Continuous ink jet auxiliary droplet catcher and method

An auxiliary droplet catcher and a method for its use in an ink jet printing apparatus of the type having charge and deflection electrodes to respectively charge selected droplets and then deflect the selected charge droplets from a normal droplet flight path towards a primary droplet catching structure. The auxiliary droplet catcher is mounted for reciprocal movements between retracted and advanced positions and is moved into the advanced position in response to a control system sensing an inability of the charge and/or deflection electrodes to respectively charge and/or deflect the selected droplets, thereby preventing flooding of a print medium.

FIELD OF THE INVENTION 
The present invention generally relates to noncontact fluid printing 
devices conventionally known as "ink jet" or "fluid jet" printers. More 
particularly, the present invention is related to an auxiliary droplet 
catching structure which is movable between retracted and advanced 
positions so as to prevent flooding of a print substrate in response to a 
sensed inability of the charging and/or deflection electrodes to 
effectively charge and/or deflect fluid droplets, respectively. 
BACKGROUND AND SUMMARY OF THE INVENTION 
Noncontact printers which utilize charged droplets are generally known as 
evidenced by U.S. Pat. Nos. 3,373,437 to Sweet et al; 3,560,988 to Crick; 
3,579,721 to Kaltenbach; and 3,596,275 to Sweet. Typically, fluid 
filaments of e.g. ink, dye, etc. are issued through respective orifices of 
an orifice plate. An array of individually controllable electrostatic 
charging electrodes is disposed downstream of the orifice plate along the 
so-called "droplet formation zone." In accordance with known principles of 
electrostatic induction, the fluid filament is caused to assume an 
electrical potential opposite in polarity and related in magnitude to the 
electrical potential of its respective charging electrode. When a droplet 
of fluid is separated from the filament, this induced electrostatic charge 
is trapped on and in the droplet. Thus, subsequent passage of the charged 
droplet through an electrostatic field having the same polarity as the 
droplet charge will cause the droplet to be deflected away from a normal 
droplet path towards a droplet catching structure. Uncharged droplets, on 
the other hand, proceed along the normal path and are eventually deposited 
upon a receiving substrate. 
A problem arises, however, in that should the charging and/or deflection 
electrode become inoperative due to, for example, loss or interruption of 
electrical power or shorting of the electrodes to ground potential, it 
would become impossible to electrostatically charge the droplets and/or to 
thereafter deflect the charged droplets from the normal droplet flight 
path. That is, should either the charging electrode or deflection 
electrode malfunction, substantially all of the droplets may behave 
similarly to uncharged droplets and will thus proceed along the normal 
droplet flight path and be deposited upon the print medium. There exists 
therefore the possibility that the print medium will become flooded due to 
the inability of the charging electrode and/or deflection electrode to 
perform their intended functions. As higher printing speeds are used, 
great waste of fluid and substrate material can occur as the result of 
electrode failure over even short intervals of time. 
Often, the charge and/or deflection electrodes of an ink jet printer can 
become electrically shorted to ground potential during the course of 
normal printing operations as a result of electrical bridging via 
particulate impurities in the printing fluid and even via the fluid 
itself. The distance between charging electrodes and fluid filaments is 
typically very small (on the order of thousandths of an inch) to ensure 
proper electrostatic charge induction. Consequently, charge electrodes may 
sometimes become wetted with a quantity of printing fluid. Printing fluid 
typically has sufficient electrical conductivity to cause current to flow 
from the wetted charge electrode to any other structure also in contact 
with the quantity of fluid, thereby decreasing the charging potential of 
the wetted charge electrode. Small particles in the fluid which become 
lodged between a charge electrode and a structure at ground potential 
(such as an electrode mounting fixture, etc.) can completely short the 
charging electrode to ground potential. 
Shorting of charging and/or deflection electrodes can usually be easily 
cured by simply drying and/or cleaning the shorted electrode (e.g. by 
using suction). Unfortunately, the method usually used to detect electrode 
shorts is for an operator to visually monitor the final printed substrate. 
An inattentive operator can thus cause extreme waste of printing substrate 
and printing fluid. Moreover, less-than-catastrophic malfunctions of the 
electrode can cause printing defects which are not readily discernible to 
the naked eye as the printing substrate is conveyed past an operator after 
printing but which should nevertheless be avoided to ensure high quality 
printing. 
The present invention overcomes such disadvantages by providing means by 
which malfunctions in the charging and/or deflection electrodes are 
automatically detected. In accordance with one aspect of the invention, 
droplets in an ink jet printing apparatus are captured to prevent the 
printing substrate from becoming flooded when the charging and/or 
deflection electrodes have malfunctioned. The auxiliary droplet catching 
structure of the present invention is mounted for reciprocal rectilinear 
movements between a retracted position wherein the catching structure is 
retracted from the generated droplet streams and an advanced position 
wherein the catching structure intercepts the droplet streams. In such a 
manner, the droplet catching structure when in the advanced position 
prevents the droplet streams from proceeding along the droplet flight path 
and being deposited upon the print medium. 
The movements of the auxiliary catching structure between the retracted and 
advanced positions are controlled so that the structure operates in a 
"fail safe" manner. Thus, when the control system senses an inability of 
the charging and/or deflection electrodes to properly charge and/or 
deflect droplets, respectively, the auxiliary catch pan will be moved into 
the advanced position so as to prevent flooding of the substrate due to 
its interception of substantially all droplets along the normal droplet 
flight path. 
INFORMATION DISCLOSURE STATEMEMT 
Attention is directed to the publications discussed below as examples of 
possibly relevant prior art. 
Movable droplet catching structures are, for example, generally known in 
the art as evidenced by U.S. Pat. No. 4,217,594 to Meece et al; 4,150,384 
to Meece; 4,305,079 to Mix, Jr.; 4,371,881 to Bork et al; 4,413,265 to 
Kockler et al; 4,266,231 to Drago et al; 4,160,982 to Keur; 4,367,479 to 
Bower; and 4,199,767 to Campbell et al. 
Meece '384 and Meece et al '594 disclose a movable gutter 24 that can be 
moved into a droplet catching position by means of an undisclosed cam when 
the charging of the droplets is to be synchronized with droplet formation. 
Mix, Jr. '079 discloses a primary ink catching gutter that is pivotally 
movable from its normal operational position at a point spaced from a 
droplet nozzle along the path of ink droplets to a position immediately 
adjacent the nozzle plate. 
A shield which is movable relative to an ink droplet writing head is 
disclosed in Bork et al '881. The shield of Bork et al '881 includes a 
resilient wiper element which, when moved relative to the openings through 
which ink droplets issue, cleanses the openings. The shield is mounted so 
as to be pivotally moved from a normal position wherein droplet printing 
is permitted to a shielding position wherein ink droplets are prevented 
from striking a printing medium. 
Kockler et al '265 also disclose a primary catching device which can be 
pivoted into a full catch position at start-up and shutdown so as to catch 
substantially all of the generated droplets. The catching device of 
Kockler et al '265 is mounted for rotation about an axis parallel to the 
row of generated ink droplets. 
A probe which is movable in a direction along the flight path of ink 
droplets is disclosed in Drago et al '231. During start-up conditions, the 
probe is moved to a position adjacent the nozzle plate and is thereafter 
displaced away from the nozzle plate along the flight path of the ink 
droplets until the print mode is operational. Once the print mode is 
operational, a primary droplet catching device catches deflected droplets, 
the movable probe thus having served its intended function. Upon shutdown 
of the print mode, the probe is once again moved along the flight path of 
the ink droplets until the nozzle-adjacent position is reached. 
A droplet catching device similar in function to Drago et al '231 is 
disclosed in Keur '982. Means are disclosed in Keur '982 to move a 
so-called accumulator, which is normally positioned adjacent the print 
medium towards the droplet ejection head during shutdown thereof. During 
start-up, the accumulator is progressively moved in a direction parallel 
to the generated ink droplet stream until its normal operation position 
(separated from the ink ejection head) is achieved. 
Bower '479 and Campbell et al '767 disclose shutter-type valving mechanisms 
which reciprocally move so as to either deflect the ink issuing from an 
ejection nozzle (Bower '479) or sealingly engage with the nozzle to 
prevent ink from issuing (Campbell et al '767). 
As briefly mentioned above, the present invention provides the means by 
which an inability of the charging and/or deflecting electrodes is sensed 
and, in response to the sensed inability, an auxiliary droplet catcher is 
moved into a droplet catching position so as to prevent flooding of a 
print substrate. These as well as other advantages of the present 
invention will become better understood by study of the following detailed 
description of a presently preferred exemplary embodiment of this 
invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENT 
As shown in FIG. 1, the fluid jet apparatus 10 generally includes a 
printhead 12 having an orifice plate (not shown) through which a linear 
array of fluid streams are issued so as to generate a sequential plurality 
of droplets which proceed along a normal droplet flight path 14 toward a 
print medium 22 moving in the direction indicated by arrow 24. Selected 
ones of the droplets are charged by means of charge electrode 16 such that 
when the selected charged droplets pass through a deflection field 
generated by deflection electrode 18, the charged droplets will be 
deflected from the normal droplet flight path 14 towards primary droplet 
catching structure 20. Uncharged droplets, on the other hand, proceed 
along droplet flight path 14 so as to be deposited upon print medium 22. 
Referring to FIG. 2, the auxiliary droplet catcher 25 is mounted so as to 
be reciprocally rectilinearly movable along a path substantially 
perpendicular relative to the normal droplet flight path 14 between a 
retracted position (shown in solid line in FIGS. 1 and 2) and an advanced 
or catching position (shown in phantom line in FIGS. 1 and 2). 
As can be best seen in FIG. 3, the auxiliary droplet catcher 25 includes a 
tray 26 having a bottom wall 28, opposing side walls 30 and a front wall 
32 to thereby define a cavity 34 for accepting fluid. Housing member 36 is 
fixed to a rear portion of tray 26 and defines a path 38 connected to a 
vacuum source 44 by means of flexible conduit 46 through which fluid may 
be removed from the tray. The flexibility of conduit 46 thus enables the 
auxiliary droplet catcher 25 to be moved between its retracted and 
advanced positions without interrupting fluid removal through path 38 by 
means of vacuum source 44. 
Tray 26 includes an opening 40 in fluid communication with cavity 34 which 
is vertically separated from bottom wall 28 by a dimenson "d" to thereby 
establish the maximum depth of fluid in cavity 34. Opening 40 also 
preferably includes a beveled edge 42 which assists in maintaining the 
level of fluid in cavity 34 at a predetermined level. Beveled edge 42 
permits the ambient air to be drawn into path 38 by means of vacuum source 
44 so as to quickly establish equilibrium flow with the fluid. In such a 
manner, beveled edge 42 assists in the selection of the maximum depth of 
fluid in tray 26 in dependence upon the bevel angle of the edge (the angle 
determining the amount of air drawn into path 38 between the surface of 
the fluid and beveled edge 42). That is, a greater amount of bevel on edge 
42 will cause equilibrium flow with the liquid in tray 26 to be more 
quickly established and thus greater depth of liquid in tray 26 will be 
maintained. 
As shown in FIG. 2, auxiliary droplet catcher 25 is mounted substantially 
transversely relative to the movement of print medium 22 (the direction of 
the movement is indicated by arrow 24) between lateral frames 50, 52 which 
extend substantially parallel to the direction of movement (arrow 24) of 
print medium 22. Frames 50, 52 respectively include support 54, 56 rigidly 
fixed to the frames. Collar members 58, 60 are fixed to the opposing sides 
of auxiliary droplet catcher 25 and are coupled to support posts 54, 56 so 
as to permit sliding reciprocal movement of auxiliary droplet catcher 25 
between its advanced and retracted positions. An air cylinder 62 or other 
like actuating means (such as an electromechanical and/or pneumatic 
actuator) is operatively coupled to auxiliary droplet catcher 25 to 
provide the actuation force necessary to move auxiliary catcher 25 between 
its extended and retracted positions. Air cylinder 62 preferably includes 
an actuator arm 64 which is extendable in response to pressurized air 
being introduced into cylinder 62. 
Pressurized air of a predetermined pressure is selectively produced by a 
conventional electrically-controlled pressurized air source 68 in response 
to a control signal V.sub.catch produced by an electronic controller 70 
(the function and operation of controller 70 will be explained in greater 
detail in connection with FIGS. 4, 5(A) and 5(B)). Pressurized air 
produced by pressurized air source 68 is introduced into cylinder 62 via a 
tube 69. Pressurized air source 68 in the preferred embodiment can be 
controlled by controller 70 (e.g., by applying a control signal 
V.sub.catch of logic level "1") to introduce pressurized air of a 
predetermined positive pressure into cylinder 62 to slide auxiliary 
droplet catcher 25 into its catching position alternatively a 
predetermined vacuum may be applied to cylinder 62 to move the auxiliary 
droplet catcher into its retracted position (e.g. by applying a signal 
V.sub.catch of logic level "0"). As will be appreciated, cylinder 62 could 
instead be a reverse acting air cylinder such that pressurized air is 
admitted into, or vacuum applied to the cylinder to retract actuator 64, 
while a spring (not shown) or other biasing means included within air 
cylinder 62 biases actuator 64 (and thus auxiliary droplet catcher 25) 
into one of the catching and retracted positions as desired. 
FIG. 4 is a schematic block diagram of a preferred embodiment of electronic 
controller 70. Controller 70 includes a digital signal processor 72, a 
deflection electrode circuit 74 and a charge electrode circuit 76. 
Deflection electrode circuit 74 selectively applies a predetermined voltage 
potential to deflection electrode 18 in response to an ENABLE signal 
produced by processor 72. Deflection electrode circuit 74 applies a signal 
V.sub.sense 1 to processor 72 whenever the potential applied to deflection 
electrode 18 falls below a predetermined level. 
Charge electrode circuit 76 applies a predetermined charging electrical 
potential to selected ones of an array of charge electrodes 16 in response 
to a print control signal produced by processor 72 and to appropriate 
pattern data supplied from a conventional pattern print control circuit 
(not shown). Charge electrode circuit 76 applies a signal V.sub.sense 2 to 
processor 72 whenever the potential supplied to charge electrodes 16 falls 
below a predetermined level. 
Deflection electrode circuit 74 includes a voltage-source or regulator 78, 
an output voltage adjust control 80, a high voltage inverter 82, a voltage 
divider 84, a comparator 86, a voltmeter 88 and, of course, deflection 
electrode 18. Voltage-source 78 in the preferred embodiment is a 
conventional regulated DC voltage source which produces an output voltage 
V.sub.1 the level of which can be adjusted by varying output voltage 
adjust control 80. The output V.sub.1 of voltage-source 78 is applied to 
the input of a conventional high voltage inverter 82. High voltage 
inverter 82 converts V.sub.1 to a high potential voltage V.sub.D having a 
level proportional to the level of V.sub.1, and applies the high potential 
voltage to deflection electrode 18. High voltage inverter 82 may include 
means for limiting the current produced at its output to prevent 
short-circuiting of deflection electrode 18 from causing the components of 
the inverter to become damaged due to over-current. 
The output V.sub.D of inverter 82 is proportional to the level of output 
V.sub.1 of voltage-source 78. The level of V.sub.1, in turn, is determined 
by the setting of output voltage adjust control 80. Therefore, the setting 
of output voltage adjust control 80 determines the voltage potential 
V.sub.D applied to deflection electrode 18, provided that deflection 
electrode 18 is not shorted to ground potential. 
A conventional voltage divider 84 is connected at one end to deflection 
electrode 18 and at its other end to ground potential. Voltage divider 84 
samples the voltage potential V.sub.D applied to deflection electrode 18 
and produces a sample voltage V.sub.s the level of which is a 
predetermined fraction of V.sub.D. The sample voltage V.sub.s produced by 
voltage divider 84 is applied as one input of voltage-source 78 to cause 
accurate voltage regulation of high voltage potential V.sub.D (e.g. 
voltage control is accomplished by a conventional feedback arrangement in 
a well known manner). Sample voltage V.sub.s may also be applied to a 
conventional voltmeter 88, which may to provide an instantaneous visual 
indication of the level of voltage applied to deflection electrode 18. 
Sample voltage V.sub.s is also applied to one input of a conventional 
voltage comparator 86. A reference voltage V.sub.ref (which may be 
proportional to V.sub.1 and be derived from voltage-source 78) is applied 
to the other input of comparator 86 (of course, V.sub.ref could be 
produced in any conventional manner, such as by an independent stable 
voltage source). 
Comparator 86 produces a TTL logic level "1" output signal V.sub.sense 1 
whenever V.sub.ref exceeds the sample voltage V.sub.s produced by voltage 
divider 84 (i.e., whenever V.sub.ref &gt;V.sub.s). Because V.sub.ref is 
derived from voltage-source 78 in the preferred embodiment, signal 
V.sub.sense 1 is produced by comparator 86 based upon a predetermined 
relationship between the actual and desired levels of the potential 
V.sub.D applied to deflection electrode 18 (the desired level being set by 
output voltage adjust control 80). Comparator 86 may produce V.sub.sense 1 
briefly following the initial generation of the deflector ENABLE signal by 
processor 72 and following any increase in the setting of output adjust 
control 80 (e.g. until V.sub.1 rises to the corresponding set level and 
V.sub.D fully tracks V.sub.1). Processor 72 is preferably programmed to 
ignore this brief generation of V.sub.sense 1, as will be explained. 
When deflector electrode 18 fully or partially short-circuits to ground 
potential, the potential V.sub.D applied to deflection electrode 18 falls 
due to the current limiting action of high voltage inverter 82. If the 
level of V.sub.D falls below a predetermined level, the level of sample 
voltage V.sub.s falls below the reference level V.sub.ref (i.e., V.sub.ref 
&gt;V.sub.s) and comparator 86 applies a logic level "1" signal V.sub.sense 1 
to processor 72. Processor 72 is thus informed that the high voltage 
potential of deflection electrode 18 has fallen below a predetermined 
determined level and may take appropriate responsive action, as will be 
explained. 
The specific threshold voltage V.sub.s at which comparator 86 begins to 
produce a logic level "1" signal V.sub.sense 1 selected by the values of 
voltage divider 84 and V.sub.ref, and is preferably a level below which 
deflection electrode 18 cannot properly deflect droplets of fluid (taking 
into account a desired safety margin). Comparator 86 continues to produce 
logical level "1" signal V.sub.sense 1 until the shorting of deflection 
electrode 18 has been corrected and deflection electrode high potential 
voltage V.sub.D rises to the level set by output voltage adjust control 
80. 
Charge electrode circuit 76 in the preferred embodiment includes a charge 
voltage source 90, a print pulse shaper/amplifier 92, a buffer 94, a 
voltage comparator 96, a charge distributor 98 and charge electrodes 16. 
Charge voltage source 90 produces a charge voltage V.sub.c of a 
predetermined level and applies this voltage to print pulse 
shaper/amplifier 92. Pulse shaper/amplifier 92 shapes, amplifies and 
buffers TTL-level print control pulses V.sub.P produced by processor 72, 
and applies the processed print control pulses to the input of buffer 94 
and to a first input of voltage comparator 96. Buffer 94 in the preferred 
embodiment is a conventional unity-gain driver amplifier which produces 
sufficient current to drive several of charge electrodes 16. Buffer 94 may 
include conventional current limiting circuitry to protect charge 
electrode circuit 76 from failure in the event charge electrodes 16 short 
circuit to ground potential. 
The output V.sub.B of buffer 94 is applied to a charge distributor 98 which 
may distribute V.sub.B to a selected plurality of charge electrodes 16 in 
a conventional fashion. V.sub.B (the output of buffer 94) is also applied 
to a second input of voltage comparator 96. Comparator 96 monitors changes 
in the difference between the level of signal V.sub.P applied to the input 
of buffer 94 and the level of signal V.sub.B produced at the input of the 
buffer. When a short circuit occurs between any of charge electrodes 16 
and ground potential (or between active and inactive ones of the charge 
electrodes), the current produced by buffer 94 rises until it reaches a 
current limiting level, and then levels off to a safe level. At this time, 
the level of the output V.sub.B of buffer 94 is no longer proportional to 
the level of the input V.sub.P of the buffer. Voltage comparator 96 
produces a logic level 1 output signal V.sub.sense 2 whenever the level of 
the output V.sub.B of buffer 94 falls below the level of the input V.sub.P 
of the buffer (i.e., whenever V.sub.B &lt;V.sub.P) (of course, comparator 96 
could compare the output V.sub.B of buffer 94 to a stable reference 
voltage level if desired). 
A buffer 94, comparator 96 and charge distributor 98 may be provided for 
each of a plurality of different (e.g. one-inch long) sections of charge 
electrodes 16 (all of buffers 94 may be driven in common by pulse 
shaper/amplifier 92). Thus, the output of a particular one of a plurality 
of comparators 96 may provide an indication of which section of charge 
electrodes 16 is shorted or has otherwise malfunctioned. 
Processor 72 may preferably comprise any conventional microprocessor or 
microcomputer including a central processing unit, a non-volatile program 
store, one or more internal registers, input/output ports etc. Processor 
72 is connected to receive V.sub.sense 1, V.sub.sense 2 and a signal 
produced by a protection override switch 100. Protection override switch 
100 may produce a logic level "1" signal whenever no short circuit 
protection is desired, and may produce a logic level "0" signal otherwise. 
Processor 72 may selectively produce an ALARM OUT signal to actuate a 
conventional audio and/or visual alarm 102, and also selectively applies 
an actuating signal V.sub.catch to air source 68 to control the position 
of auxiliary droplet catcher 25. Processor 72 also applies a deflector 
ENABLE signal to voltage-source 78 to actuate deflection electrode 18 
(thereby causing charged fluid droplets to be deflected from path 14 into 
primary droplet catching structure 20). Additionally, processor 72 may 
selectively apply a print control signal to shaper/amplifier 92 to cause a 
charging signal to be selectively applied to ones of charge electrodes 16 
(thereby determining which droplets are to be deflected by deflection 
electrode 18). The deflector ENABLE and print control signals may be 
produced by processor 72 in accordance with predetermined program 
instructions, user commands and/or appropriate pattern data supplied from 
a pattern control circuit (not shown), and may control the printing of 
material 22 in a conventional manner. 
FIGS. 5(A) and 5(B) together represent a flow chart or an exemplary 
interrupt handler 104 stored in the internal program store of processor 
72. Interrupt handler 104 is preferably executed at periodic intervals by 
processor 72, and begins execution at a START block 106. The flow of 
interrupt handler 104 as shown in the FIGS. 5(A) and 5(B) is generally 
from top to bottom. 
Decision block 108 determines whether deflection electrode 18 and charge 
electrodes 16 are active by testing whether processor 72 is presently 
applying a deflector ENABLE signal to deflection electrode circuit 74 or a 
print control pulse to charge electrode circuit 76. If deflection and 
charging voltages are not being applied to deflection electrode 18 and 
charge electrode 16, respectively, interrupt handler 104 returns control 
to other, conventional tasks to be performed by processor 72 ("return" 
block 130). Of course, if only deflection voltage V.sub.D (but not charge 
voltage V.sub.B) is being produced, interrupt handler 104 may proceed to 
test for shorting of deflection electrode 18 if desired. 
Decision block 110 determines whether protection override switch 100 is in 
a position indicating that no short-circuit protection is desired. If no 
short-circuit protection is desired, interrupt handler 104 jumps to 
"return" block 130. Otherwise, processor 72 determines if either of 
V.sub.sense 1 and V.sub.sense 2 have a logic level "1" value (indicating 
that one or both of the voltages applied to deflection electrode 18 or 
charge electrodes 16 is below a predetermined level). 
If neither of V.sub.sense 1 and V.sub.sense 2 is at logic level "1" (i.e., 
the Boolean expression V.sub.sense 1 OR V.sub.sense 2 is "False"), both of 
charging electrodes 16 and deflection electrode functioning properly, and 
interrupt handler 104 returns control to other tasks to be performed by 
processor 72 ("return" block 130). If, however, one or both of V.sub.sense 
1 and V.sub.sense 2 have a logic level "1" value (i.e., the expression 
V.sub.sense 1 OR V.sub.sense 2 is "true"), a conventional timer 
(preferably a software or hardware timer internal to processor 72) begins 
timing a first predetermined time interval (e.g. 5 seconds) (block 114). 
The first time interval preferably is programmable by a user (via, e.g. a 
keyboard not shown) to any desired value in a conventional manner. 
If neither V.sub.sense 1 nor V.sub.sense 2 is at logic level "1" at the 
conclusion of the first time interval (i.e. if the expression V.sub.sense 
1 OR V.sub.sense 2 is "False"), deflection electrode 18 and charge 
electrodes 16 are no longer shorted. Interrupt handler 104 in the 
preferred embodiment takes no further action at this time than to release 
control back to other tasks to be performed by processor 72 ("return" 
block 130). If, however, at least one of V.sub.sense 11 and V.sub.sense 2 
is still at logic level "1" at the conclusion of the first time interval 
(i.e., the expression V.sub.sense 1 OR V.sub.sense 2 is still "true"), 
either one or both of deflection electrode circuit 74 and charge electrode 
circuit 76 are still unable to operate properly and this inability has 
presumably existed for the duration of the first time interval. In order 
to prevent further wasting of printing fluid and print medium 22, 
auxiliary droplet catcher 25 is moved into its catching position (as 
previously described) by applying an appropriate control signal 
V.sub.catch to air source 68 (block 118). Alarm 102 is actuated when 
processor 72 produces the ALARM OUT signal (block 120), and other action 
may also be taken if desired (e.g. processor 72 may stop the printing 
process entirely, stop the travel of print medium 22, cause an automated 
electrode cleaning device not shown to begin operating, etc.). 
After processor 72 actuates alarm 120, it begins timing a second 
programmable time interval (block 122). At the conclusion of the second 
time interval (which may be, for example, five seconds in duration), 
processor 72 once again polls V.sub.sense 1 and V.sub.sense 2 to determine 
whether either of deflection electrode 18 and charge electrodes 16 is 
still shorted (i.e., the expression V.sub.sense 1 OR V.sub.sense 2 is 
still "true") (decision block 124). If one or both of deflection electrode 
and charge electrodes 16 are still shorted, interrupt handler 104 causes 
processor 72 to jump back to block 120 to continue to actuate alarm 102 
and to once again time the second time interval. The loop formed by blocks 
120, 122 and 124 is continually executed until either controlled 70 is 
powered off or neither of the electrodes is shorted (i.e., the expression 
V.sub.sense 1 OR V.sub.sense 2 becomes "false"). Once both of deflection 
electrode 18 and charge electrodes 16 are once again operating properly 
for the duration of the second time interval (indicated when the 
expression V.sub.sense 1 OR V.sub.sense 2 is "false" at the conclusion of 
the second line interval), the level of control signal V.sub.catch applied 
to air source 68 is changed appropriately to cause cylinder 62 to retract 
auxiliary droplet catcher 25 into its retracted position (block 126). 
Alarm 102 is also turned off at this time (block 128). 
While interrupt handler 104 has been described as sequentially performing 
all of its steps once it is invoked, it will be understood by those 
skilled in the art that the interrupt handler preferably is executed 
concurrently with other software which controls the printing process. 
Processor 72 could, of course, be a processor dedicated to detecting a 
malfunction of deflection electrode circuit 74 and charge electrode 
circuit 76. Alternatively, interrupt handler 104 could be made reentrant 
so that processor 72 could perform other tasks during the timing of the 
first time interval (block 114) and the second time interval (block 122). 
If desired, processor 72 could poll V.sub.sense 1 and V.sub.sense 2 
periodically during the timing of the first and second time intervals and 
could determine the results of decision blocks 116 and 124 on the basis of 
whether a condition had existed for the entire duration of the time 
interval (or for at least a predetermined portion of the timed interval). 
Moreover, while a digital signal microprocessor is used in the preferred 
embodiment, any other circuit configuration (e.g. an analog processor, a 
discrete logic element sequential machine, etc.) could be used instead. 
While the preferred embodiment uses direct voltage sensing to test whether 
deflection electrode 18 and charge electrodes 16 are shorted, it will be 
understood that other ways to detect short circuits could be used instead 
(for example, sensing whether excessive current is flowing to the 
electrodes, sensing the temperature of current carrying components, etc.) 
Moreover, auxiliary droplet catcher 25 could also be moved to its catching 
position in response to malfunctions other than inadequate voltage being 
applied to an electrode. For instance, it might be desirable to also move 
auxiliary droplet catcher 25 to its catching position upon start-up and 
shutdown of apparatus 10, in response to inadequate pressure or excessive 
flow rate of printing fluid at the orifice plate, in response to problems 
in the delivery or travel of print medium 22, etc. 
While the present invention has been described in what is presently 
conceived to be a preferred exemplary embodiment, those skilled in the art 
may recognize that many modifications may be made which modifications 
shall be accorded the broadest scope of the appended claims so as to 
encompass all equivalent methods, assemblies and/or structures.