Circuitry for detecting malfunction of ink jet printhead

A device and method for detecting nozzle malfunction in an ink jet printhead having a predetermined number of nozzles. The printhead is first controlled whereby each of the nozzles is activated once in a manner which is required to print on a print medium a line having a number of ink dots equal to the predetermined number. Then a photodetector detects whether each location on the print medium which opposed one of the nozzles at the time of activation of that one nozzle has an ink dot printed thereat. Each location is photodetected in time sequence by scanning. The photodetector outputs a signal having a first level in response to each photodetected location which has no ink dot thereat and a signal having a second level in response to each photodetected location which has an ink dot thereat. The output of the photodetector is then processed. In accordance with one embodiment, the length of a printed line is effectively measured to determine if it is shorter than an expected length corresponding to a printed line formed by dots equal in number to the number of nozzles. In accordance with another embodiment, a multi-bit word is formed, each bit representing the state of a corresponding nozzle. The word is input to a look-up table to generate a status signal, in response to which further action will be taken.

FIELD OF THE INVENTION 
The invention relates to circuitry for detecting malfunctioning nozzles in 
a multi-nozzle printhead of an ink jet printer. In particular, it relates 
to such a printhead incorporated in a postage meter. 
BACKGROUND OF THE INVENTION 
The use of a multi-nozzle ink jet printhead in an ink jet printer is well 
known in the prior art. Such a printhead has been used to print postage 
indicia in an electronic postage meter. Generally the nozzles of the 
printhead are linearly arrayed along a direction which is perpendicular to 
the direction of movement of the print medium relative to the printhead. 
Whether the nozzle array is vertical or at an oblique angle, each nozzle 
prints a row of dots in the course of printing the postage indicia in a 
postage meter. 
A problem associated with the use of any multi-nozzle ink jet printhead is 
that it is possible for a nozzle to malfunction so that it becomes unable 
to eject ink in response to activation. For every nozzle that is unable to 
eject ink in response to activation, there will be a corresponding row on 
the print medium which is blank, i.e., has no dots. 
This problem is particularly serious in the case of an ink jet printhead 
incorporated in a postage meter. It is possible that the failure to print 
one row of dots in the plurality of rows making up the postage indicia 
will render the postage indicia or some portion thereof indecipherable. In 
particular, it is possible that the printed postage amount will be 
indecipherable, causing the postal authorities to refuse to deliver the 
piece of mail on which the postage indicia have been printed. This is 
particularly true in the case of numerals which are printed with a stroke 
of consecutive dots in a row, such as numerals 2, 4, 5, and 7. Such 
defectively printed postage indicia will result in a monetary loss to the 
postage meter user since in the conventional secure postage meter, it is 
impossible to print postage indicia unless the postage amount being 
indicated has been accounted for within the postage meter. Since the 
postage meter user must account to the postal authorities for all postage 
printed, the meter user will also end up paying for any printed postage 
amount even if the postage indicia representing that amount was 
indecipherable, rendering the piece of mail undeliverable. 
SUMMARY OF THE INVENTION 
It is an object of the invention to overcome the foregoing problem by 
providing means for detecting whether any of the nozzles of the ink jet 
printhead have failed. 
It is a further object of the invention to provide an ink jet printer in 
which the failed nozzles are detected prior to printing of the indicia 
representing the postage amount. 
In particular, it is an object of the invention to shutdown the postage 
meter if the failed nozzles detected are such that sufficiently 
decipherable indicia representing the postage amount cannot be printed. 
Also it is an object of the invention to recognize conditions wherein the 
failure of a particular nozzle can be compensated for to ensure that 
sufficiently decipherable indicia representing the postage amount can be 
printed. 
It is a further object of the invention to provide simple and inexpensive 
circuitry for detecting whether any of the nozzles of the ink jet 
printhead have failed and whether that failure is fatal to proper 
operation of the postage meter. 
The foregoing objects of the invention are achieved in accordance with the 
preferred embodiments by providing means for photodetecting whether a dot 
has been printed on the medium in response to activation of each nozzle of 
the printhead. Each nozzle is fired only once and in accordance with a 
program such that a line of dots spaced at regular intervals will be 
formed if all the nozzles are functioning properly. This can occur either 
at the start of printing of the postage indicia, in which case the line 
forms part of the postage indicia, or prior to the start of printing of 
the postage indicia, in which case the line forms part of a test pattern. 
The photodetection circuitry produces one signal in response to detection 
of an ink dot and another signal in response to detection of the absence 
of an ink dot. 
In accordance with one preferred embodiment of the invention, the 
photodetector output is encoded into a signal having a high level when the 
absence of an ink dot has been detected and a low level when the presence 
of an ink dot has been detected. Therefore the encoded signal undergoes a 
transition from the high level to the low level when the start of the line 
of printed dots is detected. If the line has a discontinuity due to the 
failure of a nozzle to eject ink, then the encoded signal will undergo a 
transition from the low level to the high level when the start of the 
discontinuity dots is detected. Thereafter the encoded signal will again 
undergo a transition from the high level to the low level when the end of 
the discontinuity dots is detected. These transition will be repeated for 
each detected discontinuity in the line of dots. At the end of the line, 
the encoded signal will again undergo a transition from the low level to 
the high level. 
In accordance with this first embodiment, only the presence of a 
discontinuity in the line is detected, i.e., which nozzle has failed is 
not determined. In response to detection of the discontinuity indicating a 
failed nozzle, the postage meter is disabled. 
In accordance with another preferred embodiment of the invention, the 
photodetector output is encoded to produce a train of bits of binary data. 
The binary "one" represents the absence of an ink dot and the binary 
"zero" represents the presence of an ink dot. (Obviously this scheme can 
be reversed by incorporation of an inverter. The train of binary data has 
a number of bits equal to the number of nozzles in the printhead. For 
example, if the printhead has 32 nozzles, then a 32-bit work will be 
formed, each bit corresponding to one of the nozzles, each binary "one" 
representing a properly functioning nozzle and each binary "zero" 
representing a failed nozzle. 
In accordance with this second embodiment, the 32-bit word forms the 
address for a look-up table. The look-up table stores a code representing 
a command for action to be taken in dependence on the identity of any 
failed nozzles. If no nozzles have failed, then the look-up table output 
will be a control signal causing the remainder of the postage indicia to 
be printed. If one or more nozzles have failed in a manner that can be 
compensated for by reprogramming of the manner in which the nozzles are 
fired, then the look-up table output will be a control signal causing the 
central processor to carry out such reprogramming, after which the 
remainder of the postage indicia will be printed. If one or more nozzles 
have failed in a manner that cannot be compensated for by reprogramming of 
the steps by which the nozzles are fired, then the look-up table output 
will be a control signal causing the central processor to disable the 
postage meter, thereby preventing the printing of a postage amount which 
is not sufficiently decipherable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The basic components of the preferred embodiments of the invention are 
generally depicted in FIG. 1. In particular, these components are 
incorporated in a postage meter, although the teaching of the invention is 
applicable to any device which incorporates an ink jet printhead. 
The central processor 12 supplies binary data representing the characters 
and/or graphics to be printed and binary data representing the addresses 
in a bit map memory incorporated in control circuitry 14 at which the 
respective bits of data are to be stored. The control circuitry 14 
controls the firing of the nozzles of printhead 16 in dependence on the 
data stored in the bit map memory. The control circuitry 14 is in turn 
controlled by central processor 12 in accordance with various subroutines 
stored in the internal memory of the central processor. 
In particular, the central processor is programmed to cause the printhead 
to print a line of dots for testing the nozzles for malfunctions. This 
line of dots can be printed either as a separate test pattern prior to the 
start of printing the postage indicia or at the start of printing the 
postage indicia as part thereof. In the preferred embodiments disclosed 
herein, the test line is slanted. However, the test line could also be 
vertical if vertically displaceable photodetection means are provided. 
In accordance with the invention, the test line of dots is photodetected by 
photodetector 18, which generates a voltage proportional to the amount of 
light impinging on its photosensitive surface. More specifically, 
photodetector generates a signal having a voltage above a certain 
threshold when the photodetector detects the absence of dots and a signal 
having a voltage below that certain threshold when the photodetector 
detects the presence of a dot. These analog signals are input to detection 
circuitry 20, which converts them to digital form and then decodes the 
digital signals to determine the state of the printhead, i.e., whether any 
nozzles of the printhead have failed. In accordance with one embodiment of 
the invention, the detection circuitry also outputs specific status 
signals to the central processor in dependence on which of the nozzles 
have failed. In response to these status signals, central processor 12 
takes appropriate action. 
The preferred embodiments of the invention incorporate a multi-nozzle ink 
jet printhead in which the nozzles are arranged along a line which is 
canted or tilted to lie at a small angle relative to the direction of 
motion of the medium. Thus, the vertical spacing between nozzles will 
become equal to the arithmetic product of the sine of the angle between 
the direction of nozzle alignment and the direction of motion of the 
medium multiplied by the nozzle spacing. 
In the preferred embodiment of the invention, the one application of this 
printer, the angle between the direction of nozzle alignment and the 
direction of motion of the medium is 16 degrees, yielding a vertical 
resolution of about 120 dots per inch, i.e. a spacing between rows of dots 
of 0.0083 inch. 
The arrangement of the nozzles 2 in accordance with the preferred 
embodiment of the invention is partially shown in FIG. 2. It should be 
understood that the multi-nozzle printhead in accordance with the 
preferred embodiments of the invention has 32 nozzles in linear alignment, 
although FIG. 1 shows less than 32 nozzles in order to simplify the 
drawing while still depicting the geometric principle involved. 
In accordance with the preferred embodiments of the invention shown in FIG. 
2, the horizontal spacing between nozzles will be approximately 0.0288 
inch. The horizontal spacing between dots has been selected as one-third 
of this horizontal nozzle spacing, i.e., 0.0096 inch, defining the 
horizontal dot resolution as about 104 dots per inch. The "pitch" of the 
printhead is defined here as the ratio of the horizontal nozzle spacing to 
the distance between dot columns. The pitch in accordance with the 
preferred embodiment of the invention has an integral value of 3. 
It would be readily understood by the practitioner of ordinary skill in the 
art of ink jet printers that the principle of the invention can be applied 
using a printhead having a number of nozzles different than 32 and having 
an angle between the direction of nozzle alignment and the direction of 
motion of the medium which is different than 16 degrees. In particular, 
the angle between the direction of nozzle alignment and the direction of 
motion of the medium could be 90 degrees. 
A shaft encoder (not shown) attached to the medium transport mechanism 
generates a pulse each time the multi-nozzle printhead is displaced a 
distance equal to the spacing between dot columns. Thus, the shaft encoder 
generates three pulses during a displacement of the multi-nozzle printhead 
equal to the horizontal spacing between nozzles. 
The excitation of the multi-nozzle printhead is accomplished using a 
Siliconix SI9554 integrated circuit. A logic diagram for this integrated 
circuit is shown in FIG. 3. Alternatively, any other functionally 
equivalent circuitry could be used in place of the Siliconix SI9554. 
In accordance with the preferred embodiment of the invention, 32 bits of 
control data are shifted into a 32-bit shift register 4. Subsequently, 
this data is transferred to a 32-bit latch register 6. Each bit of control 
data controls whether a corresponding nozzle 2 of the printhead is turned 
on or off, i.e., ejects or does not eject ink, during the print cycle 
corresponding to those 32 bits of control data. When the medium reaches 
the next dot position, the pulse from the shaft encoder enables an output 
enable pulse which enables 32 AND gates 8, which in turn causes the 
latched 32 data bits to be respectively passed through AND gates 8, 
amplified by amplifiers 10 and output on the 32 output terminals of the 
integrated circuit. Piezoelectric actuators (not shown) energize selected 
ones of the 32 nozzles on the printhead, causing ink drops to be ejected 
onto the medium at the desired locations, in dependence on the binary 
value of the respective 32 bits of control data. 
The slanted line which is printed in accordance with the preferred 
embodiments of the invention is depicted in FIG. 4. The arrow in FIG. 4 
denotes the direction of movement of the print medium. However, at the 
outset it should be noted that the invention is also applicable in the 
case where the printhead, not the medium, is moved. 
In accordance with the preferred embodiments, only the variable data in the 
postage indicia is printed by the ink jet printhead. The remainder of the 
postage indicia 22 is printed separately using a conventional print 
roller. At the start of printing the variable postage data a slanted line 
24 is printed. In particular, the slanted line is formed by activating 
each of the 32 nozzles only once. Since the nozzles are spaced at a 
distance equal to three dot columns and the dots of the slanted line are 
to be formed in adjacent dot columns, the nozzles will be fired in timed 
sequence, adjacent nozzles being fired at a time interval equal to the 
time required for the printhead to move two dot columns. It should be 
noted, however, that different printhead configurations will require that 
the nozzles be fired in accordance with a different scheme. The invention 
is not limited to any particular configuration or number of nozzles as 
long as the printhead can be controlled to print a line of dots by firing 
each nozzle only once. 
The slanted line is scanned by a slit 28 having a width such that it 
receives light reflected from an area of blank medium having a width 
substantially equal to the diameter of an ink drop ejected by a nozzle. 
Thus during the scanning of the slanted line, the slit will pass through 
32 successive positions corresponding to the 32 dots printed by the 32 
nozzles if none of the nozzles have failed. 
The main components of the photodetection subsystem are depicted in FIG. 5. 
A light beam is projected by light source 32 onto the surface of the print 
medium 26. A lens 34 is arranged in the path of the light reflected from 
the medium surface. Lens 34 refocuses the beam to compensate for any 
divergence caused by the reflection. An opaque mask 30 is arranged on the 
far side of the lens 34. Mask 30 has the slit 28 formed therein. Slit 28 
has dimensions such that it substantially passes only light reflected from 
a vertical section of the dot column located at the point of reflection of 
the light beam. The light passing through slit 28 thereafter impinges on 
the photosensitive surface of a photodetector 36. As previously noted, the 
photodetector generates a voltage proportional to the amount of light 
impinging on its photosensitive surface. The amount of light impinging on 
the photosensitive surface varies in dependence on whether or not an ink 
dot has been printed in the vertical section of the dot column being 
scanned by the slit. The signal output by the photodetector therefore 
carries information regarding the presence or absence of an ink dot in 
each of the scanned dot columns. 
In the event that all nozzles have ejected ink in response to activation, 
the slanted line will comprise 32 dots without a discontinuity. 
Correspondingly, the photodetector output will undergo a transition from 
the high level to the low level when the dot column having the first dot 
of the slanted line located therein is scanned. The photodetector output 
will remain at the low level during scanning of the next 31 dot columns. 
Then the photodetector output will undergo a transition from the low level 
to the high level when the dot column following the dot column having the 
last dot of the slanted line located therein is scanned. A low level 
signal at the photodetector output for a time period equal to the time to 
scan 32 dot columns indicates the all nozzles have fired properly. 
FIG. 7 depicts the case wherein one nozzle in the 32-nozzle array has 
failed to eject ink in response to activation during printing of the 
slanted line. Because one nozzle has failed to fire, there will be a 
discontinuity in the slanted line, i.e. the vertical section of the dot 
column being scanned will be substantially blank, causing the 
photodetector output to return to a high level. As the slit scans to the 
next dot column, the photodetector output will again drop to the low 
level, indicating the discontinuity in the slanted line has passed. Thus a 
malfunctioning nozzle can be detected by a sequence in which the voltage 
rises and then falls at an interval equal to the time required for the 
slit to scan one dot columns. If the interval between the rise and fall 
equals the time to scan two dot columns, then this indicates that a pair 
of adjacent nozzles have malfunctioned and so forth. 
In two instances, however, a malfunctioning nozzle will not cause a rise 
followed immediately by a fall in the photodetector output voltage, i.e., 
when an end nozzle has failed to fire. In both of these instances, the 
photodetector output will be a low level signal having a duration equal to 
the time it takes to scan 31 dot columns. Because the photodetector output 
is the same in both cases, i.e., ambiguous, which nozzle has malfunctioned 
cannot be determined from scanning of the slanted line alone. 
One solution to this ambiguity is to print a second slanted line by firing 
all of the nozzles except for one of the end nozzles. This second slanted 
line is then detected to determine which end nozzle malfunctioned. If the 
photodetector output is low for a duration equal to the time it takes to 
scan 31 dot columns, then it is apparent that the end nozzle which was not 
activated during printing of the second slanted line is the defective 
nozzle. Conversely, if the photodetector output is low for a duration 
equal to the time it takes to scan 30 dot columns, then it is apparent 
that the end nozzle which was activated during printing of the second 
slanted line is the defective nozzle. 
Other schemes for resolving the aforementioned ambiguity will be readily 
apparent to the practitioner of ordinary skill in the art of 
photodetection. 
A block diagram of the circuitry in accordance with the first preferred 
embodiment of the invention is shown in FIG. 6. The photodetector output 
voltage is applied to one input of a comparator 38, the other input of 
which receives a reference voltage level from reference voltage source 40. 
The comparator 38 outputs a binary signal having a high level (hereinafter 
referred to as "one") when the photodetector output voltage is greater 
than the reference voltage and a binary signal having a low level 
(hereinafter referred to as "zero") when the photodetector output voltage 
is less than the reference voltage. 
The output of comparator 38 is connected to an input of gate 42. The other 
input of gate 42 is connected to the output of a clock 44. The clock 44 
outputs a CLOCK signal which has a period substantially equal to the time 
required for the slit 28 to scan a distance equal to the width of one dot 
column. 
When the comparator output is "zero", gate 42 is enabled. When gate 42 is 
enabled, it passes the CLOCK signal output by clock 44 substantially 
unchanged. This CLOCK signal is received by a counter 48 which counts up 
one unit for each clock pulse received. Because the counter begins to 
count when the first dot of the slanted line is detected, it effectively 
counts in real time the number of dot columns containing a dot which have 
been scanned. 
The output of comparator 38 is also connected to the input of a rising edge 
detector 46. The rising edge detector 46 outputs a STOP signal to counter 
48 in response to a transition from "zero" to "one" on the comparator 
output. This transition corresponds to either a discontinuity in the 
slanted line or the end of the slanted line, depending on the value in 
counter 48 when the count is stopped. In response to a READ signal from 
the central processor, the count is output to an encoder 50, which issues 
a SHUTDOWN signal if the count does not equal 32. A count less than 32 
indicates that less than all of the 32 nozzles were properly fired in 
response to activation. In response to the SHUTDOWN signal, the central 
processor disables the postage meter. 
The specific logic circuitry making up the components generally depicted in 
FIG. 6 are readily apparent to a practitioner of ordinary skill in the art 
of digital circuitry design. 
A block diagram of the circuitry in accordance with the second preferred 
embodiment of the invention is shown in FIG. 8. The photodetector output 
voltage is again applied to one input of a comparator 38, the other input 
of which receives a reference voltage level from reference voltage source 
40. The comparator 38 outputs a "one" when the photodetector output 
voltage is greater than the reference voltage and a "zero" when the 
photodetector output voltage is less than the reference voltage. 
The output of comparator 38 is connected to the data input of a 32-bit 
shift register 56. The output of comparator 38 is also connected to the 
input of a falling edge detector 52. In response to a transition from 
"one" to "zero" on the comparator output, the falling edge detector 52 
outputs a signal to one input of gate 42 which enables gate 42. This 
transition corresponds to detection of the start of the slanted line. The 
other input of gate 42 is connected to the output of clock 44. Clock 44 
again outputs a CLOCK signal which has a period substantially equal to the 
time required for the slit 28 to scan a distance equal to the width of one 
dot column. 
When gate 42 is enabled, it passes the CLOCK signal output by clock 44 
substantially unchanged. This CLOCK signal is received by the shift 
register 56, which shifts the incoming data by one bit in response to each 
clock pulse received. 
The CLOCK signal is also received by a count down counter 54, which begins 
to count down from 31 in response to the transition of the comparator 
output from "one" to "zero". When the count down counter reaches a value 
of zero after 32 clock pulses, it outputs a LATCH ENABLE signal to latch 
58. During this countdown, the shift register 56 has successively shifted 
the incoming data until 32 bits of data are held therein. In response to 
the LATCH ENABLE signal, the 32 bits of data held by the shift register 
are output in parallel to latch 58. 
The latched data is in turn output to a look-up table 62 via gates 60 in 
response to an OUTPUT ENABLE signal from the central processor. The 
look-up table need not be incorporated in the detection circuitry 20 and 
could equally effectively be incorporated in the central processor itself. 
In accordance with the second preferred embodiment, the look-up table is 
stored in a read only memory (ROM). The data is input to the address pins 
of the ROM. The look-up table stores the different possible states of the 
multi-nozzle printhead and outputs the state addressed by the input data. 
This output represents the status of the multi-nozzle printhead based on 
the test conducted. 
In particular, if the 32 bits received by the shift register during its 
enablement are all "zero", this indicates that all of the 32 nozzles were 
properly fired in response to activation. If the 32 bits received by the 
shift register during its enablement are not all "zero", this indicates 
that all of the 32 nozzles were not properly fired in response to 
activation. In fact, each "one" in the 32-bit word output from shift 
register 56 represents a malfunctioning nozzle. Depending on the data 
input to the look-up table, a corresponding status signal will be output. 
The subsequent steps to be performed depend on the state of the 
multi-nozzle printhead which is read from the look-up table. 
FIG. 9 shows the various steps which are taken in accordance with the 
second preferred embodiment of the invention. As already explained, in 
step 100 all nozzles are driven once in the sequence necessary to print a 
slanted line wherein the number of ink dots making up the line equals the 
number of nozzles in the printhead. The slanted line is photodetected in 
step 102. The data representing the state of the nozzles is acquired 
starting with the first transition of the comparator output from "one" to 
"zero". Upon the start of data acquisition, a 32-bit word is formed (step 
104) representing the presence or absence of ink dots in the variable data 
vertical section of the corresponding 32 dot columns. This 32-bit word is 
decoded to determine whether a nozzle malfunction has occurred (step 106). 
If no malfunction has occurred, then the remainder of the variable data, 
including the postage amount, is printed by the ink jet printhead (step 
108). If a nozzle malfunction has occurred, then the type of malfunction 
is identified (step 110). 
If the number or location of the malfunctioning nozzles is such that the 
central processor is unable to compensate by stepping the printhead in the 
vertical direction and activating selected nozzles during a predetermined 
number (preferably one) of additional passes over the print medium, then 
this condition is treated as a fatal error. The number of passes which can 
be made is, of course, limited by the number of different vertical 
positions at which the printhead can be located. In response to a fatal 
error, the postage meter is disabled (step 112). 
On the other hand, if the number or location of the malfunctioning nozzles 
is such that the central processor is able to compensate by stepping the 
printhead in the vertical direction and activating selected nozzles during 
a predetermined number (preferably one) of passes over the print medium, 
then this condition is treated as a soft error. In response to a soft 
error, the central processor reprograms the control circuitry to retrieve 
the unprinted data from the bit map memory and supply the retrieved data 
to properly functioning nozzles (step 118). The missing rows of data are 
then printed during the additional passes (step 108). 
Finally, as already discussed in connection with FIG. 7, if the data is 
ambiguous, a second slanted line is printed using fewer nozzles and the 
photodetection is repeated, except that in this mode the data is input to 
an auxiliary look-up table (not shown in (FIG. 8). 
The specific logic circuitry making up the components generally depicted in 
FIG. 8 are readily apparent to a practitioner of ordinary skill in the art 
of digital circuitry design. 
Furthermore, it is to be understood that the foregoing preferred 
embodiments are disclosed for illustrative purposes. The scope of the 
invention is defined in the appended claims.