CA1111894A - Ink jet shutoff system - Google Patents

Abstract

INK JET SHUTOFF SYSTEM
Abstract of the Disclosure Fouling of an electrostatic ink jet head is sensed and causes shutoff of the head and of the associated electron-ics. The fouling is sensed by detecting contamination of the charge electrodes or of the deflection plates by conduc-tive ink, the detection being by sensitive circuitry employing diodes.

Description

9 Background of the Invention' Field of the Invention 11 The invention relates to ink jet and, more particu-12 larly, to ink jet systems employing conductive ink.
13 Description of the Prior Art 14 In recent years, substantial effort has been -~I 15 directed toward designing nonimpact printing systems poten-,ll 16 tially having relatively high resolution and relatively high 17 printing speeds. Electrostatic ink jet is such a nonimpact ¦ 18 printing system, and significant éffort has been focused on ¦ 19 achieving high resolution and achieving high speed.
! 20 High resolution réquires fine nozzles and fine ._,,~
21 drop charging and deflection structures to form and control , 22 small drops. The inks usually employed in such systems are 23 corrosive in nature is well as conductive. Thus-, any partial . j , 24 clogging or fouling of a nozzle resulting in misdirection of ` 25 the jet may ultimately cause substantial electrical shorting .
~, 26 and cause corrosion of delicate parts.
27 A way of increasing the printing speed of ink jet 28 systems is to employ multiple nozzles and multiple charge ... .
.. - . . . . .
- ~ :
~ BO976054~ - ~

.
.. ~ ., ... ~ ~ .
.. ... .. . . . . .

, .

:
' - .:
:;

8~4 1 electrodes, all closel-~ ~paced with respect to one another.

2 With large numbers oi closely spaced an~ delicate electrodes

3 and nozzles, the partial clogging or fouling of a single

4 nozzle may cause damaging interactions with other nozzles S and electrodes. Such potentially damaging interactions may 6 result in destructio~ in the usefulness of an entire multi-7 nozzle ink jet head.
8 Prior efforts have concentrated on perfection of 9 the ink jet nozzles, upon perfection of the ink's chemical composition, and upon perfection of ink filtration systems 11 to thereby attempt to prevent any clogging or fouling of the 12 ink jet nozzles. It may be that such perfection is not 13 truly attainable in the machine production environment.
14 Summary of the Invention The present invention relates to an electrostatic 16 in~ jet head assembly, including at least one nozzle for 1~ projecting a stream of conductive ink, at least one corresponding ]~1 charge electrode for charging drops formed from the stream, 19 and deflection electrode means for deflecting charged drops from a normal path of;uncharged drops in the stream. The 21 present invention comprises sensing apparatus for detecting 22 the impingement of sufficient conductive material at certain 23 of the above electrodes to contaminate the electrodes, and 24 apparatus responsive to the operation of the sensing apparatus to shut down affected parts of the assembly, for example, to 26 shut off the ink jet head to prevent projection of the r 27 stream, and to shut off operation of the charge electrode 28 and the deflection electrode means.

Bo976054 ~ 2 ~18~4 1 Brief Description of the Drawings FIGURE 1 is an illustration of a single jet ink jet head assembly;
FIGURE 2 is a frontal view of a multi-jet ink jet head assembly ~
FIGURE 3 is a cross-section view through the ink jet :
head assembly of FIGURE 2;
FIGURE 4, shown on the same page as FIGURE 1, is a ~:
schematic representation of a sensing means for use with the ;~
charge electrode of FIGUXE 1 and with multiple charge elec~
trodes of FIGURES 2 and 3; : `
FIGURE 5 is a schematic representation of a sensing means for use with the deflection electrodes of FIGURE 1 or with the deflection electrodes of FIGURES 2 and 3; and FIGURE 6 is a schematic representation of the shutdown apparatus usable with the ink jet head assembly of FIGURE 1 or with the.ink jet head assembly of FIGURES 2 and 3. ::
Deqcription of a Preferred Embodiment 2~ FIGURE 1 illustrates an ink jet printing system and head assembly. This includes a pump 10 operated by motor 11 for directing ink from an ink supply conduit 12 to a single nozzle 13. The ink is directed through a crystal 14 which is pulsed at high frequencies, for example in the range 117 kHz. The ink emitted from the nozzle 13 breaks into drops when passed through a charge electrode 16. The drops are thus variable charged thereat in accordance with the output of a charge amplifier in order to deflect the drops in a column an amount representing the vertical height of the drop locations in any given character.

r~h 1 As illustrated, the S designated as 20 to be 2 printed on document 21 comprises a number of individual 3 vertical columns 22. The printing is such that a sequence 4 of vertical columns, each comprising a plurality of drops, is propelled from nozzle 13 toward the document 21 for the 6 printing of the character involved. If drops are not required 7 for printing, they are directed to a gutter 24 for passage 8 by means of a conduit 25 back to the ink supply. Deflection ~, 9 plates 27 and 28 are positioned respectively above and below the path of travel of the drops leaving the charge electrode 11 16. A high voltage is applied to one of the plates and the 12 other may be grounded or may have a high negative potential ~, 13 to thereby create a potential gradient therebetween. This, 14 in cooperation with the variable charges on the individual drops, determines the amount of deflection of each drop as 16 the drop is directed 'towards the document 21.
17 Specifically, the ink jet stream emitted from 18 nozzle 13 is caused to have a perturbation therein which 19 grows in accordance with the distance from the nozzle. This perturbation is the r'esult of the vibration of the crystal 21 14. Within the confines of the charge electrode 16, the 22 individual perturbations grow to the point that drops break 23 from the stream. The'charge imparted to an individual'drop 24 is dependent on the charge electrode at the moment of break-off. The variable charging and constant deflection gradient 26 between plates 27 and 28 causes characters to be formed by 27 deflecting the variably charged drops to desired locations 28 in a forty drop high raster or scan. Forty drops may Bo976054~ 4 8~4 1 represent a vertical distance of one-sixth of one inch, this 2 forming a forty drop high character box for ten pitch - -3 characters.
4 A multi-jet head assembly is illustrated in FIGURES
2 and 3. A block 30 includes a manifold 31 formed therein.
6 Within the manifold are a piezoelectric crystal 32 and a 7 nozzle orifice plate 33. The orifice includes two rows 34 8 and 35 of closely spaced ink jet orifices. A charge plate 9 36 is mounted on block 30 and is pro~ided with two rows of charge electrodes 37 and 38, each charge electrode aligned 11 with a corresponding orifice of orifice plate 33. The 12 piezoelectric crystal 32 is mounted on a backing plate 39.
13 Pressurized ink is supplied to manifold 31 and is 14 ejected through orifices 34 and 35 of orifice plate 33.
Piezoelectric crystal 32 is perturbated by an electrical 16 ~ignal to vary the internal voIume of the manifold 31. This 17 perturbates the ink pressure, causing the ink jet streams 18 emanating from orifices 34 and 35 to break into streams of 19 uniform drops. As described with respect to the stream of FIGURE 1, the ink emahates from orifices 34 and 35 in the 21 form of streams passing through openings 40 and 41 with the 22 perturbations increasing as the distance from the orifice 23 plate 33 increases. ~t a distance from the orifice plates 24 such that the streams are within corresponding charge elec-trodes 37 and 38, the drops break off from the streams. The 26 drops then assume a charge dependent upon the voltage applied 27 to the corresponding charge electrodes 37 or 38 at the 28 instant of drop breakoff.

BO976054~ 5 1 Uncharged drops E~roceed alons paths 42 and 43 to 2 im~act a recording mediurn 44. A deflection plate 45 is 3 maintained at a high voltage and is positioned intermediate 4 the drop flow paths 42 and 43. Deflection electrodes 46 and 47 are electrically grounded and are positioned respectively 6 on the opposite sides of drop paths 42 and 43 from high 7 voltage deflection electrode 45. As shown, deflection 8 electrodes 46 and 47 may curve away from the drop paths and 9 terminate in openings 48 and 49 which communicate with cavities 50 and 51. The cavities 50 and 51 may further 11 communicate with tubes 55 and 56 which are connected to a 12 vacuum source 57 for withdrawing ink therefrom for return to ~3 an ink supply.
14 Manifold 31 is further connected by means of channel 58 and hose 59 to a pump 60. The pump is further 16 connected to a conduit 61 which is connected to ~he ink 17 supply 62. Pump 60 thus is actuated to provide ink from the 18 ink supply to manifold 31 to cause ejection of the ink 19 through orifices 34 and 35 of orifice plate 33.
FIGURE 4 illustrates the circuitry of the present 21 invention for detecting the contamination of one or of a 22 plurality of charge electrodes. Specifically, exemplary ink 23 jet nozzle 70 emits ink jet stream 71 which breaks into a 24 stream of ink jet droplets 72. Assuming the exemplary ink jet nozzle is that of FIGURE 1, charge electrode 73 is the 26 same as charge electrode 16 in FIGURE 1. The voltage applied 27 thereto at input 74 by a charge amplifie~ ma~ assu~e any of 28 a wide range of applied values, possi~ly reachirlg a peak BO976054~ 6 .~4 1 v.~lu~ of 250 vol~s.
2 The circuitry of th* presen~ invention is ~lesic3ned 3 to strobe one or a plu~ality of charge electrodes to detect 4 the voltage which corresponds to a preselected ~pplied S voltage.
6 As arranged for a single charge electrode only, 7 the detection circuitry of the present invention includes a ~ diode 80 with the conduction direction towards the charge 9 electrode if the charging voltages are positive, a resistance 81, and an input 82 to which is supplied a constant voltage, 11 to establish the indicating voltage at node 83, which is 12 indicative of contamination or lack of contamination of the 13 charge electrode 73. To establish this voltage, the voltage 14 at source 82 must be slightly less than th~ preselected voltage applied at input 74 to the charse electrocle 73. The 16 voltage at node 83 is detected by an cperatior,al amplifier 17 comparator 85. Node 83 is connected to a first input 86 OL
18 the comparator 85. The other input 87 to the comparator is 19 connected to a voltage biasing source 88 The biasirlg source 88 may be a potentiometer and is connected between 21 ground 89 and a constant voltage input 90. The voltage at 22 input 90 is the same as that at input 82. Thus, the reference 23 voltage at input 87 is a somewhat lower voltage than is the 24 input voltage at input terminal 82.
So long as resistor 81 is not conducting, the 26 voltage at node 83 and at input 86 of the com.parator is 27 equal to the constant input voltage -t input 82. So lonc3 as 28 this voltage remains higher than the referel-ce voltac3e at BO976054~ 7 8$~

1 input 87, the comparator supplies a zero OUtpllt. ~lowever, 2 the applied voltage at input 74 to the charge electrode 73 3 is often at a voltac~e lower than the preselected voltaqe.
4 This lower voltage causes diode 80 to conduct, drawing a current through resistor 81, and dropping voltage at node 6 83. Inasmuch as dropping the voltage at node 83 below the 7 reference voltage ap~lied at input 87 to the comparator 8 causes the comparator to supply an output, the comparator is 9 provided with another strobe input 92. The strobe input acts as a gate, serving to keep the output of comparator 85 11 on line 91 grounded until a signal is applied at the strobe 12 input. So as to test for contamination of charge electrode 13 73 only when desired and when the preselected voltage is 14 supplied at input 74, the strobe input 92 is normally off, and is only operated at the desired testing time.
lG In operation, various charging voltages are 17 supplied ~y the charge amplifier at input 74 to the charge 18 electrode 73. Whenever this voltage is less than that 19 supplied as the constant voltage at input 82, a current is drawn via diode 80 across resistor 81. The current drawn is 21 sufficient to create a voltage drop so that node 83 follows 22 the voltage at the charge electrode 73. Diode 80 thus 23 serves as a conducting path so long as the voltage at input 24 74 is less than the constant voltage applied at input 82.
Whenever the applied voltaye 74 is greater than that at 26 input 82, diode 80 prevents conduction and the resultant 27 current draw across resistor 81. Node 83 then becomes 28 clamped at the value of the constant voltage at terminal 82.

~097~054 ~ 8 8~4 1 Th~ voltage as trac~d b~ noae 83 is blocked from prGviding 2 an Outpllt on line 91 from the comparator due to th~ lack of 3 an input at strobe input line 92.
4 To test the charge ~lectrode, the preselected voltage is applied at input 74 to the charse electrode and 6 the strobe input is applied to input 92 of the comparator 7 85. Should the charge electrode not be contaminated, no 8 current is drawn by the charge electrodQ and the voltage g thereat exceeds the constant input voltage in input 82.
Thus, diode 80 blocks that higher voltage from node 83 and 11 no current is drawn across resistor 81. The voltage of node 12 83 thus is the same as that at input 82 and is higher than 13 the reference voltage at input 87 to the comparator. Therefore, 14 although the strobe input 92 is operated, the comparator provides no output on line 91.
16 Should the charge electrode be contaminated, the 17 conductive ink will create a conduction path to ground 18 tending to reduce the resultant voltage at the charge 19 electrode and draw a current through diode 80. This current causes a voltage drop to occur across resistor 81, reducing 21 the voltage at node 83 and at input 86 to the comparator.
22 Upon its voltage dropping below that of the reference voltage 23 at input 87, the comparator provides an output on line 91 24 for the time period of the strobe 92. The signal on line 91 is an indication of contamination of the charse electrode 26 73.
27 From the above description, it is a~parent that 28 diode 80 is not absolutely necessary for operation of the 29 system. Its functions are to save a possible back voltage BQ976054~ 9 ~118~4 ~ ~

1 from occurring between charge electrode input 74 and the 2 constant voltage source 82; to allow use of relatively in- `
3 expensive low voltage. The diode is necessary, however, 4 when applying the circuitry of FIGU~E 4 to a multiplicity of charge electrodes to isolate the charge electrodes from~one ;-6 another for normal operation. The system as shown may 7 equally be used with a plurality of ink jet heads such as , ~
8 illustrated in FIGURE 1 or with the multiple charge electrodes ~
9 associated with the single head of FIGURES 2 and 3. . ~-As illustrated in FIGURE 4, a plurality of lines ~- 11 100 and diodes 101 are connected individually to each of the ~- L2 remaining charge electrodes and are connected in common to . ., ~
~ 13 input 86 of the comparator 85. As the result of this arrange~
i, ....................................................................... .
s~ 14 ment, node 83 will tend to follow the lowest voltage on any of the charge electrodes, due to the current draw toward .~ , ~, :
~r~ ~ 16 that voltage by the associated diode 80 or 101 across resistor 17 81. The diodes 80 and 101 therefore isolate the associated 18 charge electrode from node 83 whenever the charge electrode 19 voltage is higher than the node.
~J, 20 In operation, all charge electrodes are operated 21 at the preselected voltage by the associated input from the .~ .
22 voltage source on line 74. Strobe 92 is then operated.
23 Should none of ~he charge electrodes be contaminated, all of 24 tbe charge electrodes will have a voltage higher than that supplied at the constant voltage input 82. Diodes 80 and 26 101 prevent any current flow across resistor 81 so that 27 voltage at node 83 is the same as that at input 82. As this 28 voltage is greater than the reference voltage at input 87 to BO976054 ~ 10 1 the comparator, the comparator provides no output signal on 2 line 91.
3 Should any of the charge electrodes be contaminated, 4 the conductive ink again forms a conductive link towards ground potential. This causes a current drain through the 6 associated diode 80 or 101 through resistor 81. The resultant 7 potential formed across resistor 81 drops the voltage at 8 node 83. This voltage is then applied to input 86 of the 9 comparator. Upon the voltage dropping below the reference voltage at input 87, comparator 85 wil] provide an output 11 signal on line 91 so long as strobe input 92 is operated.
12 In summary, comparator 85 provides an output 13 signal on line gl only when both the strobe signal is supplied 14 at line 92 and when one of the charge electrodes 73 is contaminated.
16 With reference to FIGURE 5, the circuitry employed 17 to detect contamination of the deflection plates is somewhat 18 similar to that employed with the charge electrode, although 19 the voltages involved are considerably higher. Once again, an exemplary ink jet nozzle 70 is illustrated producing an 21 ink stream 71 which is perturbated and breaks into a stream 22 of drops of 72. Subsequent to the charge electrodes 73 and 23 in the direction of the stream or streams 72 are located the 24 deflection plates 105 and 106. Deflection plat~ 106 may be connected to a ground potential 107 or may be connected to a 26 negative potential thereat. The deflection plate 106 thus 27 corresponds to deflection plate 28 in FIGURF 1 and to deflection 28 plates 46 and 47 in FIGURES 2 and 3. Deflectlon plate 105 sos760s4~ 11 1 is connected, via node 109 and resistor 110 to input 111 of 2 a high voltage supply. Deflection plate 105 thus corresponds 3 to plate 27 in FIGURE 1 and to electrode 45 in FIGURES 2 and 4 3.
In the example chosen, the drops are charged 6 positively by charge electrode input 74 and the high voltage 7 supply is a constant high amplitude negative voltage.
8 Precisely the opposite arrangement may also be used. Should 9 the deflection plates be contaminated with conductive ink, a current might flow from deflection plate 105 towards ground 11 or towards an available positive voltage. The current flow 12 creates potential across resistor 110, raising the voltage 13 at node 109 toward ground potential.
14 The detection circuitry includes diode 114, resistance 115, input 116 from a constant voltage source of slightly 16 lesser negative amplitude than the voltage supply input 111, 17 comparator 117, and reference voltage source. The value of 18 resistance 115 is selected to be significantly less than 19 that of resistor 110. Thus any current flow through resistor 115 will be significantly greater than that through resistor 21 110, and thereby establishes a detection node 120 which is 22 connected to input 121 of the comparator 117.
23 The reference voltage source 118 may comprise a 24 potentiometer connected between ground 122 and input 123 from a voltage source. This voltage source is negative and 26 may be the same as connected to input 116. The reference 27 voltage source is connected to input 124 of the comparator.
28 Thus, a threshold voltage is established at input 124 which BO976054 ~ 12 8~4 1 is somcwhat closer to grollnci poter-tial than the ~oltage at 2 input 116. So lona as the signal at input 121 is of higher 3 negative amplitude than the threshold input 124, comparator 4 117 provides no output on line 125. Should the voltage at node 120 approach ground due to current flow through resistor 6 115, so that its value is of lesser negative amplitude than 7 the threshold voltage at input 124, comparator 117 supplies 8 an output signal on line 125.
9 In operation, first assume that the deflection -10 plates 105 and 106 are not contaminated, so that nil current 11 flows from the high negative voltage supply 111 to the 12 deflection plate. As no current is flowing through resistor 13 110, there is no voltage drop thereacross and node 109 14 assumes voltage of input 111. As this voltage is more negative than the voltage at input 116, diode llg is back 16 biased and does not conduct. With the diode not conducting, 17 node 120 assumes the voltage at input 116 for application to 18 input 121 of the comparator. As this voltage is more negative 19 than the reference voltage applied from source 118 to input 124, comparator 117 provides no output on line 125.
21 Assuming, however, that the deflection plates 22 become contaminated so as to draw a current through resistor 23 110, a potential is created thereacross to raise the voltage 24 at node 109 toward ground potential. Upon the voltage at node 109 becoming more positive than the negative vGltage at 26 input 116, diode 114 becomes conducting. As a result, the 27 current flow between input 116 and the deflection plate 28 establishes a potential across resistor 115, creating a Bo976054~ 13 ~118~4 ~:
~ -1 voltac3e at node 120, closeLy followed by the voltage at node :

2 109 and plate 10S. The voltage at node 120 is applied to ,~

3 input 121 at comparator 117. Upon this voltage being lesser 4 than that from reference voltage source 118 as applied to `

5 input 124, comparator 117 supplies an output signal on line ::

6 125.

-i 7 In summary, comparator 117 has as its comparison ~ ;
'! ~ 8 input the voltage at input 116 so long as the deflection g plates are not contaminated. Upon the deflection plates :

` 10 becoming contaminated, the voltage at input 121 follows that ;~

~i- 11 of node 120 based upon current flow in resistance 115 and of 12 deflection plate 105 so long as the voltage is closer to .~ .
,~;
~:; 13 ground and of lower negative amplitude than that at input 14 116. Upon the voltage being less than the reference voltage, comparator 117 supplies the contamination output signal on . ~
16 line 125.
~i~ Additionally, a multiplicity of deflection plates 18 could be attached and operated similarly to the charge 19 electrode5, Referring to FIGURE 6, output line 91 from FIGU~E
21 4 and output line 125 from FIGU~E 5 are supplied as inputs 22 to OR Invert circuit 130. OR Invert circuit 130 supplies an 23 output signal so long as no input signal is supplied on ~ .
24 either inputs 91 or 125. When a signal is received on either line 91 or on line 125, the OR Invert circuit 130 26 drops the output signal on line 131.

27 Output line 131 is connected to input 135 of AND
~8 circuit 136, to driver 137 and to driver 138. AND circuit BO976054~ 14 1 ~11894 r 1 136 serves as a gate-, with input l35 the controlling, or - 2 gating, input. Input 140 comprises the incoming charye 3 electrode data. The data is normally transmitted by AND
; 4 circuit 136 to a charge electrode circuit 141. The charge electrode 141 responds to the incoming data by supplying the 6 charging signals on line 74 to the charge electrode 73 in

7 FIGURE 4. In the multiple nozzle environment, such os that ~- 8 of ~IGURES 3 and 4, output line 131 of OR Invert circuit 130 ~9 is supplied to a plurality of AND circuits 136 and 142 to selectively gate data to a corresponding plurality of charge 11 electrodes 141 through 143. ~;
12 The driver 137 is connected to valve 145 in ink supply 146 and 147. The driver is arranged such that when ~-~ 14 no signal is present on output line 131, indicating that the ` 15 charge electrodes or deflection plates are contaminated, the 16 driver supplies no signal to valve 145 so the valve closes "i ,,., .. ~ , 17 as the result of spring pressure. In normal operation, OR
18 inverter 130 supplies an output signal on line 131, operating 1~ driver 137 to supply a signal to valve 145, holding the valve open to supply ink from input line 146 to output line 21 147. Output line 147 is connected to the ink jet head for 22 the supply of ink thereto. For example, the supply line may 23 comprise input 12 in FIGURE 1 or may comprise a valve at the 24 output of pump 60 in FIGURE 3. -~
Similarly, driver 138 responds to the presence of 26 a signal on line 131 by supplying a drive current to coil 27 150 of relay 151, thereby closing the relay point and connecting 28 a high voltage supply 152 to resistance 110 and node 109 ~ ~4 .:
1 also illustrated in FIGURE 5. Thus, the high voltage relay 2 151 comprises the voltage source at input llI in FIGURE 5 to supply the high voltage to the deflection plates 105 in - -FIGURE 5. Shou~d the deflection plates or charge electrodes ~; ~ 5 become contaminated, OR Inverter 130 drops the output signal 6 on line 131, in turn causing driver 138 to cease supplying a ~,: ,: :
j current through coil 150. High voltage relay 151 opens, ~ ~

8 terminating the high voltage supply to node 10g and to the -g deflection plate 105. ~ ~;
In summary, the lack of an input signal on inputs ~11 91 or 125 OR Inverter 130 indicates that no contamination is ~12 present. The OR Inverter therefore supplies an output ~ -~.:
13 signal on llne 131 gating the charging data via AND circuits ~ ;
14 136 and 142 to the charge electrode circuit, operates driver 137 to open valve 145 to allow the flow of ink therethrough ~` 16 to the ink jet head, and operate drive 138 to close high f-- 17 voltage relay points 151 so as to supply the high voltage to 7:`:
18 the deflection plate. Upon either the charge electrodes or 19 deflection plates becoming contaminated, a signal is supplied by the appropriate circuitry to either input 91 or input 125 , . . .
21 of OR Inverter 130. Upon either or both inputs having a ~`
22 signal thereat, the OR Inverter 130 drops its output on line 23 131. Dropping that output causes AND gates 136 through 142 24 to bloc~ the supply of further charging data to the charge electrode circuits, therefore terminating the application of 26 charging signal thereto. Dropping the signal on line 131 27 also causes drivers 137 and 138 to cease supplying signals 28 to the corresponding valve 145 and high voltage relay 151, BO976054~ 16 1 in turn, shutting off the ink supply through the valve 145 2 and terminating the '~igh voltage supply from source 152 to 3 the high voltage deflection plate.
4 While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the 7 foregoing and other changes in form and details may be made 8 therein without departing from the spirit and scope of the

9 invention.

BO976054 ~ 17

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an electrostatic ink jet assembly having at least one nozzle means for projecting electrically conductive ink to form drops, charging electrode means for electro-statically charging said drops; and deflection electrode means for deflecting charged drops; the improvement for detecting contamination of any of said electrode means by said electrically conductive ink comprising:
means for establishing an electrical signal whose amplitude is related to the amplitude of an electrical current flow conducted by said contaminating electrically conductive ink at any of said electrode means; and signalling means for signalling the detection of said contamination upon said electrical signal exceeding a pre-determined amplitude.

2. The improvement of claim 1 additionally including:
controlling means responsive to said detection signal from said signalling means for terminating operation of said ink jet assembly.

3. The apparatus of claim 2 wherein said nozzle means includes an ink supply means; wherein said charging means includes at least one charge electrode and at least one corresponding charge electrode circuit for applying charging signals to said charge electrode; and wherein said deflec-tion means includes at least one deflection electrode to which a high electrical voltage is applied; and wherein said controlling means comprises:
valve means responsive to said detection signal from said signalling means for blocking ink supply means;
gating circuit means responsive to said detection signal for preventing said charge electrode circuit from applying charging signals to said charge electrode; and disconnect means responsive to said detection signal for terminating said application of said high voltage to said deflection electrode.

4. The apparatus of claim 1 wherein said deflection electrode means includes at least one deflection electrode to which a high electrical voltage is applied; and additionally including controlling means responsive to said detection signal from said signalling means for terminat-ing operation of said ink assembly including disconnect means responsive to said detection signal from said signal-ling means for terminating said application of said high voltage to said deflection electrode.

5. The improvement of claim 4 wherein said establish-ing means comprises:
means for providing a first electrical signal in response to an electrical current flow conducted by said contaminating electrically conductive ink; and means for providing a second electrical signal in response to said first electrical signal reaching a pre-determined amplitude, the amplitude of said second electrical signal depending upon the amount of electrical current flow conducted by said contaminating electrically conductive ink, said second electrical signal thereby comprising said electrical signal of said establishing means.

6. The improvement of claim 5 wherein said means for providing a second electrical signal comprises:
a voltage source of a second predetermined amplitude;
a diode, interposed between said voltage source and said means for providing said first electrical signal, for providing a second electrical current to said deflection electrode means upon said first electrical signal reaching said second predetermined amplitude; and means for providing said second electrical signal in response to the amplitude of said second electrical current flow conducted by said contaminating electrically conductive ink.

7. The apparatus of claim 1 wherein said charging electrode means includes at least one charge electrode, and at least one corresponding charge electrode circuit for applying charging signals to said charge electrode; and additionally including controlling means responsive to said detection signal from said signalling means for ter-minating operation of said ink jet assembly including gating circuit means responsive to said detection signal from said signalling means, for preventing said charge electrode circuit from applying charging signals to said charge electrode.

8. The improvement of claim 7 wherein said establish-ing means comprises:
a voltage source of a second predetermined amplitude;
means, interposed between said voltage source and said charge electrode, for providing a first electrical signal in response to an electric current flow to said charge elec-trode conducted by said contaminating electrically conductive ink and to said charge electrode circuit upon the voltage thereof being of lesser amplitude than said voltage source;
and said signalling means additionally comprises means for strobe sampling said first electrical signal at predetermined times, whereat said voltage of said charge electrode circuit is at least equal to that of said voltage source.

9. The apparatus of claim 1 wherein said ink jet assembly having at least two said nozzle means and at least two corresponding said charging means, said improvement additionally comprising:
isolation means connected to each said charging means for allowing said current flow of said establishing means and for preventing current flow from one said charging means to another said charging means.