EP0362101B1 - Ink controlling and regulating device for a continuous ink jet printer - Google Patents

Abstract

PCT No. PCT/FR89/00484 Sec. 371 Date May 17, 1990 Sec. 102(e) Date May 17, 1990 PCT Filed Sep. 11, 1989 PCT Pub. No. WO90/03271 PCT Pub. Date Apr. 5, 1990.A device for controlling and regulating a continuous ink jet printer wherein a jet (J) is fractionated into droplets charged in a charge electrode (6) and which then pass between deflection electrodes, a sensor (8) is provided which includes a conductor element (8c) having two parts which are symmetrical with respect to the trajectory of the droplets. The device includes a circuit (9) which determines and processes the first I(t) and second J(t) derivatives with respect to the time of the charge induced in the conductor element (8c) by charged droplets (Gc) in order to determine their speed. The device includes means for regulating the speed of droplets and means for regulating the ink quality.

Description

The present invention relates to devices for controlling and regulating an ink and its processing in a continuous inkjet printer.

The ink-jet writing technique using a continuous jet of calibrated droplets, supplied by a modulation system, consists in electrostatically charging these droplets, by means of an appropriate electrode. The passage of these variably charged drops between two electrodes brought to a large difference in electric potential leads to a deflection of the drops proportional to their charge. This deflection combined with the displacement of the support allows the matrix printing of characters or graphics on said support.

All the parameters conditioning the operation of the printer must be controlled so as to ensure the constant quality of the printing despite the inevitable variations in the environment.

The speed of the drops constitutes the most influential parameter on the print quality, because it conditions the passage time of the charged drops in the deflector electric field (and therefore the trajectory of the printed drops), but also the phenomenon of formation and electric charge of the drops in the charging electrode.

The quality of the ink is also a very influencing factor in the operation of printers for several reasons.

First, the physical properties of the ink (viscosity, density, surface tension) condition the flow of the ink in the nozzle, as well as the physical process of formation of the drops. The main factors leading to a variation in the physical properties of the ink are the evaporation of the solvent from the ink, on the one hand, and temperature variations, on the other hand.

Secondly, the chemical properties of the ink, which result from the concentrations of the various constituents of the ink, must be kept constant over time. The dye concentration must be controlled so as to ensure consistency in the optical quality of the markings on the printed medium (optical density, color, etc.). The quantity of resin present in the ink must be controlled because it conditions, in certain formulations, the electrical conductivity of the ink, and therefore the electrical charge of the drops. The quantity of resin must be particularly controlled for applications where a physicochemical treatment is applied to the printed deposit in a phase simultaneous or subsequent to marking, such as crosslinking under ultraviolet rays, reaction under radiation, etc., in order to give it specific chemical resistance properties.

The process of formation and electrical charge of the drops also conditions the quality of the print. A spectacular characteristic of printer malfunction linked to a defect in the drop formation process is the pollution of the deflection electrodes by small parasitic droplets commonly called satellite drops. The process of formation and electric charge of the drops results from the interaction of complex hydrodynamic and electrical phenomena, still poorly described by theory. The parameters influencing this process are linked both to the physicochemical properties of the ink and to the operating characteristics of the machine: geometry, jet speed, frequency and amplitude of modulation.

The object of the invention is to allow control and regulation of the parameters most influencing the print quality of an inkjet printer: speed of the drops, quality of the ink and process of formation and charging. drops.

More particularly, an important object of the invention consists in providing control and regulation devices which are simple and compact, therefore suitable for compact inkjet printers.

Another important object of the invention consists in providing control and regulation devices which can be used reliably under severe and highly variable environmental conditions (temperature, humidity, ventilation), as well as with types of 'different ink.

• températures ambiantes très différentes selon l'activité industrielle et grandes amplitudes de variations de cette température (impression en chambre froide, impression en extérieur);
• utilisation de solvants très volatils (méthyléthylcétone, alcools, etc.), dont l'évaporation est très dépendante de l'environnement (température, ventilation, etc.);
• utilisation de formulations d'encre très différentes, généralement choisies en fonction de la nature du support à imprimer (papier, métal, verre, matières plastiques, etc.)

A field of application particularly targeted by the present invention is the field of industrial marking, where the environmental conditions are very different and very variable over time:

• very different ambient temperatures depending on industrial activity and large amplitudes of variations of this temperature (printing in a cold room, printing outside);
• use of highly volatile solvents (methyl ethyl ketone, alcohols, etc.), the evaporation of which is very dependent on the environment (temperature, ventilation, etc.);
• use of very different ink formulations, generally chosen according to the nature of the medium to be printed (paper, metal, glass, plastics, etc.)

Various devices have been developed which allow control and regulation of the parameters most influencing the print quality of an inkjet printer.

Concerning the speed of the drops, in electrostatic printers, that is to say printers using electrostatically charged drops, a conductive element makes it possible to detect the proximity of the charged drops. In US Patent ^No. 3,836,912, described a method of detecting charged drops using such a device. Furthermore, in US Patent ^No. 3,852,768 and JP 55 19 514, the printer uses two separate inductive sensors positioned along the path of drops charged, and measuring speed associated, given by the difference between the time passage of these drops opposite the detectors. In EP-A-0 124 465 in the name of the present applicant, a particular embodiment of a detection system is described, in which the two inductive detectors are integrated in a single detection electrode, split, and placed in the axis of the trajectory of the drops.

In general, most of the inventions relating to the use of inductive detectors to measure the speed of charged drops mention the need to use at least two detectors. The major drawback of these dual detector devices lies in their size.

Found in U.S. Patent ^No. 4,612,553 a description on the use of a single inductive sensor for measuring the speed of charged drops. However, in this patent, no mention is made of the conditions relating to the size of the detector and necessary for the implementation of the method. In addition, few details relate to the associated signal processing circuit. It is mentioned that the latter provides an alternating signal frequency "almost proportional" to the speed of the drops.

According to the invention, the speed of the drops is measured by means of a single detector comprising a conductive element in two parts symmetrical with respect to the trajectory of the drops, said detector being located between the charging electrode and the electrodes of deflection. The conductive element of the detector is connected to ground via a resistor across which is connected a processing circuit. A charged drop, or a charged drop train, induces a charge of opposite sign in the detector element, and this charge varies according to the position of the charged drop, or of the charged drop train, in the detector. The processing of the first derivative I (t) and of the second derivative J (t) with respect to the time of the charge Q (t) makes it possible to determine the instants of entry and exit of the charged drop, or of the train of charged drops, in the detector and, consequently, its speed, the length of the detector being known.

• des moyens de fractionnement du jet en gouttelettes équidistantes et équidimensionnelles;
• une électrode de charge où lesdites gouttelettes sont sélectivement chargées électrostatiquement;
• un détecteur de la vitesse des gouttes chargées;
• des électrodes de déflexion où lesdites gouttelettes sont défléchies en fonction de leur charge, caractérisé en ce que :
• le détecteur est unique et comprend un élément conducteur central réalisé en deux parties symétriques par rapport à la trajectoire des gouttelettes,

et en ce que le dispositif comprend de plus un circuit électronique de mesure de la vitesse des gouttes comportant :

• des moyens de mesure du courant I(t) circulant entre le détecteur et la masse et correspondant à la dérivée première par rapport au temps de la charge totale induite dans le détecteur par une ou plusieurs gouttelettes chargées;
• des moyens de calcul de la dérivée seconde J(t) par rapport au temps de ladite charge;
• des moyens de traitement des dérivées première I(t) et seconde J(t) déterminant les instants T₁ et T₂ de passage à zéro de la dérivée J(t) pour une valeur suffisante du courant, qui correspondent respectivement aux instants d'entrée et de sortie de la ou des gouttelettes chargées dans le détecteur;
• des moyens de calcul de la vitesse V des gouttelettes à partir des instants T1 et T2 et de la longueur de l'élément conducteur du détecteur.

For this, the invention relates more particularly to a device for controlling and regulating an ink and its processing in a continuous inkjet printer in which a continuous inkjet comes out of a nozzle, comprising:

• means for splitting the jet into equidistant and equidimensional droplets;
• a charging electrode where said droplets are selectively electrostatically charged;
• a detector of the speed of the charged drops;
• deflection electrodes where said droplets are deflected as a function of their charge, characterized in that:
• the detector is unique and comprises a central conductive element produced in two parts symmetrical with respect to the path of the droplets,

and in that the device further comprises an electronic circuit for measuring the speed of the drops comprising:

• means for measuring the current I (t) flowing between the detector and the mass and corresponding to the first derivative with respect to time of the total charge induced in the detector by one or more charged droplets;
• means for calculating the second derivative J (t) with respect to the time of said charge;
• means for processing the first I (t) and second J (t) derivatives determining the instants T₁ and T₂ of zero crossing of the derivative J (t) for a sufficient value of the current, which correspond respectively to the input instants and leaving the droplet (s) loaded in the detector;
• means for calculating the speed V of the droplets from the instants T1 and T2 and the length of the conductive element of the detector.

In a preferred embodiment of the invention, the length of the detector is greater than the distance between its two symmetrical parts relative to the trajectory of the drops.

With regard to ink quality control, to compensate for the permanent evaporation of the solvent in the environment, the functioning of most of the ink circuits of inkjet printing machines consists, on the one hand , to continuously measure the viscosity of the ink in the ink circuit using a viscometer and, on the other hand, to regulate the viscosity of the ink supplying by adding solvent or fresh ink the nozzle. A description of an ink circuit operating according to this principle is given in particular in US Patent ^No. 4,628,329 in the name of the present applicant. The incorporation of the viscometer function into the ink circuit significantly increases the complexity of its operation and generally leads to significant additional bulk.

In addition, the place of viscosity measurement is generally far from the print head. At a given time, the viscosity measured in the ink circuit may not be representative of the actual viscosity at the print head. this is especially true when the temperature instead of viscosity measurement is different from the temperature at the print head. To overcome this drawback, various temperature control solutions of the ink in the print head have been proposed, generally incorporating a heating element (see U.S. Patent ^No. 4,337,468 of RICOH or US ^No. 4403 227 from IBM), which increases the complexity and energy consumption of the printer.

Another object of the present invention is to measure the "quality of the ink" at the print head, without using a viscometer function proper.

This object is achieved, according to the invention, by combining the use of a device for measuring the speed of the drops, an electronic circuit and a device for supplying ink to the nozzle cooperating in regulating the speed of the drops, of an ink pressure measurement in the ink circuit associated with rules for sizing the hydraulic conduits.

Another object of the invention consists in measuring a temperature representative of the temperature of the ink at the nozzle, and in correcting the quality of the ink by adding solvents or fresh ink, according to a law which takes account of temperature.

According to the invention, it is also planned to optimize the speed of the regulation of the quality of the ink, on the one hand by taking into account the time of flow and homogenization of the ink between the place of the additions. ink (or solvents) and the nozzle and, on the other hand, using an extra ink cartridge containing an ink whose concentration is higher than the nominal use value.

Regarding the control of the formation of drops, in inkjet printers of the continuous jet type, the pressurized ink is ejected by a nozzle in the form a jet which is caused to fragment into a series of droplets to which a charge is then applied selectively and which are directed towards the printing medium or towards a gutter. Various methods can be used to control and synchronize the formation of droplets, consisting of vibrating the nozzle, or causing disturbances of the ink pressure at the nozzle by incorporating in particular a resonator excited by a piezoelectric ceramic upstream nozzle. Due to the disturbance, the jet fragments, at the frequency of the disturbance, into uniform droplets, often accompanied by smaller droplets called satellite droplets. The presence of these satellite drops must be checked because, when the load of the drops is applied, the satellites have a higher mass load than the main drops: if these pass through the deflection field, they undergo deflections significant and cause, either a fouling of the deflection electrodes leading to electrical insulation faults, or parasitic impacts on the printed support.

Known art (see the article by BOGY in Annual Review of Fluid Mechanics 1979) shows that if one fixes the physical properties of the ink, the nozzle, the frequency of the disturbance, the speed of the jet, the device resonator and the shape of the excitation signal applied to the resonator, it is possible to control the formation of the drops by the amplitude of the disturbance applied to the resonator. It is possible, in particular, to inhibit the formation of satellite droplets by choosing an appropriate amplitude for the disturbance. Furthermore, the value of this amplitude fixes the place of fragmentation of the jet at a determined distance from the position of the nozzle (and therefore from the charging electrode).

The means used to apply the electric charge chosen at each droplet generally include a charging circuit and an electrode surrounding the jet at the place of formation of the drop. The electrostatic charge of the drop is then obtained by applying a voltage of amplitude Vc between a point of electrical contact with the ink and the charging electrode. The charge Qg acquired by the drop then depends on the value of the charge voltage Vc at the time of the formation of the drop, on the electric capacity Cg of the droplet formation / charge electrode assembly, and on the ratio of the period of droplet formation at the electrical characteristic time of the jet / electrode assembly, defined by Rj.Cj where Rj is the equivalent electrical resistance of the jet between the nozzle and the drop in formation, and Cj is the electrical capacity of the jet assembly /electrode. The parameters Rj, Cj, Cg are in particular influenced by the shape of the jet during the period of formation and charge of the drop. The electrical resistance of the jet Rj also depends on the electrical conductivity of the ink, which itself generally depends on the concentration and the temperature of the ink.

For a given printhead and ink, experience shows that it is possible to determine a relationship between the physical properties of the ink at the nozzle (rheology, surface tension) and the amplitude of excitation of the resonator. , so as to obtain a correct drop formation, that is to say so that the point of separation of the drops from the jet is close to the center of the charging electrode, and that the formation of satellite drops is inhibited.

According to the invention, provision is made to control and regulate the process of formation and charging of the drops by simultaneously regulating the speed of the drops, the quality of the ink, and the place of separation of the drops from the jet. Control of the place of separation of the drops from the jet is obtained by controlling the time of flight of the drops between the place of charge of the drops and the position of the drop speed detector. Regulation of the place of separation of the drops is obtained by modifying the amplitude of excitation of the resonator so as to maintain the place of separation of the drops at a place called operating point, which depends on the quality of the ink measured at the nozzle.

• la Fig. 1 est une vue schématique représentant les éléments principaux d'une tête d'impression dans une imprimante à jet d'encre continu selon l'invention,
• la Fig. 2 est une vue schématique, à échelle agrandie, représentant la buse, une électrode de charge et le détecteur pour mesurer la vitesse des gouttes de la tête d'impression de la Fig. 1,
• les Figs. 3a à 3d sont des vues de structure associées à des diagrammes de la densité de charge linéique induite dans le détecteur par une goutte chargée en fonction de sa position par rapport audit détecteur,
• la Fig. 4 est un diagramme représentant la charge Q(t) induite dans le détecteur par une goutte chargée par rapport au temps,
• la Fig. 5 est un diagramme représentant la dérivée première I(t) de Q(t) par rapport au temps,
• la Fig. 6 est un diagramme représentant la dérivée seconde J(t) de Q(t) par rapport au temps,
• la Fig. 7 regroupe en superposition les diagrammes de I(t), J(t), Q(t), ainsi que deux diagrammes représentant les valeurs de trois signaux numériques F1, F2 et F3 fonction de I(t) et de J(t) et servant à déterminer les instants d'entrée et de sortie d'une goutte chargée dans le détecteur,
• la Fig. 8 est une vue semblable à la Fig. 3b, à la différence près qu'un train de gouttes chargées au lieu d'une goutte chargée unique est utilisé pour la mesure de vitesse,
• la Fig. 9 est une vue regroupant les diagrammes de I(t), de J(t) et des signaux F1, F2 et F3 pour le cas où un train de gouttes chargées est utilisé pour la mesure de vitesse,
• la Fig. 10 est une vue schématique représentant sous forme de blocs le circuit associé au détecteur pour déterminer la vitesse des gouttes,
• la Fig. 11 est une vue détaillée du circuit de la Fig. 10,
• la Fig. 12 est une vue regroupant des diagrammes concernant le fonctionnement du circuit de la Fig. 11,
• la Fig. 13 est une vue schématique illustrant le dispositif de contrôle et de régulation de l'invention dans son ensemble,
• la Fig. 14 est un diagramme représentant des pressions de référence en fonction de la température concernant les composants de l'encre et un mélange approprié desdits composants, et
• la Fig. 15 regroupe les diagrammes de I(t), J(t) et un diagramme du chargement des gouttes Vc(t) illustrant comment est mesuré le temps de vol des gouttes entre le lieu de leur formation et l'entrée du détecteur et, par suite, la longueur entre la buse et ledit lieu de formation des gouttes.

The characteristics of the invention mentioned above, as well as others, will appear more clearly on reading the following description of a preferred embodiment, made in relation to the accompanying drawings, among which:

• Fig. 1 is a schematic view showing the main elements of a print head in a continuous ink jet printer according to the invention,
• Fig. 2 is a diagrammatic view, on an enlarged scale, showing the nozzle, a charging electrode and the detector for measuring the speed of the drops of the print head of FIG. 1,
• Figs. 3a to 3d are structural views associated with diagrams of the linear charge density induced in the detector by a charged drop as a function of its position relative to said detector,
• Fig. 4 is a diagram representing the charge Q (t) induced in the detector by a charged drop with respect to time,
• Fig. 5 is a diagram representing the first derivative I (t) of Q (t) with respect to time,
• Fig. 6 is a diagram representing the second derivative J (t) of Q (t) with respect to time,
• Fig. 7 superimposes the diagrams of I (t), J (t), Q (t), as well as two diagrams representing the values of three digital signals F1, F2 and F3 as a function of I (t) and J (t) and used to determine the instants of entry and exit of a drop charged in the detector,
• Fig. 8 is a view similar to FIG. 3b, at difference is that a train of charged drops instead of a single charged drop is used for speed measurement,
• Fig. 9 is a view grouping together the diagrams of I (t), J (t) and of the signals F1, F2 and F3 for the case where a train of charged drops is used for the speed measurement,
• Fig. 10 is a schematic view representing in the form of blocks the circuit associated with the detector for determining the speed of the drops,
• Fig. 11 is a detailed view of the circuit of FIG. 10,
• Fig. 12 is a view grouping together diagrams relating to the operation of the circuit of FIG. 11,
• Fig. 13 is a schematic view illustrating the control and regulation device of the invention as a whole,
• Fig. 14 is a diagram representing reference pressures as a function of the temperature relating to the components of the ink and an appropriate mixture of said components, and
• Fig. 15 groups together the diagrams of I (t), J (t) and a diagram of the loading of the drops Vc (t) illustrating how the time of flight of the drops is measured between the place of their formation and the entry of the detector and, by next, the length between the nozzle and said place of formation of the drops.

Fig. 1 illustrates the main mechanical and electrical elements of an ink jet print head 1 of the continuous jet type. It has in particular a nozzle 2 supplied with ink under pressure by an ink circuit 3 and creating a continuous jet J. Under the influence of the vibration of a resonator 4 supplied by a modulation circuit 5, the continuous jet J splits in the center of a charge electrode 6 into a continuous sequence of Equidistant and equidimensional G droplets. The charging electrode 6 is connected to a charging circuit 7. The drops G, driven by a speed V substantially equal to the average speed of the liquid in the jet J, then pass through a detector 8 used as phase detector and jet speed, and connected to an electric drop speed detection circuit 9. The charged drops are then deflected by a constant electric field maintained between deflection electrodes 10. The uncharged or lightly charged drops are recovered by a gutter 11 , while the others continue their flight to a recording medium, not shown. The drops recovered by the gutter 11 are recycled to the ink circuit 3.

Fig. 2 schematically illustrates the charged drop speed detection electrode 8, placed immediately downstream of the drop formation and charging site. In the figure, the passage of a single charged drop Gc, of charge Qg, shown in black and being close to the active conducting element 8c of the detector 8, is illustrated. The latter is electrically connected to the electric detection circuit. of speed of drop 9. The speed detection electrode 8 comprises a central conducting element 8c, preferably protected from the influence of external electric charges (present on the charging electrode 6 in particular), thanks to a thickness d insulator 8i and to an external conductive element 8e, said guard electrode, electrically connected to ground. In a preferred embodiment, the detector 8 has a plane symmetry and the drops G move in the axis of a slot made along the axis of symmetry of the detector. However, any other configuration of the symmetrical detector with respect to the axis of the trajectory of the drops G may be suitable. The droplets G have a substantially uniform translation speed V in the detector, and oriented along the axis of the detector.

, $\text{t2 = x2/V}$

, $\text{t3 = x3/V}$

, $\text{t4 = x4/V}$

, où les temps et les abscisses sont comptés positivement à partir de l'entrée du détecteur 8 et sont liés par la relation $\text{x = Vt}$

. Sur ces figures, la gouttelette chargée Gc est représentée en couleur foncée et les autres gouttelettes non chargées situées en amont et en aval sont représentées en clair. La distance entre les gouttelettes G, notée λ, est par ailleurs liée à la vitesse V et à la fréquence de modulation f par la relation $\text{λ = V/f}$

. D'autre part, l'art connu montre que pour des conditions de fonctionnement nominales d'une imprimante, cette distance est liée au diamètre de la buse par une relation du type :

$\text{λ = 4,5 à 6 ⌀B}$

où ⌀B est le diamètre de la buse. Pour simplifier, on retiendra la valeur 5 ⌀B.In the schematic representation part of Figs. 3a to 3d, or upper part of the figures, the charged droplet is represented at four different relative positions with respect to the detector 8, denoted x1, x2, x3 and x4, and corresponding to the times $\text{t1 = x1 / V}$

, $\text{t2 = x2 / V}$

, $\text{t3 = x3 / V}$

, $\text{t4 = x4 / V}$

, where the times and the abscissa are counted positively from the input of the detector 8 and are linked by the relation $\text{x = Vt}$

. In these figures, the charged droplet Gc is shown in dark color and the other uncharged droplets located upstream and downstream are shown in clear. The distance between the droplets G, denoted λ, is moreover linked to the speed V and to the modulation frequency f by the relation $\text{λ = V / f}$

. On the other hand, known art shows that for nominal operating conditions of a printer, this distance is linked to the diameter of the nozzle by a relation of the type:

$\text{λ = 4.5 to 6 ⌀B}$

where ⌀B is the diameter of the nozzle. For simplicity, we will retain the value 5 ⌀B.

où Li est la longueur totale d'isolant mesurée suivant la trajectoire des gouttes. Avec les simplifications ci-dessus, dans le cas d'une goutte de petite taille par rapport à la dimension transversale R du détecteur 8 (largeur de la fente du détecteur), la densité linéique de charge peut être approchée mathématiquement par la fonction :

   La courbe de densité linéique de charge est symétrique par rapport à la position x_i de la goutte. Comme l'indique la relation (2), les charges électriques induites par la gouttelette sur le détecteur sont plus concentrées à proximité de la goutte et pratiquement inexistantes à grande distance de la goutte. La longueur S de la zone influencée électriquement par la goutte Gc est représentée sur les Figs. 3a à 3d. D'après la relation (2), la longueur S de ladite zone vérifie la relation :

$\text{S = 2R (3)}$

   A un instant donné, la charge totale portée par l'élément actif 8c de longueur efficace Le est notée Q. Elle est définie par :

   Q correspond aux surfaces hachurées dans les Figs. 3a à 3d. Q varie avec la position x de la goutte dans l'électrode de détection 8c. L'évolution de la charge Q est représentée à la Fig. 4 en fonction du temps $\text{t = x/V}$

compté le long de la trajectoire de la gouttelette chargée Gc. Selon l'invention, les dimensions de l'électrode de détection 8c vérifient la relation :

$\text{S/2 < Le, soit, d'après (3)}$

$\text{R < Le (5)}$

   Ceci correspond à une largeur R de la fente suffisamment faible pour que la moitié au moins de la zone de longueur S influencée électriquement par la gouttelette Gc soit contenue dans la longueur efficace Le de l'élément conducteur 8c. Selon l'invention, la gouttelette chargée Gc dont on veut mesurer la vitesse est précédée en aval par au moins n1 gouttes non chargées, où n1 vérifie la relation :

$\text{(n1+1) > (Le+ R)/λ}$

ou encore, compte tenu de (1)

$\text{n1 > (Le + R)/(5 ⌀B) -1 environ (6)}$

   Cette condition permet à la goutte chargée d'entrer dans le détecteur de vitesse 8 alors que les gouttes précédemment chargées sont suffisamment éloignées pour ne pas influencer la mesure.The proximity of the charged drop Gc (the charges are represented by signs - around the charged drop Gc in Figs. 3a to 3d) leads by electrostatic influence to the appearance of electric charges of opposite sign on the surface of the detector (charges represented by signs + in Figs. 3a to 3d). The quantity of electric charges present on the detector varies according to the axial distance x. If the influence of the insulator 8i is neglected, this amount of charge can be represented in the form of a linear charge density σ (x) given diagrammatically on the ordinate for different positions x1 to x4 of the charged drop Gc. In reality, in the vicinity of the insulator 8i, the distribution of electrical charges is significantly modified and cannot be calculated in all rigor only with cumbersome numerical calculation methods to be implemented. However, to simplify the explanations which follow (text and figures), the process which is the subject of the invention will be described while ignoring the effects of the presence of the insulator 8i on the distribution of electrical charges. In practice, the influence of the insulator will be taken into account by replacing the length L of the active element 8c of the detector with an effective length. $\text{Le = L + Li / 2}$

where Li is the total length of insulation measured along the trajectory of the drops. With the above simplifications, in the case of a small drop compared to the transverse dimension R of the detector 8 (width of the detector slot), the linear charge density can be approximated mathematically by the function:

The linear charge density curve is symmetrical with respect to the position x _i of the drop. As indicated in relation (2), the electric charges induced by the droplet on the detector are more concentrated near the drop and practically nonexistent at a long distance from the drop. The length S of the area electrically influenced by the drop Gc is shown in Figs. 3a to 3d. According to the relation (2), the length S of said zone verifies the relation:

$\text{S = 2R (3)}$

At a given instant, the total charge carried by the active element 8c of effective length Le is denoted Q. It is defined by:

Q corresponds to the hatched areas in Figs. 3a to 3d. Q varies with the position x of the drop in the detection electrode 8c. The evolution of the charge Q is shown in FIG. 4 as a function of time $\text{t = x / V}$

counted along the trajectory of the charged droplet Gc. According to the invention, the dimensions of the detection electrode 8c verify the relationship:

$\text{S / 2 <Le, either, according to (3)}$

$\text{R <The (5)}$

This corresponds to a width R of the slot sufficiently small so that at least half of the zone of length S electrically influenced by the droplet Gc is contained in the effective length Le of the conductive element 8c. According to the invention, the charged droplet Gc whose speed is to be measured is preceded downstream by at least n1 uncharged drops, where n1 checks the relationship:

$\text{(n1 + 1)> (Le + R) / λ}$

or, taking into account (1)

$\text{n1> (Le + R) / (5 ⌀B) -1 approximately (6)}$

This condition allows the charged drop to enter the speed detector 8 while the previously charged drops are far enough apart not to influence the measurement.

où Lt est la distance qui sépare la buse de l'électrode de détection 8c et Lb est la longueur du jet J entre la buse et le point de formation des gouttes, ces distances étant représentées à la Fig. 2. On en déduit :

$\text{n2 > (Lt + Le - Lb)/5 ⌀B -1 environ (7)}$

   La condition (7) permet de s'assurer qu'aucune goutte n'est chargée pendant la durée où le détecteur 8 est influencé par la goutte Gc utilisée pour la mesure de vitesse. En effet, malgré le blindage de l'électrode de détection de vitesse 8c, celle-ci peut être parasitée par les tensions de charge appliquées sur l'électrode de charge 6. Il est d'ailleurs préférable, lors de la charge des gouttes utilisées pour la détection de vitesse, d'appliquer la tension de charge sur l'électrode de charge pendant la moitié, ou moins, de la période de formation des gouttes. Ceci permet de charger correctement les gouttes, tout en minimisant le parasitage de la mesure.Still according to the invention, the number n2 of uncharged drops according to the drop used for the measurement of the speed checks for equality:

$\text{(n2 + 1)> (Lt + Le - Lb) / λ}$

where Lt is the distance between the nozzle of the detection electrode 8c and Lb is the length of the jet J between the nozzle and the point of formation of the drops, these distances being shown in FIG. 2. We deduce:

$\text{n2> (Lt + Le - Lb) / 5 ⌀B -1 approximately (7)}$

Condition (7) ensures that no drop is charged during the time that the detector 8 is influenced by the drop Gc used for the speed measurement. In fact, despite the shielding of the speed detection electrode 8c, it can be parasitized by the charging voltages applied to the charging electrode 6. It is moreover preferable, when charging the drops used for speed detection, to apply the charging voltage to the charging electrode for half, or less, the period of drop formation. This allows the drops to be loaded correctly, while minimizing interference from the measurement.

   Dans la relation 8, Le est la longueur équivalente de l'électrode 8c, caractéristique de la mesure obtenue par calibration en utilisant une autre méthode de mesure de vitesse de goutte.If conditions (5), (6) and (7) are respected, the drop speed V is then obtained by measuring the duration between the instants T1 and T2 corresponding to the two inflection points of the function Q (t), either the relation:

$\text{V = Le / (T2 - T1) (8)}$

In relation 8, Le is the equivalent length of the electrode 8c, characteristic of the measurement obtained by calibration using another method of measurement of drop speed.

. Le même circuit électronique 9 mesure aussi la dérivée $\text{J(t) = d(I)/dt}$

de ce courant, donc la dérivée seconde de Q(t) représentée à la Fig. 6. J(t) s'annule aux temps T1 et T2 définis plus haut.A practical embodiment of the measurement is shown in Figs. 5 to 7. The electronic measurement circuit 9 detects the current I (t) flowing between the detector 8e and the ground. This current is shown in FIG. 5 and corresponds to the derivative with respect to the time of Q (t), that is $\text{I (t) = dQ (t) / dt}$

. The same electronic circuit 9 also measures the derivative $\text{J (t) = d (I) / dt}$

of this current, so the second derivative of Q (t) shown in Fig. 6. J (t) is canceled at times T1 and T2 defined above.

A means of implementing the measurement of T2 - T1 is described in FIG. 7. A count is triggered when simultaneously J (t) takes a negative value and I (t) is greater than a threshold + i _o . The counting is stopped when simultaneously J (t) takes a positive or zero value and I (t) is less than -i _o . The content of the counter then corresponds to the value T2 - T1 to be measured. The representation of the digital processing is given by the diagrams of the digital signals F1, F2 and F3. The counting lasts the time that the digital signal F3 is at the high logic level. The digital signal F1 is at the high logic level when I (t) is greater than the threshold i _o or less than the threshold -i _o . The digital signal F2 is at the high logic level when J (t) is positive or zero. The signal F3 goes to the high logic level during the falling edge of F2, F1 being at 1. F3 returns to zero during the next rising edge of F2 while F1 is at 1.

$\text{N < 1 + 2(Le - R)/5 ⌀B, environ (9)}$

   Cette condition stipule que la longueur SN du détecteur influencée électriquement par la train de N gouttes doit être inférieure à deux longueurs Le de l'électrode.The method for measuring the speed of charged drops, described above for the case of a charged drop, requires loading, and therefore deflecting, drops which are not useful for printing. In order not to print unnecessary drops on the recording medium, the drops loaded to perform the speed measurement are sufficiently light to be recovered by the gutter 11. Given the low level of charge of these drops, it is necessary, in order to increase the signal / noise ratio of the device, to carry out the measurement on a train of N equally charged and equidistant droplets. The linear density of charge σN on the electrode 8c of the detector 8 corresponds, in this case, to the sum of the contributions of the N charged drops of the train of drops (the case for three charged drops is represented in Fig. 8). The sum of the contributions of the N charged drops is symmetrical with respect to the center of the train of drops. In general, the speed measurement method is similar to that described above for the case of a single charged drop. The generalization to the case of N drops of the relation (5) can be written as a first approximation:

$\text{SN = (N - 1) λ + 2R <2Le or}$

$\text{N <1 + 2 (Le - R) / 5 ⌀B, around (9)}$

This condition stipulates that the length SN of the detector electrically influenced by the train of N drops must be less than two lengths Le of the electrode.

$\text{n2 > (Lt + Le - Lb)/λ - 1/2 - N/2 (7′)}$

   En fonction du rapport λ/R, la densité linéique σN peut présenter plusieurs maxima, comme le montre la Fig. 8. On a représenté à la Fig. 9 les évolutions des grandeurs I(t) et J(t) correspondantes et utilisées pour faire la mesure. On remarque que la grandeur I(t) présente une allure similaire à la densité linéique σN. Il en résulte que les points de passage à zéro de la fonction J(t) peuvent être multiples. Une variante de traitement de la mesure consiste à effectuer un comptage du temps s'écoulant entre les instants correspondant aux fronts montants du signal logique F2 au niveau haut lorsque J(t) est supérieur à une valeur J_o ou inférieur à une valeur -J_o, comme représenté à la Fig. 9. Cependant, dans un mode de réalisation préféré de l'invention, on effectue à l'aide d'un circuit électrique adapté une mise en forme du signal qui permette de s'affranchir de ces inconvénients. Le circuit électrique de mesure est décrit plus en détail ci-dessous, en relation avec les Figs. 10 et 11.Furthermore, the other relationships (6) and (7) characteristic of the implementation of the method become:

$\text{n1> (Le + SN / 2) / λ - 1 (6 ′)}$

$\text{n2> (Lt + Le - Lb) / λ - 1/2 - N / 2 (7 ′)}$

Depending on the ratio λ / R, the linear density σN can have several maxima, as shown in Fig. 8. There is shown in FIG. 9 the evolutions of the quantities I (t) and J (t) corresponding and used to make the measurement. We note that the quantity I (t) has a shape similar to the linear density σN. It follows that the zero crossing points of the function J (t) can be multiple. A variant of processing the measurement consists in counting the time elapsing between the instants corresponding to the rising edges of the logic signal F2 at the high level when J (t) is greater than a value J _o or less than a value -J _o , as shown in Fig. 9. However, in a preferred embodiment of the invention, the signal is shaped using a suitable electrical circuit which makes it possible to overcome these drawbacks. The electrical measurement circuit is described in more detail below, in relation to Figs. 10 and 11.

The processing of the signals necessary to carry out the measurement results in a shaping of the temporal variations of the electrical signals I (t), J (t). In practice, it turns out to be necessary to filter the electrical signal delivered by the electrode 8c, in order to control the transmission of the signal and minimize the influence of parasitic random signals. The electric circuit for measuring the speed of drops 9 is schematically in the form described in FIG. 10. The current I (t) resulting from the temporal variations of the electric charge Q (t) carried by the sensitive electrode 8c flows between this electrode and the ground through a resistor 12. The voltage U (t) at the terminals of the resistor 12 is treated successively by a bypass then by filtering, giving a signal W (t). The filtering solution adopted is a 5-fold filtering towards the high frequencies and of order 1 towards the low frequencies. This filtering towards high frequencies makes it possible in particular to eliminate in the processed signal W (t) the multiple zero crossing points present in the raw signal J (t), which result from the presence of several charged drops in the train of charged drops: compare J (t) in Fig. 9 and W (t) in FIG. 10.

A detailed description of the operation of the circuit is given below, in relation to FIG. 11. The function of the circuit is to determine the difference of the two characteristic instants T2 and T1 corresponding to the zero crossings of the voltage W (t) of FIG. 10.

A preamplification of Q (t) is carried out using an amplifier with FET input 13 whose spectral density of input current noise is very low, of the order of 10⁻¹⁴ Amperes / √ hertz . The input resistor 12 determines a first derivation of the signal. The components comprising the resistor 14 and the diodes 15 and 16 provide protection of the input. The components comprising resistors 17, 18, 19 and the capacitors and 21 contribute to the filter function.

A capacitor 22 creates a second bypass of the signal. The components comprising the resistors 23, 24, the capacitor 25 and the amplifier 26 constitute the continuation of the filter function.

A comparator 27 changes state by passing to a high level at its output when the first derivative of the charge of the electrode 8c exceeds an amplitude VL, determined by resistors 28 and 29.

The components comprising resistors 30 and 31 and diodes 32 and 33 adapt the output voltages of the comparators to the voltages of the logic circuits.

A comparator 34 changes state at its output at zero crossings of the voltage UH (t). Resistors 35 and 36 create the shift. A resistor 37 and a diode 38 create a voltage offset on W (t) in the waiting phase of the measurement, and a resistor 39 creates a voltage offset on W (t) in the measurement phase. It is necessary to use the "offset" function to prevent the comparators from changing state randomly at times when the amplitude of the load derivative is low, and to avoid bouncing of the logic signals in the search for zero crossings of the voltage UH (t). In this regard, the resistors 35, 36 and 39 are involved in the quality of the measurement, so the offset voltage generated must be sufficiently low and also distributed around the zero potential.

The operation over time can be followed in FIG. 12. Note that the measurement can only start if the voltage V (t) is sufficiently negative (-VL). Then, the signal E at the output of the comparator 27 is at the high level. At this stage, a flip-flop 40 has a high level on its input D. Via NAND gates 41 and 42, the level CL / goes to the high level and puts the flip-flop in operating state, the offset is reduced to that necessary for the measurement. When the rising edge of the signal C coming from the comparator 34 comes, the flip-flop 40 copies the state present on the input D on the output QL, the output QL / takes the opposite state, ensuring the offset of the voltage UH (t) necessary for hysteresis during the measurement. The instant T1 being thus defined, the counting of time begins. During the passage of the signal C at the low level, via the gates 41 and 42, an offset is defined for the measurement wait, the level CL / goes to the low level and puts the scale in frozen state with the output QL low. The QL / output takes the opposite state, ensuring the voltage offset UH (t) necessary during the measurement standby phase. The instant T2 is thus defined. The time counting is stopped and the information T2 - T1 is made available to the computer.

Fig. 13 schematically represents the various mechanical and electrical elements constituting an ink jet printer, including a print head 1 and an ink supply circuit. The various elements are also represented: sensors, electrical circuits, allowing the implementation of the ink quality control process, object of the present invention. Fig. 13 illustrates in particular a print head 1 comprising a nozzle 2 making it possible to form a succession of droplets G, a charging electrode 6 and electrical means 7 making it possible to charge these droplets, a drop speed detector 8, deflection electrodes 10, and a gutter 11, already described in connection with FIG. 1. An ink circuit comprises a constant ink flow generator 43, independent of variations in the environment, said generator 43 being hydraulically connected to the nozzle 2 by lines 44 and 45 in series, from a mixing tank 46 containing the ink intended for the nozzle. Two tanks 47 and 48 respectively containing fresh ink and solvent are hydraulically connected to the tank 46, so as to adjust the quantities of ink and solvent of the latter. Finally, a reservoir 49 contains the ink coming from the drops not used for printing and recovered in the gutter 11.

In the particular case of the exemplary embodiment shown in FIG. 13, the constant flow generator 43 consists of a positive displacement pump 50 driven by a motor 51, a speed measuring device according to the invention, and a drop speed regulation circuit 52.

In particular, the positive displacement pump 50 may be a multifunction unit having a variable volume chamber, as described in the French patent application ^No. 2608225 in the name of the present applicant. The drop speed regulation circuit 52 acts on the motor 51 driving the pump 50, so as to increase (or decrease) the flow rate of the pump 50, depending on whether the measured drop speed is lower (or higher) than a setpoint Vo. A similar method drop of speed control is described in particular in US Patents ^No. 4,045,770 and US ^No. 4,063,252 for the case of a jet printer magnetic ink.

The generator 43 is connected to the nozzle 2 by a single line defined by the series of lines 44 and 45. The regulation of the drop speed substantially amounts to regulating the flow of ink leaving the generator 43, circulating in the lines 44 and 45.

• du dénivelé (zp - zj) existant entre le lieu de mesure de pression et le jet J;
• des caractéristiques géométriques (sections, longueurs et formes) de la canalisation 45 située entre le lieu de mesure de pression et le jet J, et de la buse 2;
• des caractéristiques de l'encre présente dans la canalisation 45 entre le lieu de mesure de pression et le jet J (viscosité, densité), et dans la buse 2.

According to the invention, there is a device 53 for measuring the pressure of the ink Pe delivered by the pump 50, placed between the generator 43 and the nozzle 2, and dividing the pipe into an upstream part and a downstream part with respect to the direction of circulation of the fluid, already referenced respectively 44 and 45. The pressure Pe necessary to maintain a fixed jet flow rate Qo (or a drop speed Vo) depends on the following parameters:

• the difference in height (zp - zj) existing between the pressure measurement location and the jet J;
• geometric characteristics (sections, lengths and shapes) of the pipe 45 located between the place of pressure measurement and the jet J, and of the nozzle 2;
• characteristics of the ink present in line 45 between the place of pressure measurement and the jet J (viscosity, density), and in nozzle 2.

où ρ représente la densité moyenne de l'encre dans la canalisation 45 et dans la buse 2;
η représente la viscosité moyenne de l'encre dans la canalisation 45 et dans la buse 2,
g représente l'accélération de la pesanteur,
K1 et K2 sont des coefficients caractérisant la géométrie de l'écoulement d'encre le long de la canalisation 45 et dans la buse 2.The relationship between the ink pressure and these different parameters can, in particular, be written in the following form:

$\text{Pe = K1}\overline{\text{ρ}}\text{.Qo² + K2}\overline{\text{η}}\text{Qo -}\overline{\text{ρ}}\text{g (zp - zj) (10)}$

or ρ represents the average density of the ink in line 45 and in nozzle 2;
η represents the average viscosity of the ink in line 45 and in nozzle 2,
g represents the acceleration of gravity,
K1 and K2 are coefficients characterizing the geometry of the ink flow along the pipe 45 and in the nozzle 2.

   la densité de l'encre ρ contribue à la perte de pression Pe* par le premier terme du membre de droite de la relation (11), qui correspond à une perte par inertie; celui-ci dépend (par l'intermédiaire du coefficient K1) de l'amplitude des changements de sections de l'écoulement de l'encre dans les conduits situés entre le lieu de mesure de pression et le jet. La viscosité η de l'encre contribue à la perte de pression Pe* par le second terme du membre de droite de la relation (11), qui correspond à une perte par frottement; celui-ci dépend (par l'intermédiaire du coefficient K2) du diamètre et des longueurs des conduits situés entre le lieu de mesure de Pe* et le jet.For a given installation, the height difference (zp - zj) is known (by construction or measurement on the site). The pressure Pe * taking into account the difference in height, and defined below, then only depends on the characteristics (density and viscosity) of the ink circulating in the conduits (the pipe 45 and the nozzle 2) between the place of pressure measurement and the jet.

$\text{Pe * = Pe +}\overline{\text{ρ}}\text{g (zp - zj) = K1}\overline{\text{ρ}}\text{Qo² + K2}\overline{\text{η}}\text{Qo (11)}$

ink density ρ contributes to the pressure loss Pe * by the first term of the right member of the relation (11), which corresponds to a loss by inertia; this depends (via the coefficient K1) on the amplitude of the changes in cross-sections of the flow of the ink in the conduits located between the place of pressure measurement and the jet. The viscosity η ink contributes to the pressure loss Pe * by the second term of the right member of the relation (11), which corresponds to a loss by friction; this depends (via the coefficient K2) on the diameter and lengths of the conduits located between the place of measurement of Pe * and the jet.

où K_1B et K_2B sont des paramètres représentatifs de la géométrie de la buse 2, caractérisée par un diamètre d'orifice ⌀B et une longueur d'orifice LB. Dans ce cas, la viscosité η et la densité de l'encre ρ apparaissant dans la relation (12) sont représentatives des valeurs à la buse. La mesure de la pression Pe* permet alors, pour un type d'encre et une buse donnés, de contrôler la qualité de l'encre circulant dans la buse, immédiatement en amont du lieu de formation des gouttes. La pression Pe* mesurée à l'aide du principe décrit plus haut résulte d'un effet combiné de la densité ρ et de la viscosité η de l'encre en écoulement dans la buse, tel que donné par la relation (12). Ces deux paramètres dépendent essentiellement de la concentration en solvant dans l'encre et de la température de l'encre. Ils diminuent tous deux lorsque la température de l'encre augmente et lorsque la quantité de solvant dans l'encre augmente.In a preferred embodiment, the diameter of the pipe 45 is much larger (more than ten times) than the diameter 0̸B of the nozzle 2 located at the end, and the length of the pipe is relatively small, so that the pressure loss in these conduits is negligible compared to the pressure loss in the nozzle, and thus the relation (11) can be written:

${\text{Pe * = K}}_{\text{1B}}{\text{Qo² + K}}_{\text{2B}}\text{Qo (12)}$

where K _1B and K _2B are parameters representative of the nozzle 2 geometry, characterized by an orifice diameter ⌀B and an orifice length LB. In this case, the viscosity η and the density of the ink ρ appearing in relation (12) are representative of the values at the nozzle. The pressure measurement Pe * then makes it possible, for a given type of ink and a nozzle, to control the quality of the ink circulating in the nozzle, immediately upstream from the place of formation of the drops. The pressure Pe * measured using the principle described above results from a combined effect of the density ρ and the viscosity η of the ink flowing in the nozzle, as given by equation (12). These two parameters depend essentially on the solvent concentration in the ink and the temperature of the ink. They both decrease as the temperature of the ink increases and as the amount of solvent in the ink increases.

For a given variation in ink concentration of 1%, for example, there is generally a greater relative variation in viscosity (30%) than in density (1%). In order to increase the sensitivity of the measurement of Pe * to a variation in concentration of the ink, a nozzle 2 is preferably used, the slenderness (defined by the ratio of the length of the orifice to the diameter of the orifice) is at least equal to 1, in order to increase the value of the coefficient K2B in relation (12) and to obtain a measurement more sensitive to variations in ink quality, which mainly results from variations in viscosity.

The ink quality control device is schematically shown in FIG. 13. According to the invention, a temperature sensor 54 is arranged in the ink circuit in order to carry out a temperature measurement representative of the temperature Te * of the ink at the nozzle. With the assumptions made above on the diameter of the pipe 45, the average speed of the ink in the pipe is low (of the order of a few cm / s), so that the ink temperature is identical to room temperature as soon as the length of the conduit is more than approximately 50 cm. A simple measurement of the ambient temperature is then sufficient to implement the method described below.

The measurements of the pressure Pe * and of the temperature Te * of the ink are transmitted to a control circuit 55. The latter, according to a quality instruction of the ink to be maintained, which can in particular be defined by a curve Pe * (setpoint) -Te * as shown in FIG. 14, permanently regulates the quality of the ink by adding to the mixing tank 46 determined quantities of fresh ink coming from the tank 47, or of solvent coming from the tank 48, or of ink recycled to the gutter 11 coming from the tank 49, thanks to an action on one of the solenoid valves, respectively 56, 57 and 58.

According to the invention, the ink present in the fresh ink reservoir 47 is of a higher concentration than the nominal concentration of use. The Pee curve characterizing the quality of this fresh ink as a function of temperature is shown in FIG. 14, as well as that of the solvent Pes. The main advantages of using concentrated make-up ink are, on the one hand, a faster response time of the ink quality regulation and, on the other hand, a greater autonomy of the machine. in terms of new ink replenishment.

In a particular embodiment, the positive displacement pump 50 consists of a variable volume chamber closed by a membrane, the latter being set in reciprocating motion by a motor of the stepping motor type. The pump 50 continuously supplies the print head 1 with ink, through the mixing tank 46, the flow rate Qo being kept constant using of the regulating circuit 52. The regulation of the quality of the ink is obtained by varying the opening times of the solenoid valves 56, 57 and 58, controlled by the regulating circuit 55. The latter also functions d 'a sampled period period dt. In order to take into account the time of mixing and transit of the ink between the mixing tank 46 and the nozzle 2, the regulation takes account, not only of the quality of the ink measured at the present moment, but of all the history of the ink quality measured from the moment the machine was started. The ink quality control mode is then ensured as follows:

• la durée d'ouverture De(i) de l'électrovanne d'encre fraîche 56,
• la durée d'ouverture Ds(i) de l'électrovanne de solvant 57,
• la durée d'ouverture Dg(i) de l'électrovanne 58 d'encre recyclée provenant de la gouttière 11,
• la température mesurée de l'encre Te*(i),
• la pression mesurée Pe*(i) à la température Te*(i),
• la courbe de consigne Pec(T) en fonction de la température T (Fig. 14);
• la courbe Pee(T) caractéristique de l'encre fraîche (Fig. 14);
• la courbe Pes(T) caractéristique du solvant (Fig. 14);
• le temps de réponse tr du circuit compris entre les réservoirs 47, 48, 49 et la buse 2 défini par le rapport volume de la canalisation sur le débit volumique du jet Qo.

The following average values are defined over a sampling period dt of the regulation circuit 55, over the ith sampling period:

• the opening time De (i) of the fresh ink solenoid valve 56,
• the opening time Ds (i) of the solvent solenoid valve 57,
• the opening time Dg (i) of the solenoid valve 58 of recycled ink coming from the gutter 11,
• the measured temperature of the ink Te * (i),
• the pressure measured Pe * (i) at the temperature Te * (i),
• the setpoint curve Pec (T) as a function of the temperature T (Fig. 14);
• the Pee curve (T) characteristic of fresh ink (Fig. 14);
• the Pes (T) curve characteristic of the solvent (Fig. 14);
• the response time tr of the circuit between the tanks 47, 48, 49 and the nozzle 2 defined by the ratio of the volume of the pipe to the volume flow rate of the jet Qo.

   On définit l'écart dynamique de la qualité de l'encre H(i) :

où n = 0 correspond à l'instant de mise en route du circuit d'encre de l'imprimante. La régulation s'écrit :

si |H(i)| < Ho

encre de qualité satisfaisante
alors De(i) = 0
Ds(i) = 0
Dg(i) = dt

si H(i) > Ho

encre trop concentrée
alors De(i) = 0
$\text{Ds(i) = dt.Ks.|H(i) - Ho|}$

$\text{Dg(i) = dt.(1 - Ks).|H(i) - Ho|}$

si H(i) < - Ho

encre trop peu concentrée
alors $\text{De(i) = dt.Ke.|H(i) - Ho|}$

Ds(i) = 0
$\text{Dg(i) = dt.(1 - Ke).|H(i) - Ho|}$

où
Ke est proportionnel à |Pec(To) - Pee(To)|
Ks est proportionnel à |Pec(To) - Pes(To)|
To est une température moyenne d'utilisation,
Tp est de l'ordre de 3tr.Let DP (i) the instantaneous ink quality deviation from the set value:

$\text{DP (i) = Pe * (i) - Pec (Te * (i))}$

We define the dynamic difference in the quality of the ink H (i):

where n = 0 corresponds to the time when the printer's ink circuit starts up. The regulation is written:

if | H (i) | <Ho

ink of satisfactory quality
then De (i) = 0
Ds (i) = 0
Dg (i) = dt

if H (i)> Ho

ink too concentrated
then De (i) = 0
$\text{Ds (i) = dt.Ks. | H (i) - Ho |}$

$\text{Dg (i) = dt. (1 - Ks). | H (i) - Ho |}$

if H (i) <- Ho

too little concentrated ink
so $\text{De (i) = dt.Ke. | H (i) - Ho |}$

Ds (i) = 0
$\text{Dg (i) = dt. (1 - Ke). | H (i) - Ho |}$

or
Ke is proportional to | Pec (To) - Pee (To) |
Ks is proportional to | Pec (To) - Pes (To) |
To is an average usage temperature,
Tp is around 3tr.

Fig. 13 also illustrates the operating diagram of the device for controlling the formation of drops, object of the invention. The device implements the print head 1, comprising the nozzle 2, supplied by the ink circuit comprising the constant-flow generator 43. The jet J coming from the nozzle 2, the speed of which is fixed (regulated), fragments at a distance Lb, Fig. 2, of the nozzle 2 in a succession of equidistant and equidimensional droplets G, under the action of the pressure disturbance applied by the resonator 4 placed upstream of the nozzle 2, and fed by the modulation circuit 5. The circuit charge 7 cooperating with the charge electrode 6 makes it possible to charge the drops intended for printing.

où Tf est un temps de retard caractéristique du filtrage électronique et indépendant des autres paramètres.According to the invention, an electrical circuit 59 measures a time of flight tv of the drops used for the speed measurement. This flight time tv is defined by the duration between the instant of charge of these drops and the instant of detection of their passage at the input of the speed detector 8. A chronogram of the operation of the detector 59 is presented in FIG. . 15. The number of drops of the train used for speed detection being known (5 in Fig. 15), a simple processing of the charge signals Vc (t) (the charge voltage Vc is applied to the charge electrode for half a drop period for the case shown in Fig. 15) and speed detection I (t), J (t) allows to obtain the time tv. The distance Lt, Fig. 2, between the nozzle 2 and the input of the detector 8 being known by construction, the distance Lb separating the nozzle from the place of formation and charging of the drops is obtained by the relationship below, which implements both the drop speed V and time of flight tv, both controlled by the printer:

$\text{Lb = Lt - V. (tv - Tf) (13)}$

where Tf is a delay time characteristic of the electronic filtering and independent of the other parameters.

Experience also shows that, the speed of the drops being fixed by the regulation described above, there is a unique relationship between the quality of the ink at the nozzle (measured by the pressure Pe *) and the length of breakage Lb, ensuring optimal formation and charge of drops. The circuit for regulating the formation of the drops 59 acts on the amplitude of the excitation signal of the resonator 4, in order to maintain the breaking point Lbopt, ensuring optimal formation of the drops, depending on the type of ink used and the quality of the ink circulating at the nozzle. Another advantage of the invention lies in the fact that, by such a method, it overcomes possible disparities in the characteristics of the resonators, from one machine to another.

In the control means described above, all the parameters controlled are measured (or representative of the measurable values) at the level of the nozzle. This allows very precise regulation of the operation of the printer. The precision achievable by these control means allows their use in inkjet printers used for high quality marking applications. In general, it contributes to improving the quality of printing and the reliability of inkjet printers.

The following table gives indicative values for three models of printhead according to the invention: Example 1 Example 2 Example 3 f 125 kHz 83.33 kHz 62.5 kHz Drop frequency R 0.6 mm 0.6 mm 0.6 mm Electrode slot width L 2 mm 2 mm 2.8mm Detector length 8c Li 1 mm 1 mm 1.2mm Insulation length The 2.39 mm 2.375 mm 3.25mm Effective length of 8c NOT 7 6 7 number of charged drops ⌀B 40 µm 55 µm 70 µm Nozzle diameter

Claims (18)

1. Device for checking and regulating an ink and for processing it in a continuous ink jet printer in which a continuous ink jet (J) exits from a nozzle (2), comprising:

   - means (4, 5) for dividing the jet (J) into droplets (G) an equal distance apart and of equal dimensions;

   - a charging electrode (6) at which the said droplets are selectively charged electrostatically;

   - a velocity detector (8) for the charged drops;

   - deflection electrodes (10) at which the said droplets (G) are deflected in dependence upon their charge,

   characterised in that:

   - the detector (8) is a single detector and comprises a central conductive element (8c) having a length (L) and being manufactured in two parts which are symmetrical relative to the trajectory of the droplets (G),

   and in that the device further comprises an electronic circuit (9) for measuring the velocity of the drops comprising:

   - means for measuring the current I(t) flowing between the detector (8) and earth and corresponding to the first derivative, with respect to time, of the total charge Q(t) induced in the detector (8) by one or more charged droplets;

   - means for calculating the second derivative J(t) of the said charge Q(t) with respect to time;

   - means for processing the first derivative I(t) and second derivative J(t) determining the moments T₁ and T₂ when the derivative J(t) changes to zero, for a sufficient value of the current I(t), which correspond to the respective moments when the charged droplet or droplets enter and leave the detector (8);

   - means for calculating the velocity V of the droplets from the moments T1 and T2 and from the length (L) of the conductive element (8c) of the detector (8).
2. Device according to claim 1, characterised in that the charging voltage at the charging electrode (6) is only applied during part of the period of formation of the droplets used for measuring velocity.
3. Device according to claim 2, characterised in that the charging voltage at the charging electrode (6) is only applied during at most half of the period of formation of the droplets used for measuring velocity.
4. Device according to one of claims 1 to 4, characterised in that the conductive element (8c) of the detector (8) is protected from the influence of external electrical charges by an insulating element (8i) having a total length (Li) measured along the trajectory of the droplets, and by a guard electrode (8e) electrically connected to earth.
5. Device according to claim 4, characterised in that the conductive element (8c) of the detector (8), having the length (L), with the distance separating the two parts thereof being (R), satisfies the condition:
   
   $\text{R < Le}$

   

   
   
   in which Le is the effective length of the detector, defined by the relationship:
   
   $\text{Le = L + Li/2}$

   
6. Device according to claim 4 or 5, characterised in that it comprises a means for dividing a charged droplet (Gc) serving to carry out a measurement of velocity, relative to other charged droplets, so that the said charged droplet (Gc) is preceded by at least n1 uncharged droplets and is followed by at least n2 uncharged droplets, with n1 and n2 satisfying the relationships:
   
   $\text{n1 > (Le + R)/(5 ⌀B) - 1, approximately}$

   

   
   $\text{n2 > (Lt + Le - Lb)/(5 ⌀B) - 1, approximately,}$

   

   
   
   in which ⌀B is the diameter of the nozzle (2),
   Lt is the distance between the nozzle (2) and the entrance to the detector (8), and
   Lb is the distance between the nozzle (2) and the place where the drops (G) are formed.
7. Device according to claim 4 or 5, characterised in that it comprises a means for dividing a series of N successive charged droplets serving to carry out a measurement of velocity, relative to other charged droplets, so that the said series of N successive charged droplets is preceded by at least n1 uncharged droplets and followed by at least n2 uncharged droplets, with n1 and n2 satisfying the relationships:
   
   $\text{n1 > ((Le + SN/2)/λ) - 1}$

   

   
   $\text{n2 > ((Lt + Le - Lb)/λ) - 1/2 - N/2}$

   

   
   
   in which λ is the distance between two successive droplets and S is the length of the zone electrically influenced by a charged drop (Gc).
8. Device according to one of claims 1 to 7, characterised in that, with the means for measuring the velocity of the droplets, there is associated a means for controlling the charge of the droplets (Gc) which are used for measuring velocity, so that the said droplets have a sufficiently low charge for them to be recovered in the gutter (11) of the printer.
9. Device according to one of claims 1 to 8, characterised in that it comprises a means (52) for regulating the velocity of the drops, this means acting on a motor (51) which drives a pump (50) in the ink supply circuit to the nozzle (2), according to whether the measured velocity of the drop is less (or greater) than a reference value (Vo).
10. Device according to claim 9, characterised in that it comprises a sensor for the ink pressure (Pe*) on the pipes (44, 45) between the constant ink flow generator (43), formed by the pump (50) and the motor (51), and the nozzle (2), immediately upstream from the nozzle (2), in order to derive the ink concentration in dependence upon the said pressure, a pre-determined operating temperature and the velocity of the drops.
11. Device according to claim 10, characterised in that the pipes between the flow generator (43) and the nozzle (2) have a diameter which is approximately ten times greater than the diameter of the nozzle (2).
12. Device according to claim 10 or 11, characterised in that the ratio between the length and the diameter of the opening of the nozzle (2) is at least equal to 1.
13. Device according to one of claims 10 to 12, characterised in that it comprises a further temperature sensor (54) for measuring a temperature representative of the temperature (Te*) of the ink at the nozzle (2).
14. Device according to claim 13, characterised in that it comprises a further means (55) for constantly regulating the quality of the ink in dependence upon measurements of pressure (Pe*), temperature (Te*) and a reference curve of the pressure in dependence upon temperature, for a given drop velocity (Vo).
15. Device according to claim 14, characterised in that the ink quality regulation takes place in a mixing reservoir (46), from a fresh ink reservoir (47), a solvent reservoir (48) and a reservoir (49) of ink recycled at the gutter (11), with the regulating means (55) selectively actuating respective solenoid valves (56, 57, 58) on the pipes between, on the one hand, the reservoirs (47, 48, 49) and, on the other hand, the reservoir (46).
16. Device according to claim 15, characterised in that the ink in the reservoir (47) has a higher concentration than the nominal operating value.
17. Device according to one of claims 14 to 16, characterised in that the regulating means (55) comprises a processing circuit taking into account the quality of the ink at the present moment and the history of the quality of the ink since the moment when the machine started up.
18. Device according to one of claims 1 to 17, characterised in that it comprises a further means (59) for determining the distance between the place where the drops (G) are formed and the nozzle (2), and for acting on the amplitude of the excitation signal of the means (5) serving to form the said drops (G) so that the distance (Lb) ensures an optimum formation of drops in dependence upon the type of ink used and the quality of the ink flowing at the nozzle.

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