Note: Descriptions are shown in the official language in which they were submitted.
The invention rel.ates to a method and apparatus for
checking the printing quality o~ matrix or liquid ~et printers
with the aid o~ photo-sensitive sensing means.
In ma-trix printer printing, the readability is
dependent, inter alia, on the condltion.of the printing ribbon
and on whether all the printing elements of the printing head are
functioning. Problems arise when printers are used without
continuous supervision. Examples of such printers are computer-
lo controlled printers operating at certain times during the day,
and cash dispensers which print out details of transactions.
Different methods exist for checking the printing
quality of matrix printers. For example, in PCT application US
81tO1207, there is described a device for detecting every dot
that is printed by means of optical sensors following the
printing head. The result of the detection is compared in a
computer with a signal pattern corresponding to the printed
character. A fault signal is produced on deviation from the
signal pattern. The device is also intended for use with ink ~et
writers. Since the two most important factors affecting printing
quality of a matrix printer are the condition of the printing
ribbon and the functioning of the printing elements, the device
acccrding to the mentioned patent is unnecessarily sophisticated
and expensive.
The present invention provides a simple and effective
check on the printing quality o~ a matrlx printer or ink ~et
printer. Each printing element simultaneously prints a dot pat-
tern within an area o~ the paper where no normal printing takesplace. I.n cooperat:lon with the line shift, a photo-sensitive
device reads the printed pattern. The signals thus generated are
processed and compared with a predetermined signal pattern corre-
sponding to correct printing quality. The difference between
these patterns activates an alarm circuit.
. , -- 1 --
;,t~
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According to the presen-t inventlon there is provided a
method of checking the printin~ quality of a matrix printer which
has a plurality of printing elements by means of a photosensitlve
circuit, the printer effecting line shifting of a record medium
to permit printing of a plurality of lines of print on said
record medium, said method comprising print1ng an indicium on
said record medium in an area related to at least one line of
prin~ on said record medium, said indicium being generated by
said printing elements as rows and columns of dots, in which each
lo row is printed by means of different printing elements, scanning
the rows of dots in sequence by means of the photosensitive
circuit in the course of relative movement between the indicium
and the photosensitive circuit during lineshift, th~reby generat-
ing a signal pattern, comparing ~he signal pattern from the photo
sensitive circuit with a signal pattern corresponding to a cor-
rectly printed indicium and generating an error signal iE there
is a difference between the compared signal patterns.
The present invention also provides a matrix printer
comprising a plurality of printing elements for printing a
plurality of lines of print on a record medium while performing
line shifts on said medium, a photosensitive sensing means, said
printer including means for printing an indicium on sald medium
in an area related to at least one llne of print, said lndicium
belng formed as rows and columns of dots in which the dots in
each row are printed by one printing element and in the columns
are printed by dif~erent printing elements, said photosensitive
sensing means being disposed such that, during a line shlft, the
rows of dots of said indicium pass in sequence under the sensing
means so that an electrical signal is generated, and circuit
means includin~ means for generating a reference signal,
comparator means for making a comparison between said electrical
signals and said reference signals, and means for generating an
error signal in response to a dlfference between the electrical
and reference signals. Suitably said printing means feeds the
medium in a given direction durlng line shifting, said prlnting
-- 2 --
means forming said row of dots in alignment with the direction of
feed o~ the medium, the formation of said indiclum by all of the
printing elements enabling determination of whe-ther all of the
printlng elements are operating correctly. Desirably the print-
ing elements are in a sequence of odd and even elements and theprinting means operates such tha-t for every second line said
indicium originates from the odd printing elements and for the
other lines said indicium originates form the even printing
elements.
The invention will now be described in more detail, by
way of example only, with reference to the accompanying drawings,
in which:-
Figure 1 schematically illustrates the part of a matrix
printer which are of interest to the invention; and
Figure 2 illustrates the associated electricalcircuits.
Figure 1 schematlcally illustrates writlng paper 1
bearing characters printed with the aid of a printing head 2 of
the matrix printer type. The printing head has also printed a
dot pattern 3 in the left hand margin of the paper for each line
of print. The pattern is shaped as a row of dots at rlght angles
to the direction o~ movement of the printing head, and has been
produced by all the writing elements of the printing head. An
optical sensing means ~ is disposed such that the dot pattern 3
passes under the sending means ~ during the line shift.
Figure ~ illustrates in more detall a device in
accordance with the present invention. The optical sensing means
~ includes a light emitting diode 7 illuminating the writing
paper under a row of photo diodes 8 provided with a shift
register, e.g., of the type EG & G Reticon RL128G. On a line
shift, the dot row 3 passes in the direction of the arrow P past
- 2a -
the photo diode row 8, where the diodes, at a spacing of 25~m,
are arranged at right angles to the direction of movement of the
dot row 3. By means of the shift register, each diode is read
out in turn
- 2b -
~,
,: .
with a speed determined by a control means 12 included in a com-
puter 6. In this case, the clock frequency of the control means
has been selected at 50 KHz, and the sampling frequency at 300
HZ, which gives a scanning time of 2.56 ms and about 12 scans
per dot of the dot pattern 3. The siynal from each photo diode
is transferred to a comparator 5, where it is amplified in a DC
amplifier F and compared with the vol-tage from a reference volt-
age source R in a differential a~plifier D. Signals with higher
amplitude than the reference voltage are assigned a "one" in the
differential amplifier, while to signals with lower amplitude are
assigned a "zero". The digitized signals are read by the computer
6, where a counter 9 adds all the "ones'l according to given con-
ditions. One condition, for example, may be that at least four
consecutive "ones" are required for the counter to add them.
After three scans of all diodes without the above-mentioned con-
dition being filled, the counter is read, and then set to zero.
A condition for a dot to be registered on passage of the dot pat-
tern 3 past the sensing means is that the counter 9 added 40
"ones", for example. The number of registered dots is compared
in a register 10 with the number of printing elements in the
printing head 2. If there is a difference between these numbers,
a signal circuit 11 is actuated to give an alarm signal.
