detector and said further ink droplet charge detector is tuned for the measurement of charges on ink droplets involved in phasing while the other of said charge detectors is tuned to detect a collapse of said deflection field. In this embodiment a lower gain is applied to a signal from said other of said charge detectors such that said other of said charge detectors is substantially insensitive to charges carried by passing ink droplets.

In yet a further alternative, the method involves establishing a signal threshold level; comparing the duration of an input signal above said threshold with the expected duration of a phasing signal received as part of a phasing exercise; and assuming deflection field collapse if the duration of said input signal exceeds the duration of a phasing signal by a predetermined amount.

In still a further embodiment the method involves comparing the timing of signals detected by said ink droplet charge detector and said further ink droplet charge detector; and assuming deflection field collapse if the time difference is less than a pre-determined level.

In a second aspect the invention provides a continuous inkjet printer when adapted to perform any of the methods set forth above.

Preferably said printer includes two charge detectors mounted on one surface of a non-conducting substrate, said substrate further including signal modifying electronics on a further surface thereof Preferably said modifying electronics include an amplifier and a filter.

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

The invention will now be described with reference to the accompanying drawings in which: Figure 1: shows a schematic view of a charge electrode and deflection

field in a typical, known, CIJ print head;

Figure 2: shows a known schematic circuit for detecting deflection field collapse in a CIJ print head; Figure 3: shows an elevation of a CIJ print head adapted according to the invention; Figure 4: shows a sectional view along the line Z-Z in Figure 3; Figure 5: shows a front-side view of a phase/velocity sensor assembly incorporated in the print head shown in Figures 3 & 4; Figure 6: shows a view of the rear of the sensor plate included in the assembly shown in Figure 5; Figure 7: shows a rear view of the assembly shown in Figure 5; Figure 8: shows a side view of the assembly shown in Figures 5 & 7; and Figure 9: shows a circuit for implementing one means of discriminating a high-voltage breakdown of the deflection field in a CIJ print head.

Description of Working Embodiments

This invention employs the capacitive elements in the phase/velocity sensor to detect catastrophic field collapse. That is to say, collapse of the deflection field (the field between the deflection plates) in a continuous inkjet printer.

To achieve this end, the phase/velocity sensor, which may be broadly of the form shown in our European Patent 0 951 393, is placed in the deflection field and the signals there-from are processed to discriminate between the phase signals and catastrophic field collapse. Given the required tasks, and the presence of system noise, it is necessary to modify the signals from the phase/velocity signal at source. To this end, and as described in greater detail below, the form of sensor described in EP 0 951 393 is modified by the addition of amplifiers and other signal modifying components on the reverse side thereof. Although the phase/velocity sensor requires modification, the resulting design results in a saving in cost and space, as well as eliminating any handling of high voltages by the sensor, thus providing a reliable and sensitive trip sensor.

To set the scene for a description of how the invention may be reduced to practice, some basic aspects of CIJ printing will be described.

Charging synchronisation is achieved by sensing drops which have a fixed negative charge applied at a specific (but variable) phase of the modulation voltage. When the optimum drop position within the charge electrode is achieved, ink droplets will be charged to the maximum level. If these drops then pass a charge sensor, the level of charge on the drop can be measured. A schematic of a conventional charge sensor is shown in Figure 1. The charge sensor comprises an assembly of a sensing electrode(s) 10 and a grounded surface 11, the assembly being held sufficiently close to the charged ink droplets 12 to be able to detect a charge from the droplets that is at least three times higher than the system noise. The placement of the sensing electrode 10 within an aperture 13 in the grounded surface 11 creates a uni-directional sensor. The grounded surface 11 is also arranged so that it screens the sensing electrode(s) 10 from electromagnetic noise sources. The signal from such a device will be relatively small and requires conditioning. This is generally done in the printer for reasons of cost andlor simplicity.

Referring now to Figure 2, a known method of sensing a breakdown of the sensing field involves the coupling of a comparator 15 to the plate 16 at high potential. This coupling is usually achieved by means of a small capacitance 17 (a few pico farads) that can withstand the high voltage. This capacitor is placed in series with a higher value capacitance 18 to form a potential divider.

The trip event is then detected by comparator 15 sensing that the voltage has risen above a pre-determined threshold.

Velocity sensing is achieved using two droplet charge detectors that are held a set distance apart. The difference in time between the two sensors receiving the signal is then measured to give ajet velocity. It is important that the two sensors give clear signals and are maintained at a known distance apart. This is enabled by the device described in EP 0 951 393.

As stated above, the invention uses one or more drop charge detector(s) to detect the collapse of the field, and hence a HV trip event. The invention calls upon a design of drop charge detector capable of sensing and discriminating between the charge on droplets and the collapse of deflection field. Three means for discrimination are described herein: firstly by comparison of the strength of signals; secondly by assessing the timing of the signal; and thirdly, if two sensors are used, by assessing whether the signal occurs simultaneously on the two sensors.

The central component preferably used in the performance of the invention is a phase/velocity/trip sensor 20 as shown in Figures 5 to 8. The sensor 20 is mounted in a CIJ print head 19, one form of which is shown in Figures 3 & 4.

The print head 19 includes a droplet generator shown in dotted outline at 21 in Figure 4. The form of the droplet generator 21 is not essential to the performance of the invention but it may, for example, be as shown in our published International Patent Application No. WO 2006/0300 18.

As with all CIJ print heads, drops emitted by the droplet generator 21 pass a charge electrode 22 and then enter a deflection field 23 defined between a ground plate 24 and a high voltage plate 25. Uncharged drops are not deflected as they pass through the deflection field 23, and are collected by the gutter 26 for re-circulation through the ink supply system of the printer.

As can be seen, the sensor is mounted on, or incorporated in, the ground plate 24. The sensor 20 is a hybrid sensor based on thick film technology as described in EP 0 951 393, and has two charge detection pads on the front face 30 thereof, pad 32 being for phase setting and pad 33, in combination with pad 32, being for velocity detection. Signals from the pads 32 and 33 are preferably modified or conditioned at source with the addition of electronics integrated into the sensor; in particular, on the reverse side 31 thereof as shown in Figure 6, In the form shown the electronics mounted on the device consist of two instrument amplifiers Ui, U2 and combinations of resistors and capacitors to perform some frequency filtering and, thereby, reduce system noise. The precise configuration of the sensor electronics does not form part of the invention and will be readily ascertainable by one skilled in the art.

Referring to Figures 5, 7 & 8, the sensor plate is combined with a shielding can 35 to reduce electrical pick-up from other devices in the print head. The shielding can 35 is a simple folded metal section sized to substantially overlie the rear surface 31 of the sensor plate 20. The shielding can may, for example, be formed from 0.2mm copper plate. An indent 36 may be provided in one edge of the can to accommodate wires 37 leading from the connecting pads 38 provided on the surface 31, the wires 37 terminating in a connector 39.

As stated above, the phase/velocity sensing elements are capacitors formed from layers of a conducting material (gold) and a dielectric laid down on a ceramic substrate as described in EP 0 951 393. The sensing pads 32 and 33 are formed on one side 20 of the substrate and protected by a cover-glaze.

The electronic components Ui, U2 etc are mounted on the reverse side 31 of the substrate. A plated through-hole (not shown) provides connection between the sensor and the electronics.

Amplif'ing the phase signal at source provides the best possible signal to noise ratio, and noise rejection characteristics. The addition of conditioning electronics on the same ceramic substrate, to provide pre-amplification and filtering, produces a much better response.

It will be appreciated that the signal created by a catastrophic breakdown of the deflection field is much larger than that created by a drop, the former signal being approximately an order of magnitude larger in size. This allows simple discrimination between signals based on size. The circuit illustrated in Figure 9 shows one means of discrimination although others will be easily discernable to those skilled in the art.

In the embodiment shown in Figure 9, the signal from velocity sensor 33 is used, because the sensor 33 is physically located further from the charging electrode 22 and is thus less prone to cross-talk. The processed velocity + signal is capacitively coupled, via Cl, into a voltage divider formed by Ri and R2. This provides a DC bias which is, in turn, fed into a line driver IC 1. If the signal on the Velocity + signal is sufficiently large, it will cause the switching threshold of the line driver IC! to be exceeded thereby toggling the outputs of LC 1 and generating a trip signal which is passed to the system processor to shut the printer down.

As already described, the preferred embodiment of sensor assembly 20 incorporates two charge detectors 32 and 33 conventionally optimized for phase measurement and arranged at a fixed distance apart so that jet velocity information can be obtained. In the discrimination method described above, just the signal from charge detector 33 is used.

An alternative discrimination method employs the signals from both charge detectors. In this alternative method, one detector, preferably detector 32, is tuned for the measurement of small charges on droplets involved in the phasing process, while the other detector is tuned specifically for detecting the collapse of the field. In this embodiment the signal from the second detector is subjected to a lower gain and is relatively insensitive to the signal produced by passing droplets. Again a signal measured by the second detector which is above a pre-determined threshold will be assumed to indicate deflection field collapse and will thus be directed to cause printer shutdown. It will be appreciated that, in this configuration, the sensor 33 will not be able to, in combination with sensor 32, function as a velocity detector. If, in this embodiment, a velocity detection function is desired, a third sensing electrode must be provided.

A third alternative for sensing deflection field collapse relies on the observation that phase measurement is deterministic; a phasing pulse will only be detected if a drop has been charged. The logic in the printing system can be configured so that any signal received from a charge detection electrode, which is of a certain magnitude above the noise level of the system, and when the timing of the signal is not within a time period in which a phasing signal is expected, will be interpreted as a field collapse event. This is a viable means of detection as the time duration of a field collapse event far exceeds the time duration of a phasing signal (a few hundred microseconds for phasing) and also the elevated signal caused by field collapse will persist beyond the duration of the phasing window. The advantage of this alternative is that its implementation requires only one sensor and does not need a line driver for driving the HV lines to the printer, nor indeed the lines themselves.

In a fourth alternative, if two charge detectors are employed then the printer can further discriminate between phasing signals and deflection field collapse by examining the timing of signal generation by the two charge detectors. As previously disclosed the sensor arranges the two detectors a fixed space apart so that it can derive droplet velocity information by measuring the time for a packet of charged drops to pass between the sensors. A field collapse event would cause both sensors to register a signal simultaneously. Thus the printer logic can be programmed to discriminate between a phase signal, which would be separated in time, and an HV trip event signal, which would be simultaneous. This alternative can also be combined with the first alternative described above with reference to Figure 6.

It will thus be appreciated that the present invention provides effective methods of employing one or more droplet charge detectors, already provided in a CIJ print head for phasing and/or drop velocity determination, to also

sense deflection field collapse.

Claims

Claims A method of determining deflection field collapse in a continuous inkjet printer, said printer having an ink droplet charge detector positioned within said deflection field, said method being characterized in that it includes processing a signal derived from said drop charge detector to discriminate between a charge on an inkdroplet and collapse of said deflection field.
2. A method as claimed in claim 1 wherein said printer includes a droplet charging electrode and a further ink droplet charge detector positioned further from said charging electrode than the first-mentioned ink droplet charge detector, said ink droplet charge detector and said further ink droplet charge detector combining to provide an ink droplet velocity detector, said method comprising processing a signal derived from said further ink droplet charge detector to determine collapse ofsaid deflection field.
3. A method as claimed in claim 2 further including conditioning signals derived from said ink droplet charge detector and said further ink droplet charge detector at positions closely adjacent to said detectors.
4. A method as claimed in claim 3 wherein a signal from one of said ink droplet charge detectors is amplified and compared to a threshold wherein a signal above said threshold is taken as indicating deflectionfield collapse.
5. A method as claimed in claim 4 wherein the signal derived from said further ink droplet charge detector is used for the comparison.
6. A method as claimed in claim 3 wherein the signal from one of said ink droplet charge detector and said further ink droplet charge detector is tuned for the measurement of charges on ink droplets involved in phasing while the other of said charge detectors is tuned to detect acollapse of said deflection field.
7. A method as claimed in claim 6 wherein a lower gain is applied to a signal from said other of said charge detectors such that said other of said charge detectors is substantially insensitive to charges carried by passing ink droplets.
8. A method as claimed in claim 3 including the step of establishing a signal threshold level; comparing the duration of an input signal above said threshold with the expected duration of a phasing signal received as part of a phasing exercise; and assuming deflection field collapse if the duration of said input signal exceeds the duration of a phasing signal by a predetermined amount.
9. A method as claimed in claim 3 including the step of comparing the timing of signals detected by said ink droplet charge detector and said further ink droplet charge detector; and assuming deflection field collapse if the time difference is less than a pre-determined level.
10. A continuous inkjet printer when adapted to perform any of the methods claimed in any one of claims 1 to 9.
11. A printer as claimed in claim 10 including two charge detectors mounted on one surface of a non-conducting substrate, said substrate further including signal modifying electronics on a further surface thereof.
12. A printer as claimed in claim 11 wherein said modifying electronics include an amplifier and a filter.