US 3836912 A
An inductive charge sensing device is disclosed in accordance with the teachings of the present invention for use with an ink jet printing system wherein ink under pressure is applied to a nozzle and ink emitted by the nozzle thereafter breaks up into a series of drops which are electrostatically charged and subsequently deflected in order to achieve controlled printing upon a recording surface moved in front of said apparatus. The charging and deflection of individual droplets is effected under control of an applied video signal. In order for the proper information to be recorded, the charging and deflection operation must be performed in precise synchronization with the ink droplet formation. As droplets are emitted from the nozzle, the charge sensor detects charges impressed on said droplets passing adjacent to but in non-impinging relationship with said sensor and a signal is developed which may be used to control an electrical or electromechanical drop forming means associated with said nozzle and ink supply.
Description (OCR text may contain errors)
United States Patent [191 'Ghougasian et al.
[451 Sept. 17, 1974 DROP CHARGE SENSING APPARATUS FOR AN INK JET PRINTING SYSTEM  Assignee: International Business Machines Corporation, Armonk, NY.
 Filed: Dec. 11, 1972  Appl. No.: 313,913
 US. Cl 346/75, 317/3, 324/71 CP  Int. Cl. G0ld 15/18  Field of Search 346/75; 324/71 CP, 32;
73/194 E, 432 PS; 128/214 E; 317/3  References Cited UNITED STATES PATENTS 12/1968 Hertz et al. 346/75 8/1971 Bischoff 346/75 X OTHER PUBLICATIONS Charged Single Droplets; The Review of Scientific lnst., Vol. 38, No. 3, March 1967, pp. 325-327.
Primary Examiner-Joseph W. Hartary Attorney, Agent, or Firm--Roy R. Schlemmer [5 7 ABSTRACT An inductive charge sensing device is disclosed in accordance with the teachings of the present invention for use with an ink jet printing system wherein ink under pressure is applied to a nozzle and ink emitted by the nozzle thereafter breaks up into a series of drops which are electrostatically charged and subsequently deflected in order to achieve controlled printing upon a recording surface moved in front of said apparatus. The charging and deflection of individual droplets is effected under control of an applied video signal. In order for the proper information to be recorded, the charging and deflection operation must be performed in precise synchronization with the ink droplet formation. As droplets are emitted from the nozzle, the charge sensor detects charges impressed on said droplets passing adjacent to but in nonimpinging relationship with said sensor and a signal is developed which may be used to control an electrical or electromechanical drop forming means associated with said nozzle and ink supply.
3 Claims, 14 Drawing Figures SOURCE OF SYNC. INK SIGNALS UNDER DROP PRESSURE PRODUCING 10 TRANSDUCER 56 PHASE 1 CHANGING i j NETWORK i i t VIDEO SIGNAL 20 a SOURCE DROP CHARGE UNE 24 8 HI 2 AMP.
1 2e 2 -ss 27 a o FIELD 26 POTENTIAL SOURCE 28 PAIENTEDSEPWW I SOURCE Q v OF SYNC.
50 SIGNALS UNDER DROP PRESSURE PRODUCING 1;. I PHASE CHANGING TRANSDUCER I r v NETWORK,
VIDEO SIGNAL SOURCE I HIAZ AMP.
FIELD POTENTIAL SOURCE INSTANT or DROP SEPARATION TRANSDUCER VOLTAGE CHARGING SIGNAL'I- (m PHASE WITH v.
SEPARATION TIMEH CHARGING SIGNAL 1 (OUT OF PHASE WITH l SEPARATIONTTIME) FIG. 6B
PAIENTEDSEP'I CHARGE 1st DROP FIG. 6A
INDUCED 2nd DROP LAST DROP s uMfQF; Y 'moucso CHARGE laeo CURRENT FIG. 7 5
INDUCED CHARGE )3 My a or 4 s t -+1 8(t) =TlM E BETWEEN-DROPS 4 t 1 TIME glam TIME 1 t2 +At TIME I TIME PAremwsw m 3.836.912
INK-STREAMS 448aoooaopne 1 DEFLECTION PLATES 094:6 naaa A CHARGING NOZZLE ELECZTORODES A 14 A LK gb MN Q a a A I v I T0 TO OTHER FEEDBACK --1 AMPLIFIERS CIRCUITS DROP CHARGE SENSING APPARATUS FOR AN INK JET PRINTING SYSTEM CROSS-REFERENCE TO RELATED APPLICATIONS The present application is related to U.S. Patent application Ser. No. 3l3,9l'4 filed concurrently with the present application of H. Juliusburger et al. entitled Digital Ink Jet Pulse Edge Phase Control System (now U.S. Patent No. 3,769,632). This referenced patent discloses a particular phase control system which utilizes the inductive proximity drop charge sensor of the present invention in a specific synchronization control system. As such it represents a preferred utilization of the device of the present invention.
BACKGROUND OF THE INVENTION The need for improved low volume extremely. high speed printers has increased drastically in recent years. A particular application for such printers is in the computer printout area wherein the actual printing devices utilized to produce human readable records has long been a major bottleneck in the overall computer system wherein data which'is produced by the system must often be held in temporary storage such magnetic tapes, discs, drums, etc. for manyhours before the particular printing devices can produce the required outputs. Most currently available printers in this general area today are of the impact type where a printing element must actually be moved forceably against a record member to produce a visible letter or symbol. In recent years ink jet printing has been developed wherein ink is applied under pressure to a suitable nozzle. The ink is caused to break up into individual droplets. The droplet formation is controlled by a number of different methods available in the art including physical vibration of the nozzle, pressureperturbations introduced into the ink supply feed to the nozzle, etc. The result of applying such external perturbations to the inkjet apparatus is to cause the jet stream emerging from the nozzle to break up into uniform drops at a predetermined frequency and at a somewhat variable distancefrom the tip of the nozzle. It is necessary, however, that this droplet formation and the application of video charging signals to the ink dropletstream be synchronized. The rate of drop formation in such systems is determined by a signal applied to the physical perturbation means. e.g., vibrating the nozzle. A means for applying an electrostatic charge to each drop produced by the nozzle is provided in such systems adjacent to the location where the ink stream begins to form such droplets. Conventionally, this means is a hollow tube. channel, plates, etc. surrounding the emerging stream and connected to a suitable charging source. Video signals are applied between the nozzle and the charging electrode in response to which a drop will assume a charge determined by the amplitude ofthe particular video signal onthe charging electrode at the time that the drop breaks away from the jet stream.
The drop thereafter passes through a fixed electric field and the amount of deflection is determined by the amplitude of the charge on the drop at the time it passes through said deflecting field. A suitable recording surface is positioned downstream from the deflecting means with the result that the droplet strikes such recording surface and forms a small spot. As will be appreciated, the position of the drop on the writing surface is determined by the deflection the drop experiences which in turn is determined by the charge on the droplet. Thus, by suitably varying the charge, the location at which the droplet strikes the recording surface may be controlled with the result that by applying suitable video signals to such a system, a visible human readable printed record may be formed upon the recording surface. U.S. Pat. No. 3,596,275 of Richard G. Sweet entitled Fluid Droplet Recorder discloses-such a recording or printing system.
As will be further appreciated with such a system, the time that the drop separates from the fluid stream emerging from the nozzle is .quite critical since the charge carried by the droplet is normally produced by electrostatic induction. The field established by the video signal is maintained while the drop separates. The drop will carry a charge determined by'this video signal and proportional to the magnitude thereof. However, if at the time of separation the video'signal is in the process of either rising or falling or is not present at all at thetime of drop separation, the exact charge on the drop will be some time function of'the maximum video signal rather than being proportional thereto in accordance with somepredetermined and fixed relationship..lt is thus necessary in order to place exact predetermined charges on individual droplets in accordance with successive video signals, to know exactly the time of droplet separation in relationship to the timing of the video signal. Stated differently, the droplet separation time and the application of the video signal must be very precisely synchronized. Failure to properly synchronize droplet formation and the video signal results in very imprecise control of the printing process with attendant severe degradation of the uniformity, clarity. and generally thequality of the final printer re- .sult.
A number of devices have been used in the past to .detect the timing of droplet formation in such ink jet printing systems. Two of these are U.S. Pat. Nos. 3,465,350 and 3,465,351 of Keur et al., both of which are entitled lnk Drop Writing Apparatus." In both of these systems, however, actual impingement of the ink droplet on some receiving member is required, such as an ink gutter or the like. in virtually all such ink jet printing operations where the droplet formation occurs rapidly, extremely precise control of the synchronization of the system is mandatory. Such detection elements as disclosed in the two above-referenced Keur .et al patents cause problems due to ink buildup on the receiving element which decreases the sensitivity of the system. This causes attendant lack of precise control over the synchronization of such systems in extremely high speed applications. The buildup also causes serviceability problems as the ink must be periodically removed.
SUMMARY AND OBJECTS OF THE INVENTION It has now been found that improved sensing is possible with such ink jet printing systems for the purpose of detecting the precise times at which droplets are formed by utilizing an inductive charge sensing element comprising a rod-like conductor placed proximate to the ink jet stream. but in such a position that the ink droplets do not impinge upon said member but which allow charging of said member by inductive electrostatic effects in much the same way that the ink droplet is initially charged.
lt is accordingly a primary object of the present invention to provide an improved element for detecting ink drop separation times for use in an ink jet recording system.
lt is a further object to provide such an element utilizing a charge carried by individual droplets to detect such separation times.
It is yet another object of the invention to provide a switching element wherein the charge on the droplets is sensed by electrostatic induction.
It is yet another object to provide such an element which avoids problems of ink fouling of the element itself.
It is a still further object to provide such an element which allows an improved signal-to-noise ratio in the sensing circuitry associated with such element.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.
BRlEF DESCRIPTION OF THE DRAWINGS FIG. 1 comprises a combination functional block diagram and a simplified perspective organizational view illustrating the principle mechanical and electrical components of an ink jetrecording system utilizing the inductive drop charge sensing element of the present invention.
FlG. 1B comprises a series of curves illustrating certain of the critical time relationships in the system of FIG. 1A.
FIG. 2 comprises a set of curves illustrating a preferred method of charging ink drops during a test phase portion of the system operation cycle.
FIG. 3 comprises a set of curves illustrating a different embodiment for applying a test signal to various ink droplets during the test phase portion of an operating cycle.
FlGS. 4A and 4B illustrate the location ofa given ink droplet with respect to the sensing element tip at times f and [2.
FIG. 5 is a graphical illustration of the charge produced in the sensing element as a result of a charged drop passing said element.
FIGS. 6A-6D are curves representing the series of individual induced charges in the sensing element and the cumulative charge effect induced in said element.
FIG. 7 is a graph illustrating the currents which would be induced in the sensing circuitry as a result of the induced charge illustrated in FIG. 6D.
HO. 8 is a perspective view of a preferred embodiment of the present invention comprising a matrix of sensing elements and illustrating the application of the invention to monitor the synchronizing ofa plurality of individual ink jet streams.
FIG. 9 comprises a fragmentary cross-sectional view of FIG. 8 illustrating a single detecting element of the type shown and also the nozzle, charging apparatus, and deflection means as they would be located in such a system.
DESCRIPTION OF THE DISCLOSED EMBODlMENT The objects of the present invention are accomplished in general by a sensing element for use in an ink jet printing system wherein individual ink droplets are electrostatically charged and subsequently deflected electrostatically. The element comprises a conductive member placed downstream from the charging station proximate to, but in non-impinging relationship with the droplet stream. A pair of shielding plates are located on each side of said sensing element substantially perpendicular to said ink stream and each plate has a small hole therein adapted to allow the ink droplet stream to pass therethrough.
The sensor is connected to a suitable current amplifier which develops a pulse output in accordance with the inductive charges detected by the element. According to a preferred embodiment of the invention, a test cycle is utilized wherein a special test charge is placed on the ink droplets and a plurality of successive droplets are utilized to cumulatively build-up a charge in the sensing element which greatly improves the signal-tonoise ratio and thus the accuracy of the element.
Referring now to FIG. [A a more or less typical ink jet recording system utilizing electrostatic deflection of the ink droplets is shown with the inductive sensing element of the present invention in place between the charging station and the deflecting station. The mechanical portion of the system comprises a reservoir 10 in which the ink is stored under pressure from whence it flows through the conduit 12 to a nozzle 14. The ink jet stream 16 emanates under pressure from the nozzle 14 and subsequently forms droplets 18. The details of the formation of the droplets from the stream are shown more clearly in FIG. 9, it being noted that the droplet formation is not immediate but occurs at some point downstream from the nozzle and within the charging station 20. This as will be readily appreciated, is necessary since in order for the charge to be placed on the drops inductively, a current path must exist within the ink while under the influence of thecharging electric field just prior to the formation of the droplets 18. This feature, as stated above, is clearly illustrated in FIG. 9.
Subsequent to leaving the charging station, the droplet passes through shielding plates 24 having appropriate holes therein and pass in close proximity to the drop charge sensor 22 which in the disclosed embodiment comprises an elongated metal rod. The details of the placing ofthe rod with respect to the droplet stream are shown more clearly in figures which will be described subsequently. The droplets then reach the deflection plates 24 wherein the deflection of individual droplets is a result of the charge on each droplet together with the magnitude of the charge on the deflection plates. In the more widely used system such as exemplified in the previously referenced Sweet U.S. Pat. No. 3,596,275, the deflecting field is maintained constant and the charge upon individual droplets is varied. However, it should be understood that the present sensor would work equally well with a system wherein the charge was fixed and a variable field was placed on the plates. The ink droplets impinge upon the recording surface 28 subsequent to passing through the deflecting field, their particular location being determined by the video signal applied to the charging element 20.
It will be noted that a drop producing transducer is shown attached to the nozzle 14. In conventional apparatus as referenced above this transducer is apiezoelectric crystal which mechanically vibrates the nozzle to form the subsequent droplets 18. The frequency of the vibration of the crystal determines the rate of the droplet formation. Obviously the phase of. the vibration of the crystal will determine the exact time, especially with respect to the video signal source, at which droplets are produced. As stated previously, if the time of droplet formation is not in correct synchronization with the applied signals from the video signal source 32 given droplets will either not be charged at all or have an incorrect charge thereon which will result in very poor print quality. In the disclosed system a source of synch signals at 34 is utilized to drive both the transducer 30 and act as the basic timing source for the video signal source 32. This source would normally be a conventional high stability oscillator producing the desired frequency for drop formation. The synch signals going to the video signal source 32 pass through a phase changing network 36 wherein a phase change may beintroduced in accordance with the signal received from the amplifier 38 connected to the detecting element 22. Thus, if the droplets are breaking off too soon or too late with respect to the application of the charging signal, a phase change may be introduced into the network 36 to correct this situation as will be understood.
Referring briefly to FIG. 1B, the three curves illustrate just what happens when a drop separates from the Y ink stream in phase or completely out of phase with the charging signal. In this FIG.. curve A represents the-signal applied to the drop producing transducer 30. The two arrows on this curve indicate the exact instant of drop separation from the stream. Curve B illustrates the charging signal placed on the charging element 20 by the video signal source 32 substantially in phase with the drop separation and curve C illustrates the charging signal source being applied completely subsequent to drop separation. As will be understood when the situation of curve C exists. virtually no charge will be placed on the droplet with resultant erroneous operation of the system.
It is thus necessary to have accurate and reliable synchronization means in such an ink jet recording system to insure optimum operation of the system. As stated previously, prior systems required direct interception of ink droplets to make such timing or synchronization measurements. As will be well understood. with ink buildup on these elements. various types of problems are possible. One problem is lack of sensitivity as ink builds up and dries on the element. It can also interfere with adjacent apparatus. In order to avoid such buildup or to minimize its effects. periodic shutting down ofthe machine and cleaning of the sensing element has been found necessary. In accordance with the teachings of the present invention, the inductive charge sensor never intercepts the individual ink droplets but merely picks up the signal inductively which is then amplifed by an appropriate amplifier.
In a preferred embodiment it has been found that best results can be obtained if a special test cycle is utilized periodically with the system to test the current synchronization of the droplet formation and charging.
FIGS. 2 and 3 represent two possible ways of applying test signals to the droplets. In both FIGS., curves A represent the transducer signal with the arrow indicating the moment of drop separation and curves B indicate the signal applied to the charging plates. In FIG. 2 the test signal comprises a series of short duration pulses successively displaced across the transducer signal period. Thus, the FIG. 2 cycle requires a series of test pulses applied to a predetermined number of ink droplets with each series being slightly displaced. When a predetermined signal magnitude is detected by the sensor and amplifier, it will be apparent that the droplet is being separated at some particular time with respect to the phase ofthe transducer signal. The phase changing network 36 may be controlled to, in effect, center the charging video signal appropriately around the detected separation point.
By known means, such as the gutter 27 a way is provided to intercept the charged, or partially charged test drops, so they do not interfere withthe recording operation. If requried, the test drops would be charged opposite to those used for normal recording.
In FIG. 3 an analog system is disclosed wherein a ramp signal is applied to a successive series of droplets during the test phase and depending upon the magnitude of the charge detected, the timing of the ink droplet separation is readily resolvable with respect to the phase of the transducer signal. Thus, in the illustrated case the drop separation occurs fairly low down on the ramp; however, if the drop were found to be separating later in phase a larger ramp or charging signal would be placed on the droplets with an attendant larger charge detected by the sensor. A corrective signal would then be fed into the phase changing network as described with respect to .the embodiment of FIG. 2.
It is believed that the electronics of the present system would be obvious for one skilled in the art to build the necessary phase changing network for controlling the moment of application of the video signal to the charging electrode. It will be noted that a particular preferred embodiment of a digital system is disclosed in previously referenced copending application Ser. No. 313,9]4 of H. .luliusburger et al. It 'is the intent of the present application to set forth and claim the details of the sensing element per se, it being apparent that the element could be utilized with a number of different types of actual control circuits and mechanism without essentially modifying the element.
Referring now to FIGS. 4-7, the actual operation or the charging effect will be described. FIGS. 4A and 4B merely indicate the location of an individual ink droplet with respect to time at the time periods I, and t Referring concurrently to FIG. 5 as well as FIGS. 4A and 48, it will be noted that at time 1 the maximum induced charge developed when the ink droplet reached the leading edge of the sensor so that its full charging effect is felt therein. This is shown in FIG. 5 at time I; which is the beginning of the plateau region. This maximum charge effect is felt until time after which time the charge starts to fall off as the drop leaves the area of the sensor. Similarly, with respect to the leading edge a charge slowly builds up as the droplet approaches the sensor element.
FIGS. 6A-6D illustrate the manner in which a series of droplets utilized during a test phase can be utilized to maximize the total induced charge (1,, shown in FIG. 6D. to increase the sensitivity or signal-to-noise ratio of the system. The successive curves 6A, 6B and 6C merely duplicate curve and are shown to illustrate the charging effect of a successive series of drops passing the sensing element.
FIG. 6B shows the total charge built-up in the sensing element during a sequence of detected charged droplets. The signal or charge built-up in the sensing element and thus in the sensing element detection circuit which would preferably be an amplifier having a very high input impedance is essentially the wave form shown in FIG. 6D. When applied to the amplifier, this charging signal wave form will produce a signal current in the input circuit of the amplifier as exemplified by the curve of FIG. 7.
Thus, depending on the exact circuitry utilized, the signal appearing at times I; or t may be utilized for controlling the phase changing network.
It will be readily appreciated that the particular preferred embodiment of the invention shown in crosssection in FIGS. 4A and 4B is a single elongated rodlike element constructed of a highly conductive material such as copper or possibly aluminum. It should be readily understood that the inductive charge sensor could have other shapes such as a plate, ring-shaped member, or U-channel surrounding the path of the ink jet and suitably connected to the detection circuitry.
The shielding plates could be any thin metal such as aluminum, brass, copper, etc. The opening should be large enough so that there is no possibility of interfering with the passage of the ink droplets therethrough. The thickness and spacing of the plates is not overly critical. For example, the plates may be spaced [/16 of an inch on either side of the sensing element.
Referring now to FIGS. 8 and 9, there is shown an embodiment of the present invention particularly adapted for use with an array ofindividual ink jet printing nozzles as would be used in a matrix type printing operation as is well known in the art. In effect, in this embodiment a separate sensing element is provided for each ink jet stream making up the matrix. In FIG. 8 only the sensing element is shown as the overall configuration would be similar to that of FIG. IA; however, obviously there would be a multiplicity of nozzles, electronics, etc.
Referring now to FIG. 8, it will be noted that a plurality of individual rod-shaped elements 22 are shown mounted in a non-conductive plastic block 40. The shielding plates 42 are cemented or otherwise fastened on either side of block 40 and the apertures 44 therein provide a path for the ink droplet stream to pass through the sensing assembly. A plurality of individual ink droplet streams are clearly illustrated in the drawing and are designated by the numeral 46. As will also be apparent, each ink stream subsequently passes to its own set of deflection plates which for the case of most matrix type printing systems will either cause the individual ink droplets to strike a gutter or some other shield or allow them to pass through and form a dot at a predetermined location on the recording surface.
Similarly, as indicated in the FIG., an amplifier 48 is connected to each of the sensing elements 22 through an appropriate impedance element such as the resistance 50. Alternatively the sensing element may be connected to the input ofa current to voltage amplifier such as described in the book Operational Amplifiers, Design and Application, Toby, Graemp, and Huelsman, McGraw Hill, New York, 1971. The output of each amplifier, as described previously, is fed to an associated phase shifting or control network for controlling the phase of the charging signal for the associated nozzle and charging electrode.
The relative configuration of the overall system is shown more clearly in FIG. 9 which illustrates the nozzle, the ink stream breaking up into droplets, the charging plate, the sensing station including the sensing element and finally the deflection plates. This FIG. clearly shows the droplets forming within the area of the charging electrode 20. As stated previously, this is necessary since, for the charging voltage to have the desired effect on a droplet, the droplet just prior to breakoff must be under the full influence of the charging field to allow electrons to flow out of the affected droplet through the ink stream and to ground, as will be well understood. The placement of the charging electrode is fairly standard and the point of droplet fomation is relatively constant since the primary system physical variables are in viscosity, temperature, and the like. The variable which causes the problem which is solved by the present invention is synchronizing the precise drop break-off time with the application of the charging signal to the charging electrode. As will be apparent from the FIG., and also with reference to FIG. 1B and specifically curve C thereof, if there is no charging signal on the electrode at the time of drop break-off, the droplet will either receive no charge or a very slight charge due to the effects of the preceding charging signal.
The shape and location of the sensing element 22 together with the shielding plates 42 having apertures 44 therein, is clearly shown in FIG. 9. As stated, the element must be placed sufficiently close to the stream of ink droplets to be able to sensitively detect charges thereon and yet be spaced far enough away that there is no reasonable likelihood of the ink droplet impinging upon the sensing element. The magnitude of the signal is not materially affected by the width of the sensor, since only approaching and departing charged drops contribute to the sensor output. In practice, sensors between about 5 and 20 times the ink drop diameter have been used for ease of alignment. The spacing of a droplet from the stream should be between about 5 and 10 times the diameter of a droplet. Typical values of voltage developed across the 10 megohms resistor 50 are 50-l50 mV for 16 drops at I00 KHz and for dimensions as shown above.
While for the purpose of describing the preferred mode of the invention a plurality of drops during the test period has been disclosed, this is not absolutely necessary. For example, a single droplet could be used, however, the signal-to-noise ratio of the sense amplifier would be more important. A test signal of increased magnitude with respect to the print signal may also be utilized with a single or a plural test drop sequence to provide a layer detected signal.
Having thus described the operation of-the sensing element of the present invention, it is reiterated that the essential elements are a conductive charge sensing member and a set of shield plates. The conductive element is located in an inductive charging relationship with the ink stream but is sufficiently spaced therefrom to insure that the ink stream cannot impinge upon said element. Thus, in operation, the element senses the occurrence ofa charge on the droplet inductively and further senses the amount of charge on the droplet which can be utilized to give a positive indication of the effectiveness of the charging signal. In the latter case the amplitude may be utilized to time synchronization of the charging signal with the precise break-off time of the ink droplet from the stream. it is possible to readily enhance the overall accuracy of the system by accumulating energy from consecutively charged droplets to provide larger output signals and better signal-to-noise ratios. It is also possible to utilize a high impedance in the sensing circuitry which also improves signal-tonoise ratios. Finally, the element can be utilized in such systems to in effect, monitor other parameters than the specific charge synchronization; such, for example, as velocity measurements by utilizing a plurality of such elements spaced at precisely known distance apart along the ink stream.
While the invention has beendisclosed utilizing cer- 4 tain preferred embodiments, it will be readily understood that various substitutions may be made by a person skilled in the art without departing from the essential spirit and scope of the invention.
What is claimed is:
1. In an inkjet recording system including an ink supply, a nozzle, means for projecting a high pressure ink stream from said nozzle which breaks up into droplets downstream therefrom, means for applying an electrical charge to individual droplets as they break off from said stream, means for deflecting said droplets and a recording medium on which said droplets impinge to produce a visible record, the improvement which comprises an inductive charge sensing means for detecting a charge on said droplets,
said means comprising an elongated rod shaped conductive member mounted downstream from said charging means, having a substantially flat end in close proximity to, but in non-impinging relation to said droplet stream to receive, by induction, a charge on said member corresponding to a charge on said droplets, whereby the magnitude of said charge may be determined, and wherein the spacing of the end of said rod shaped member from said stream is between 5-l0 drop diameters and wherein the width of said rod shaped member is between 5-20 drop diameters, and
means connecting said rod shaped element to a signal utilization means for comparing the signal with a predetermined norm.
2. An inductive charge sensing element as set forth in claim 1 including a pair of shield plates disposed on either side of said element and substantially perpendicular to the path of said ink droplet stream, each of said shield plates containing a hole therein through which said ink droplet stream can freely pass and circuit means connected to said shield plates whereby the charge sensing element is protected from stray fields.
3. In an ink jet printing system including a linear array of closely spaced nozzles, each producing a high pressure ink jet which breaks up into a series of droplets shortly after leaving said nozzle, individual charging means located adjacent each nozzle for placing appropriate charges on said droplets, separate deflection means for each said droplet stream and a recording medium upon which said droplets impinge, the improvement which comprises an array of inductive charge sensing elements for individually detecting charges placed upon the droplets of each individual stream by their respective charging elements, said inductive charge sensing array comprising:
a plurality of rod-shaped elements constructed of a highly conductive material mounted in a common insulating support member such that the end of each rod is located adjacent to but in a nonimpinging disposition with respect to each stream of ink droplets;
means connecting each said element to individual utilization circuitry means therefor and,
two shielding plates closely spaced to and on opposite sides of said linear array of charge sensing elements, individual holes located in said plates so that each said ink droplet stream can pass through a pair of holes in said plates in non-impinging relationship thereto and, passes the end of an associated charge sensing element located between said plates, said plates being disposed substantially perpendicular to said stream of ink droplets, and means for grounding said shield plates whereby said inductive charge sensing elements are shielded from stray field effects.