Patent Application
20140354725
1. Technical Field
The invention relates to a directivity detection device of trajectories of drops issuing from a liquid jet.
More particularly, it deals with control of the functioning of a continuous ink jet print head.
The invention detects whether the drops not printed and issuing from a continuous ink jet are effectively or not directed to the recovery gutter of these drops. It likewise determines the charge synchronisation of drops and allows to know the speed of drops issuing from the continuous jet.
The invention likewise relates to an associated electrostatic sensor, print head and printer with continuous ink jets.
2. Prior Art
Continuous ink jet printer heads comprise functional means well known to the person skilled in the art.
The functionality of these different means is described herein below. The ink contained in the drop generator 1 is issued from at least one calibrated nozzle 10 forming at least one ink jet 11. Under the action of a periodic stimulation device placed upstream of the nozzle (not illustrated), constituted for example by a piezo-electric ceramic placed in the ink, the ink jet breaks off at regular time intervals, corresponding to the period of the stimulation signal, at a precise point of the jet downstream of the nozzle. This forced fragmentation of the ink jet is usually caused at a so-called “break-up” point 13 of the jet by periodic vibrations of the stimulation device. At the location of this break-up point the continuous jet transforms into a spatial sequence 11 of evenly spaced identical ink drops. This drop sequence is directed according to a trajectory colinear to the axis of ejection of the jet which theoretically joins the centre of the recovery gutter 20, by geometric construction. Without the effect of external forces, the real trajectory of the drops follows a so-called “static” direction which can be slightly different from the theoretical direction in question, on the one hand because of the imprecisions in manufacture which produce an error of fixed orientation, and on the other hand, due to a drift of the orientation of the jet during operation due to changes in operating conditions of the jet by the nozzle. These changes can be caused in particular by modification of the surface conditions in and around the nozzle caused by accumulation of ink fouling. This problem becomes particularly sensitive after long periods of operation of the printer.
The charge electrode 4, located near the break-up point of the jet, is intended to selectively charge each of the drops formed at a predetermined electric charge value. To do this, with the ink being kept at a fixed electrical potential in the drop generator, a determined electric tension is applied to the charge electrode, different to each drop period. In order for the drop to be charged correctly, the instant of application of the electrical tension must take place slightly prior to instant of break up of the jet so that the electric continuity of the jet is ensured and a given quantity of charges is attracted by electrostatic influence to the tip of the jet. It is therefore necessary to synchronise perfectly the instant of application of the charge tension with the breakup process of the jet.
The two deflection plates 2, 3 are electrically driven to a relative fixed potential of a high value which produces an electrical field Ed substantially perpendicular to the trajectory of the drops. This field can deflect the electrically charged drops which engage between the plates, by an amplitude which is a function of their charge and of the speed of these drops. These deflected trajectories 12 are not collected by the gutter 20 and impact the medium to be printed 30. The placement of the drops on the drop impact matrix to be printed on the substrate is obtained by the combination of individual deflection given to the drops of the jet with relative displacement between the head and the medium to be printed. These two deflection plates 2, 3 are in general flat. One of them can also have an incurved profile or can be arranged at a certain angle. A more elaborate construction is that revealed in application FR 2 821 291 filed by the applicant and illustrated in
The recovery gutter 20 comprises at its inlet an opening 21 whereof the effective section is the projection of its inlet surface onto a plane perpendicular to the nominal axis of the non-deflected jet, placed just upstream in contact with the gutter. In the context of the invention this plane will be called an inlet plane of the gutter. Within the scope of the invention the nominal axis of the non-deflected jet is understood to mean the theoretical axis of the jet when all the sub-assemblies of the head are manufactured and placed relative to one another nominally once the head is assembled. In a print head with curved plates such as described in application FR 2 821 291, the gutter 20 can be positioned more upstream than the lower end of the deflection plates 2, 3 due to the presence of the slot 16, as illustrated in
It is known that control of the functioning of a continuous jet print head further requires functional means described earlier, using a certain number of complementary means allowing on the one hand deflection of the drops (which is determined to a large part by the electric charge and the speed of the drops) to be controlled and on the other hand the monitoring of the proper functioning of the recovery of non-printed drops.
With respect to controlling deflection of the drops, it is known to implement dedicated means especially to ensure, on the one hand, synchronised application of the charge signal of the drops with the instant of break-up of the jet (called synchronisation of the charge), and on the other hand, to measure the speed of the drops Vg in order to servo control it them to a preset value. To do this, the print heads according to the prior art generally comprise a measuring device of a representative magnitude of the charge carried by the drops. This measuring device is arranged downstream of the charge electrode. As this charge measuring is carried out, in general, when the specifically charged drops pass in front of this device, the method usually adopted to select the synchronisation instant of the charge relative to the break-up consists of performing repeated trials for changing sequences of drops with a succession of electrical charge signal of shorter duration than the drop period, but with different charge instants (also called “phases”) differently distributed throughout a drop period, and for each assigned phase, to measure the level of charge carried by the drop. The charge level is representative of the efficiency of the charging process of the drops and therefore of the suitability of the charge synchronisation. Some phases produce mediocre or even very poor charge synchronisation, but in general, a certain number of phases permits maximum charge. The charge phase to be used in printing will be selected from the latter. According to the solutions operated for measuring the charge of the drops in view of charge synchronisation, it is generally possible to deduce, in addition to these measurements of charge of the drops, an effective measurement of the speed of the charged drops. In fact, by detecting certain characteristic instants corresponding to the presence of drops identified at different characteristic geometric locations of the print head, it is possible to deduce there from an average travel time of the drops between these known locations, and therefore an average speed of the drops between these locations.
Among all the devices of the prior art, electrostatic sensors are generally used to fulfil this function.
Such a sensor is described for example in patent U.S. Pat. No. 6,357,860 assigned to Linx company and is constituted by two flat electrodes spaced along the trajectory of drops and forming an integral part of one of the deflection plates. This double-electrode sensor provides a signal when charged drops pass in front of each electrode: the amplitude of the signal is representative of the quantity of embedded charge per drop and the time offset between detection by each of the two electrodes give the duration of flight. The speed of the drops of the jet between these two points whereof the separation distance is known can thus be deduced. The advantage of this solution of sensors placed at the level of the deflection plates is to not increase the distance of flight of the drops in the head between the ejection nozzle and the medium to be printed. On the contrary, the disadvantage here is to expose the sensor to significant electrostatic perturbations, especially generated by the noise produced by the circulation of charged drops in the internal environment of the print head and by the noise emitted by the different internal components of the head, which are subjected to variable or noisy electric voltages. These conditions do not allow very precise measurements due to the very noisy signal of the sensor.
Patent EP 0 362 101 B1 in the name of the applicant describes a single electrostatic sensor placed between the charge electrode and the deflection plates, as well as the processing of the associated signal. The sensitive core of this sensor and the circulation space of the charged drops in front of this sensitive core are protected from electrostatic perturbations by electrostatic shielding. The presence of specifically charged drops is detected by their electrostatic influence on the sensitive core of the sensor. The exploitation of the signal obtained from these drops passing in front of this sensor takes very precise measurements of the charge level of these drops and defines the instants of their entry to and exit from the sensor, therefore the transit duration of these drops in the detection zone of the sensor. If the effective length of the zone travelled through is known, the average speed of the drops passing in front of the sensor can be deduced.
With respect to monitoring of the collection of non-printed drops, it is known to use dedicated means to detect that the ink not used for printing is properly recovered. If this ink escapes the gutter, the jet must be stopped to avoid fouling of the print head and its environment, fouling being generally unacceptable to the user of the printer. These problems can be created by deficiency of the recovery device which is incapable of evacuating the ink of the non-printed drops or by abnormal behaviour of the jet. In fact, the orientation of the jet can vary, such as for example be set at start-up at a value different to the nominal value or can move away from the nominal value during operation. No functional problem occurs as trajectories of the drops not intended for printing reach the interior of the gutter. On the contrary, dysfunctioning appears when the trajectory of the jet exits from the gutter or when drops strike its edge. Recovery detection can be done in different ways, especially by analysis of the resistivity of the fluid vein of the return circuit of the ink immediately downstream of the inlet of the gutter. Unfortunately, the system can be faulty since it cannot generally make the difference between the case of correct functioning and that where the jet, when improperly oriented, strikes the edge of the gutter. In this case, part of the ink enters the gutter to create the conditions which the resistivity sensor will interpret as a jet partially recovered by the gutter, a situation also characteristic of normal printing. So, in a situation where the jet is improperly oriented all or part of the ink of the jet contaminates the immediate environment of the edge of the gutter, or flows inside the gutter, which generally results in major dysfunction after it accumulates. The detection of correct recovery of the ink inside the gutter is therefore not reliable with solutions of the prior art.
This is why a certain number of solutions using sensors for locating the drops in this case has already been proposed. The localisation of ink drops by physical contact on a pressure sensor or by means of optical barriers is not reliable under industrial conditions of use of ink jet printers, due in particular to the sensitivity of such solutions to fouling by ink.
Other solutions according to the prior art consist in using electrostatic sensors, in so far as the liquid which makes the drops have come is conductive, the latter able to be charged electrically. The general principle uses the property according to which the level of the signal detected by an electrostatic sensor, during the passage of electric charges, depends on the distance between the active surface of the sensor and the charged drops. The localisation principle of the charged drops according to the state of the art consists in using two electrostatic sensors placed symmetrically on either side of the trajectory of drops the spacing of which relative to a nominal trajectory is to be evaluated. The difference in amplitude of the current signals delivered when charged drops pass in front of the sensors indicates the real position of the drops relative to the sensors in a certain single direction.
U.S. Pat. No. 3,886,564 assigned to IBM company describes several types of arrangement of pairs of electrostatic sensors, delivering signals whereof the differential processing determines the relative position of the drops passing in front of the sensors. The detection of position of charged drops in two directions defining a plane cutting the trajectory of these drops requires an arrangement of four electrostatic sensors arranged in two pairs and the implementation of electronics and the associated signal processing.
U.S. Pat. No. 4,551,731 and EP 0 036 789 assigned to Cambridge Consultants company describe this type of arrangement definitively requiring four sensors per trajectory of drops to be monitored for evaluating the drift, in two directions of the real trajectory of the drops relative to a nominal trajectory in passing in front of the sensors. Using this principle on a continuous ink jet print head leads to complex, bulky and costly implementation. This realisation causes other disadvantages:
In summary, the major disadvantages of recovery detection solutions of drop coming from liquid jet according to the prior art are the following:
The aim of the invention is therefore to eliminate the drawbacks of the prior art.
A particular aim of the invention is to propose a reliable and inexpensive solution for detection of the directivity of trajectories of ink drops issuing from a continuous jet in a print head, which ensures rapid detection of operating defects and optimal management of these possible defects to limit the harmful consequences for the user of the printer equipped with the head.
To do this, the invention relates to a directivity detection device of trajectories of drops issuing from liquid jet, the drops being charged electrically.
The device according to the invention comprises an electrostatic sensor comprising a portion for electric charge detection, made of electrically conductive material, said sensitive zone, surrounded by a portion made of electrically insulating material, said insulating zone, itself surrounded by a portion made of electrically conductive material and connected to earth to create electric shielding, said shielding zone; the zones of the sensor delimiting at least one continuous flat surface, the sensitive zone of the sensor comprising at least four edges including an upstream edge and a downstream edge connected to one another by two lateral edges, the arrangement of the sensor being such that:
The device likewise comprises means to process electrical signals created by the electrical charges of the drops in movement that are detected by the sensor, said means being adapted respectively for:
In the device according to the invention, the first comparison allows to know the actual position of a trajectory of drops in the plane parallel to the flat surface of the sensor and the second comparison allows to know the actual position of the same trajectory of drops in the plane perpendicular to the flat surface of the sensor.
It is specified here that within the scope of the invention the terms “upstream” and “downstream” must be understood by reference to the direction of flight of the drops issuing from liquid jet. Accordingly, the upstream edge of the sensitive zone is the part of the sensitive zone in front of which a given drop first passes.
Similarly, the term “height” is to be understood by reference to the direction of flight of the drops issuing from liquid jet: the height of the sensor zones according to the invention is the dimension according to the straight line H which is the projection of the nominal trajectory.
Advantageously, the signal-processing means comprise means to evaluate the time-interval T between the inlet peak Pe and the outlet peak Ps to deduce therefrom the speed of the drops Vg at the level of the sensor. In fact, from knowing the effective length Leff of a sensor according to the invention, it is possible to deduce the speed of the drops by the relationship Vg=Leff/T. As specified herein below, the effective length is defined substantially as being the distance separating the centres of the two strips of the insulating zone whereof one is situated adjacent to the upstream edge of the sensitive zone and whereof the other is situated adjacent to the downstream edge of the sensitive zone.
According to an embodiment, the arrangement of the sensor is such that its sensitive zone is symmetrical relative to the straight line H which is the geometric projection of the nominal trajectory of drops.
According to an alternative, the arrangement of the sensor is such that its sensitive zone is non-symmetrical relative to the straight line H, which is the geometric projection of the nominal trajectory of drops.
Thus, detection according to the invention can be implemented with a sensitive zone not necessarily symmetrical relative to the straight line H. In other terms, the electrostatic sensor according to the invention can have a non-symmetrical shape but with an arrangement such that the upstream edge and the downstream edge are substantially parallel to one another and the segments of each of these edges located on the same side of the straight line H have different lengths.
According to one characteristic, the difference in length, in absolute value, between the segment of the upstream edge and the segment of the downstream edge located on the same side relative to the straight line H is at least greater than one diameter of the drops.
The arrangement of the sensor is advantageously such that its flat surface is distant from the nominal trajectory of the drops by a distance comprised between twice the diameter of the drops and the height of the sensitive zone of the sensor. The distance between the drops of the nominal trajectory and the flat surface of the sensor is the result of a compromise to be found for making reliable the detection while functioning in a harsh environment.
Therefore, in the internal environment of a continuous ink jet printer head it is necessary to find a balance between two technical necessities:
The height of the sensitive zone is advantageously between 3 and 100 times the distance between successive drops in the jet.
The height of the insulating zone enclosing the sensitive zone at the level of the upstream and downstream edges is between 0.5 and 10 times the diameter of the drops. The selection of the heights of the sensitive and insulating zones produces substantial detection resolution. In fact, these heights are determined to produce on the signal highly distinct inlet and outlet peaks, that is, without possible overlap, and with maximum amplitude for given drop characteristics (length of the train of drops, speed and charge).
Also, the dimension of the flat surface delimited by the sensitive zone must advantageously be relative to the electrostatic influence area of the drops. This area depends on the distance of the drops relative to the sensor according to the invention. In fact, the quantity of charge caused on the sensor must be sufficient to generate a current exploitable by the signal-processing means. According to a preferred embodiment, the width of the sensitive zone is greater than twice the diameter of the drops.
The invention likewise relates to an electrostatic sensor comprising a portion for electric charge detection, made of electrically conductive material, said sensitive zone, surrounded by a portion made of electrically insulating material, said insulating zone, itself surrounded by a portion made of electrically conductive material and connected to earth to create electric shielding, said shielding zone; the zones of the sensor being delimited by at least one continuous flat surface, the sensitive zone of the sensor comprising, in a frontal view of the flat surface, at least two edges substantially parallel to one another, the straight line perpendicular to these edges which passes through the middle of one of these edges cuts the other edge in delimiting two segments of different lengths on either side.
According to an alternative, the invention likewise relates to an electrostatic sensor comprising a portion for electric charge detection, made of electrically conductive material, said sensitive zone, surrounded by a portion made of electrically insulating material, said insulating zone, itself surrounded by a portion made of electrically conductive material and connected to earth to create electric shielding, said shielding zone; the zones of the sensor being delimited by at least one continuous flat surface, the sensitive zone of the sensor comprising at least, in a frontal view onto the flat surface, at least two edges substantially parallel to one another and of different lengths, the straight line perpendicular to these edges which passes through the middle of one of these edges likewise passes through the middle of the other of these edges.
In a frontal view of the flat surface, the sensitive zone of the sensor, according to this embodiment of the invention, has a trapezoidal geometric shape, the insulating zone which surrounds the sensitive zone defining a quasi-homothetic trapezoidal shape.
In a frontal view of the flat surface, the lateral edges of the sensitive zone, which join the two edges parallel to one another, can have a curved, rectilinear or stepped profile. The profile could be selected as a function for best adapting the specification of the detection zone.
With respect to manufacturing an electrostatic sensor according to the invention, a conductive pass through is preferably made in a small insulating plate in a zone intended to be the sensitive surface. The assembly is preferably metallised on the two faces and at least on one edge of the plate, then etched locally to remove the metallisation on the patterns representing the insulating zones of the flat functional surface and to insulate the area where the conductive pass through terminates on the rear face. The shielding of the flat functional surface extends therefore over the majority of the rear face, ensuring optimal electric protection of the sensitive zone. The conductive pass through transfers the electric continuity of the sensitive zone to the rear of the plate where it is taken up by an adapted terminal. The plate is then preferably fixed tightly and in reference on a casing. When the electrostatic sensor according to the invention is implanted in a continuous jet print head, this casing will itself be mounted in reference on the one hand relative to the gutter and on the other hand relative to the nominal trajectory of non-deflected jet (in fact, the mechanical reference structure of the head).
The small insulating plate in which the conductive pass through is made is preferably made of Al2O3 ceramic at 99.7% purity. It can also be made of any type of insulating material which can be metallised.
The conductive pass through is preferably constituted by a stuck metallic insert, but can also be constituted by a metallised via.
The metallisation step is preferably carried out by depositing thin layers made by metallic vapor deposition. The metallised layers preferably comprise a sub-layer of chrome covered by a layer of gold. Other metallisation techniques leading to the same results can be used.
The etching step of the conductive layer can advantageously be ablation by laser, but can also be chemical etching or machining. The person skilled in the art will ensure that significant precision during this etching step is respected.
The terminal preferably consists of a ribbon cable of “flex” type (printed circuit on flexible Kapton®), connected by conductive adhesion. It can also consist of welded cables or electric connection by conductive spring contacts.
Other technologies are also feasible for manufacturing a sensor according to the invention, such as:
The invention likewise relates to a continuous ink jet print head comprising a drop generator fitted with an ink-ejection nozzle from which a continuous jet is issued, a charge electrode arranged downstream of the ejection nozzle for electrically charging drops issuing from the jet, a pair of deflection electrodes spaced apart from one another and arranged downstream of the charge electrode for selectively deflecting the charged drops intended for printing, a recovery gutter for non-deflected drops and at least one electrostatic sensor as described previously.
The deflection electrodes each preferably have an incurved active surface, the active surface of one of them comprising a pass through slot for letting the non-deflected drops pass, the electrostatic sensor being arranged between said slot and the recovery gutter. The deflection electrodes disclosed in patent EP 0 362 101 B1 cited in the preamble are particularly specified.
The electrostatic sensor is arranged preferably close to and upstream of the recovery gutter of non-deflected drops. Thus, the downstream edge of the sensitive zone is preferably distant from the inlet plane of the gutter by a minimum distance between 0.5 mm and 5 mm, for a drop diameter of between 70 μm and 250 μm. In fact, the downstream edge of the sensor must be as close as possible to the opening of the gutter to have maximum precision in the evaluation of the detection surface. This also helps enlarge the sensitive zone to the maximum with gains on several parameters, such as the Signal/Noise ratio, the jet/sensor distance, . . . . On the contrary, there is a risk of fouling when the drops arrive at high speed on contact inside the gutter: droplets can splash out of the gutter and foul the sensor. The sensor must therefore be sufficiently far from the gutter to be out of reach of these splashing droplets. In practice, the compromise of distance defined hereinabove has proven optimal for a drop diameter of between 70 μm and 250 μm, effectively corresponding to the types of drops issuing from a continuous ink jet of a printer. A first arrangement of the sensor made in the print head is such that its flat surface is substantially perpendicular to the deflection plane of the drops and opposite the directions of deflection defined as being the directions between zero deflection trajectory and the plurality of deflection trajectories caused by the deflection electrodes during printing.
Another arrangement made of the sensor is such that its flat surface is substantially parallel to the deflection plane of the drops and to the rear of the ink jet, the front of the ink jet being defined in reference to the front face of the head. With these two arrangements, accessibility for maintenance of the print head is optimal.
There is also the feasibility of using the combination of two electrostatic sensors each arranged in one of the two perpendicular positions mentioned hereinabove. The two sensors are not mandatorily positioned at the same distance from the gutter along the trajectory of the jet. This extends the detection zone in a print head by determining solely, for the two sensors, the representative function of the difference between the levels of the inlet peak Pe and outlet peak Ps. In fact, the evaluation of the distance between the drops and a single sensor is limited by attenuation of the signals and degradation of the signal/noise ratio when the drops move away from the face of the sensor: therefore, using a second electrostatic sensor arranged perpendicularly relative to the first extends the detection zone.
The invention finally relates to a continuous ink jet printer comprising a print head described previously and signal-processing means of the detection device likewise described previously.
The drops detected by the detection device according to the invention are preferably drops called test drops charged by the charge electrode during normal operation of the printer and inserted within a sequence of drops deflected by the deflection electrodes with a view to being printed. The test drops can be charged with inverse polarity to that of the drops deflected with a view to being printed.
The signal-processing means can advantageously be connected to an alarm which is triggered if at least one of the comparisons results in confirming that one of the values or the range of predetermined values has been exceeded, the triggering of the alarm signalling the risk of non-recovery of all the non-deflected ink drops by the gutter.
A printer according to the invention can advantageously comprise means for varying the charge phases of the drops. The signal-processing means are adapted, during variation of the charge phases, to determine the highest peak of the representative signal of the electrical current derived from a charge in movement detected at the level of the same edge of the sensor, the charge electrode then being set during operation of the printer on the charge phase causing this highest peak.
The invention defined hereinabove enables detection and monitoring of the bidirectional displacement of a jet of drops around a nominal trajectory.
In fact, the processing of the signal issuing from an electrostatic sensor according to the invention allows at the same time to evaluate the value of the lateral displacement of drops parallel to the sensor, relative to their nominal trajectory and the distance between the trajectories of these drops and the flat surface of the sensor. This results in evaluation of bidimensional directivity of the drops around a nominal trajectory at the level of the location where the drops pass in front of the sensor.
As specified hereinabove, the invention is applied in a print head and in particular to monitor the trajectories of non-printed drops, to verify that they are well directed to the interior of the gutter. Detection by the sensor of the real location of the trajectory of drops makes possible to trigger an alarm when the drops of the jet have a trajectory too close to the edge of the gutter.
On the other hand, without increasing the complexity of a print head such as described earlier, that is, by using a single electrostatic sensor, the processing of signals from the sensor likewise searches for the best phase of charge synchronisation and measures the speed of drops in the jet.
In the context of a continuous ink jet print head the inventors have thus attempted to ensure via automatic measuring that the jet is directed systematically at the gutter inlet, in determining its real orientation.
Other advantages and characteristics of the invention will be more apparent at the reading of the following detailed description, given in reference to the figures, of the detection device according to the invention in its application in a continuous ink jet print head and to the particular monitoring of the recovery of non-printed drops, in which:
The problem with which the inventors have been confronted is the following: theoretically, the trajectory of non-deflected drops referenced by 11 in these
The inventors have therefore decided to use a detection device which can locate the passing position of ink charged drops, so-called test drops, across a plane substantially perpendicular to their trajectory and situated between the charge electrode 4 and the recovery gutter 20.
Here, in the embodiment illustrated, the test drops 310 are drops emitted during normal operation of the print head: they are therefore inserted in a sequence of deflected drops intended for printing. Yet, during normal operation of the print head, the deflection plates 2, 3 are permanently fed by continuous high voltage and the deflection field between plates is therefore present throughout the trajectory of the test drops 310. For the test drops 310 to undergo minimal deflection and for them to behave in the closest possible way to non-deflected drops to be monitored (those which must return to the recovery gutter), a minimal charge level is produced with the charge electrode 4. In the mode illustrated, a charge level is placed on the test drops 310 such that their trajectory no longer deflects more than a drop diameter at the level of the sensor, relative to that of the trajectory of non-deflected drops, the directivity of which is to be monitored.
The inventors have first attempted to geometrically define a detection zone. The precise constraints defining the detection zone at the location of the recovery gutter will now be explained in reference to
These figures illustrate the inlet plane 411 of a gutter whereof the edge has a thickness e, the plane is viewed according to the direction of the nominal theoretical trajectory of the jet. It is specified here that the circular form of the inlet 21 of the gutter illustrated constitutes only one example and that it can take any shape, oval for example. For the sake of clarity, two axes X, Y perpendicular to one another are illustrated in the inlet plane 411: the axis Y is the nominal axis of deflection of the drops (that is, from one deflection electrode 2 to the other 3) and the axis X is an axis directed to the front of the print head. In other words, the axis X is parallel to the flat surface of the sensor and perpendicular to the axis Y. The axes Y and X therefore illustrate a system of axes for defining the relative position of the trajectories of drops relative to the centre of the gutter and relative to the sensor.
In the nominal conditions of
The relative positions of the points 300, 310 and 320 are largely independent of the orientation of the non-deflected jet and remain identical for any given application.
Yet, by definition, an electrostatic sensor can only detect charged drops: the detection zone is therefore the surface delimited by the curve 340 in
Also, the sensor according to the invention cannot be physically situated at the level of the inlet plane 411 of the gutter due to its intrinsic size: it is therefore located at the level of an intermediate plane situated between the charge electrode 4 and the gutter 20, preferably closer to the latter.
In concrete terms, as evident in
In practical terms, this means to detect the passing of test drops 310 through a surface 420 delimited by the intersection of the conical space 400 (defined earlier) and flat plane 421 (parallel to the inlet plane 411) perpendicular to the nominal trajectory 402 of the jet. This surface 420 is the conical projection of the surface 410 on the plane 421. The electrostatic sensor according to the invention is therefore arranged in this plane 421.
The detection device according to the invention is based on the principle of a single electrostatic sensor constituted and arranged such as shown in longitudinal sectional view in
With the passing of electrically charged drops 600 in the vicinity of the sensor, each drop 600 causes thereon a variation in the quantity of charges per unit surface. This charge variation is illustrated on the curve 620 as a function of the relative position of the charged drop in its direction of displacement (
The current circulating between the sensor and the ground, which is the derivative of the charge curve 620 gives a signal whereof the representative curve 630 has a inlet peak 631 and an outlet peak 632, the polarity of the two peaks are opposite.
The dynamic and the level of the signals depend on multiple factors, inter alia: the charge level of the drop, the distance between drops and sensor, the speed of the drop, the width of the insulating zone, the surface of sensitive zone present in the electrostatic influence area of drop. This electrostatic influence area 602, illustrated in
Since the other parameters are fixed, the absolute value of the level of the inlet or outlet peaks is representative of the embedded quantity of charges per drop. For a charge phase correctly synchronised with the instant of break-up of the jet, the levels in absolute value of the peaks are maximum. Their amplitude however depends on the conditions of use of the sensor and the characteristics of the application (ink, jet speed, drop frequency, sequence of test drops 310, . . . ).
Knowing the effective length Leff of the sensitive zone 612 of the sensor gives the average passing speed Vg of the drop in front of the sensor with the formula Vg=Leff/Tvol, by determining the time lapsed Tvol between the instants of extremums of the two inlet and outlet peaks. The effective length is defined within the scope of the invention as being substantially the length between the middles of the two insulating portion zones 610, one situated adjacent to the upstream edge 701 and the other adjacent to the downstream edge 702 of the sensitive zone 612.
The continuous flat surface 750 of the sensor is constituted by three distinct zones: a sensitive conductive zone 700 separated from a surrounding shielding zone 710 by an insulating zone 720.
The sensitive zone 700 is delimited by four edges: an upstream edge 701 and a downstream edge 702 connected by two lateral edges 703 and 704, which are rectilinear in
The shielding zone 710 is conductive and connected to ground. It extends over the whole face of the sensor, except for a reserved-out part including the sensitive surface 700 augmented by a margin over its entire periphery.
The insulating zone 720 corresponds to the margin in question, defined hereinabove. The width of the part of the insulating zone vis-à-vis each edge of the sensitive surface can be different, and can even be variable along each edge.
The arrangement of the sensor is such that the upstream 701 and downstream 702 edges are substantially perpendicular to the nominal trajectory of the drops 402 issuing from the non-deflected jet.
The straight line H, which is the projection of the nominal trajectory 402 of the non-deflected jet on the flat surface 750 of the sensor perpendicularly to the latter, separates the upstream edge into two segments 705, 706 and the downstream edge into two segments 707, 708 on either side of the straight line H. As illustrated, the electrostatic sensor is symmetrical relative to the straight line H.
The upstream and downstream segments, located on the same side relative to the straight line H (705 and 707 on the one hand, or 706 and 708 on the other hand), are different in length. On the same side of H, on the one hand, the length of the shorter segment is less than or equal to the maximum permissible amplitude of trajectory offset of the jet along the axis X in the direction to the side of H considered, and on the other hand, the length of the longer segment is substantially greater than this same amplitude.
In the preferred embodiment illustrated in
Application of the constraints expressed hereinabove, in the preferred embodiment illustrated in
A length of downstream edge 702 equal to around ⅔ of the inner diameter of the gutter 20 is preferably selected. This internal gutter diameter is in the present case greater than 10 times the diameter of a drop.
A length of the upstream edge 701 equal to around 4/3 of the inner diameter of the gutter is also preferably selected. The insulating zone vis-à-vis the upstream and downstream edges is a strip of constant width of the order of 3.5 drop diameter.
The insulating zone vis-à-vis the lateral edges 703 and 704 is preferably a strip of constant width equal to around twice the diameter of the drops. This width is less than that of the insulating zones vis-à-vis the upstream and downstream edges.
The height of the sensitive zone 700 is adjusted as according to the operating setting of the printer, specifically: drop size, drop frequency and jet speed. Given the values of the other parameters of the operating setting of the printer, this height has a preferred value of around 15 times the distance between drops in the jet.
The distance between the nominal trajectory of the non-deflected jet and the flat surface of the sensor delimited by the sensitive, insulating and shielding zones 700, 710, 720 is preferably the greatest possible to produce maximum tolerance to the instabilities of a jet which risk polluting the sensor; here it is substantially equal to ⅙ of the height of the sensitive zone.
As mentioned hereinabove, in the preferred embodiment, the test drops 310 are charged with inverse polarity to that of the drops intended for printing and at a value of the lowest possible electric charge causing the least possible deflection, while remaining measurable.
Given the relative upstream position of the sensor relative to the gutter 20 and the nominal distance d between the least deflected drop and the outer edge of the gutter which is here greater than around twice the diameter of the drops, at the level of the sensor the test drops 310 must remain in a surface of shape substantially identical to the test section 420 of
For an average drop diameter of the order of 150 μm, there are the following values respectively for an electrostatic sensor illustrated in
Operation of the drop trajectory directivity detection device will now be described.
The processings applied to the signal measured from the sensor are different to produce evaluation of the offset of the jet trajectories along the axis X (parallel to the sensor) or along the axis Y (perpendicular to the sensor), and are successively described.
In the examples of
It is also noted that for offsets greater than that of
One can therefore evaluate the lateral offset along the axis X of the jet being displaced parallel to the sensor by a representative function of the difference between the levels of the inlet Pe and outlet Ps peaks extracted from the representative signal of the current circulating to the sensor as the test drops 310 pass close by. The decision to trigger an alarm signalling excessive offset of the jet, that is, non permissible, is the result of a test on the value provided by this function.
The function in the preferred embodiment is the ratio in absolute value between level of inlet peaks and outlet peaks Ps/Pe and the test consists of verifying that the value obtained is greater than a single predetermined threshold value R. In a configuration where the shape of the sensitive zone of the sensor discriminates the direction of displacement of the drops, the value of the function of the levels of inlet Pe and outlet Ps peaks can be compared to two predetermined threshold values which correspond respectively to the instances of trajectories offset to the right and left of the straight line H.
An offset of the jet along the axis Y causes approach or distancing of the test drops 310 relative to the flat surface of the sensor. The nominal trajectory of the test drops 310 is entered at a distance of 700 μm from the sensor. The expected effect of the offset of the jet along the axis Y is a variation of the amplitude of the inlet and outlet peaks of the representative signal of the current circulating in the sensor.
If this offset of the jet along the axis Y is considered while it remains in the plane of symmetry of the sensor (X=0), the test drops 310 will remain in the permissible safety zone if they do not approach one another less than 400 μm from the sensor (or −300 relative to the nominal test drops trajectory situated at 700 μm from the sensor) and if they do not move away from one another more than 1300 μm (+600 μm relative to the nominal trajectory). The nominal trajectory is indicated in vertical dotted lines in
In
each of the levels of the inlet Pe and outlet Ps peaks decreases progressively as a function of the distance of the trajectories relative to the sensor,
the difference between the levels of the two inlet Pe and outlet Ps peaks remains approximately constant.
The calculated ratio Ps/Pe illustrated on the curve of
As explained earlier, evaluation of an offset of the jet in a predetermined safety surface 501 (
Therefore, the level of the inlet peak indicates the distance between the flat surface of the sensor and the trajectory of test drops 310 and for this distance, the ratio Ps/Pe indicates the lateral offset of the trajectory of test drops 310.
According to the invention an alarm procedure can also be established from the jet offset evaluations. This alarm procedure must lead to a binary output form between two situations:
Preferably, the alarm procedure is launched after assurance that the best charge phase is utilised, resulting in optimal signals. In fact, poor charge synchronisation relative to the break-up of the continuous ink jet could lead to aberrant and unstable peak levels, unusable for tests and alarm.
The steps of the procedure which triggers an alarm when the jet approaches the limit of the permissible safety zone are the following:
1—emission of a sequence of test drops 310;
2—elaboration of the representative signal of the current generated in the detection device when test drops 310 pass in front of the electrostatic sensor;
3—evaluation of the level of the inlet Pe and outlet Ps peaks present in the signal and calculating of the absolute value of the ratio Ps/Pe (|Ps/Pe|);
4—comparison between the higher level of the peak P (of Pe or Ps) with predetermined values Nmin and Nmax: if P>Nmax or P<Nmin the alarm is triggered and the procedure is abandoned. The higher peak is the inlet peak Pe with a sensor and arrangement according to
5—OTHERWISE (Nmin>P>Nmax) selection of a predetermined value R (from a memorised table or a function of calculation) as a function of the level of the peak P;
6—comparison between the ratio |Ps/Pe| and the value R: if |Ps/Pe|<R the alarm is triggered and the procedure is abandoned;
7—OTHERWISE the procedure terminates. In this step 7/, the trajectory of the jet is therefore considered permissible.
Phase Searching and Measuring the Speed of the Drops Issuing from the Jet:
With the same detection device illustrated in
Thanks to the invention, the combination of the phase search, speed measuring and evaluation of real position of jet can therefore be carried out in the same test sequence. The advantage of this is to reduce the time allocated to control measurements of a printer according to the invention equipped with an electrostatic sensor and signal-processing means as hereinabove. This is all the more significant since during this control time normal operation of the printer, that is, the production of printing, is interrupted. Otherwise expressed, in reducing the control time dedicated to implement the steps according to the invention, the availability of the printer is increased.
An advantageous arrangement of an electrostatic sensor according to the invention in a continuous jet print head is shown in
In the prior art, the implementation of electrostatic sensors in print heads required the length of the flying path of the drops to be increased in the print head, as it was necessary to physically interpose a sensor between the charge electrode and the gutter. The bulkiness of a sensor of the prior art was increased by the necessity to apply shielding around the sensitive core. For example, the patent EP 0 362 101 describes an electrostatic U-shaped sensor whereof the sensitive zone is placed at the bottom of the slot. The exterior of this U-shaped sensor is completely shielded, allowing effective protection vis-à-vis the electrostatic environment prevailing in the head. Similarly, for flat sensors, exposed directly to the electrostatic environment, the prior art proposes applying a shielding surface vis-à-vis the functional surface of the sensor with jet trajectories passing between the flat surface of the sensor and the applied shielding surface. Such a configuration is for example that of print heads marketed under the brand “Serie Imaje Serie 9020”.
But this increasing of the flying path length of the drops is not desirable, as it can result in degradation of printer performance, especially imprecision on the position of the printed drops.
The print head illustrated in
above the gutter 20, as far as possible from the nozzle to maximise measuring precision but likewise at sufficient distance from the gutter inlet to minimise the risk of pollution generated by the splashing coming from the gutter;
the flat surface 750 of the sensor is perpendicular to the deflection plane of the drops;
behind the deflection plate kept at 0V and at a very close distance to the latter. As explained hereinabove, the deflection electrode therefore plays the role of effective shielding vis-à-vis the sensor plane, without adding a additional shielding function.
The gutter can advantageously be placed more upstream than the lower end of the deflection plates. The casing of the sensor and the gutter can be mechanically linked for easier mutual positioning and to make the specifications of the detection zone solely defined by construction (without adjustment during assembly).
Implementation of the sensor in the head, as in
The invention which has just been described improves in particular directivity detection of trajectories of drops due to possible precise real-time evaluation of the actual bidirectional shifted position of a trajectory of charged drops relative to a nominal trajectory at a given location of the latter (advantageously close to the recovery gutter).
The advantages of a continuous ink jet printer according to the invention relative to ink jet printers of the prior art are the following:
precisely evaluating the bidirectional shift of trajectories of ink drops issuing from the jet of the drop generator of the print head;
triggering an alarm if the position of drop passing near a given sensor location with a monitored drop trajectory approaches limits or exits from a safety zone and in particular, exits from the inlet of the recovery gutter;
providing the user of a continuous jet printer, with reliable information on the recovery of non-printed drops, if required as a complement to information from a flow sensor in the gutter (any drops caught by the gutter with sufficient safety margin, or any significant risk for some drops of striking the edge of the gutter is detected) searching for the best charge phase synchronisation and measuring the drop speed.
In addition, executing the invention increases neither the complexity of the head nor its bulk. The flight time of drops circulating in the print head is not modified by detection according to the invention: printing performances are therefore preserved. Arrangement of the sensor does not impair accessibility in the print head which therefore remains optimal for maintenance. Integration of the sensor according to the invention in a print head with curved deflection electrodes creates effective shielding of said sensor vis-à-vis electromagnetic perturbations without disturbing passing of the deflected drops.
Other improvements can be made without as such departing from the scope of the invention.
In particular, if in the detailed description the trajectory of which the directivity has been detected is the trajectory of the non-deflected ink drops leading the former to the centre of the recovery gutter, the invention can also be applied to monitore the directivity of drop trajectories around a nominal trajectory, optionally deflected, not necessarily directed to the recovery gutter.
Also, the polarity of the charged drops detected according to the invention can be identical to that of the deflected printed drops or alternatively take on opposite values.
Also, the electrostatic sensor described precisely hereinbefore is a sensor whereof the sensitive zone and the insulating zone have trapezoidal shapes on its flat surface: detection can be adjusted by adapting the shape of the flat surface delimited by the sensitive zone and of the insulating strips, for example according to the shapes illustrated in frontal view in
The present application is a continuation of U.S. application Ser. No. 13/967,184 filed Aug. 14, 2013, which is a continuation of U.S. application Ser. No. 13/387,354 filed Apr. 5, 2012 and now issued as U.S. Pat. No. 8,511,802, which is a national stage filing of PCT/EP2010/060942 filed Jul. 28, 2010, which claims priority to U.S. Provisional Application No. 61/243,513 filed Sep. 17, 2009, all of which are incorporated by reference herein and in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61243513 | Sep 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13967184 | Aug 2013 | US |
| Child | 14444941 | US | |
| Parent | 13387354 | Apr 2012 | US |
| Child | 13967184 | US |
