Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a liquid-jet printer
for franking and value-stamping machines that generates script
and graphic characters and symbols on a receiving surface by
ejecting droplets from nozzles, these nozzles and the associated
control means being held in a guide plate of a print head, and
arranged at an angle to the direction of movement of the receiv-
ing surface.
It is known that spray nozzles used in printers can be
installed in a fixed print head in such a way that these are
either parallel to each other or such that their longitudinal
axes intersect at a point in the print plane (DE-30 04 530 Al).
In order to achieve a close nozzle raster pattern,
other ink mosaic printers have the nozzles installed with their
tips close together. When this is done, the number of nozzles,
combined with a plurality of ink reservoirs, permits variegated
colouring of the print image (DE-34 20 921 Al).
In li~uid-jet printers of this kind, precise adjustment
of the print heads is essential in order to ensure a pri.nt image
that is always faultless. In addition, it is necessary to use a
receiving surface that is guided and of a standard thickness so
that there is always an e~ual distance between the receiving
surface and the print head or the outlet orifices of the spray
nozzles, respectively.
When such printers are used in franking machines, one
encounters the problem that the receiving surfaces are of
different thicknesses, and thus the distance between the print
nozzles and the receiving surface has to be determined anew on
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each occasion and the necessary adaptations made.
It is an object of the present invention to create a
liquid-jet printer that always ensures the same distance, even
in the case of receiving surfaces that are of different
thicknesses, and that ensures a differentiated, close print
raster by the arrangement of the nozzles within the print head.
There~or~ the present invention seeks to provide a
liquid-jet printer for franking and value-stamping machines that
generates script and graphic characters and symbols on a
receiving surface by ejecting droplets from nozzles, these
nozzles and the associated control means being held in a guide
plate of a print head, and arranged at an angle to the direction
of movement of the receiving surface, characterized by a concave
arrangement of the nozzles in the print head relative to the
receiving surface, the liquid jets from the individual nozzles
lntersecting at a fictive focal point beneath the receiving
surface, a distance measurlng system that is associated with the
print head measuring the distance between the nozzles and the
receiving surface and serving to compensate for different
distances.
The sub-claims describe additional, advantageous
measures according to the present invention.
The advantage of arranging the nozzles on a concave
curve relative to and with a fictive focal point of the liquid-
jets beneath the receiving surface is seen in enhanced resolution
of the print image as it corresponds to the interval between the
nozzles in the print head.
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27260-18
The present invention will be described in greater
detail below on the basis of the drawings appended hereto. These
drawings show the following:
Figure 1: The construction, in principle, of a print
head;
Figure 2: A schematic diagram showing the distance-
measuring system.
In these drawings, identical parts bear the same refer-
ence numbers.
The nozzles 10 are held in the guide plate 20. In the
example shown in figure 1, the guide plate 20 is configured so
as to be concave. Instead of a guide plate of concave shape,
this can also be planar. In this case, the outlet openings of
the nozzles 10 are arranged concavely relative to the receiving
surface 30. The nozzles 10 are so orientated in their position
relative to the receiving surface 30 that the fictive focal
point 50 of all the liquid-jets 40 lies~below the receiving
surface 30. The printing width is determined by the interval
between the edge jets 41, ~2 at the level of the receiving
surface 30. Because of the position of the fictive focal point
50 beneath the receiving surface 30, high resolution of the
character is achieved despite the relativel~ wide interval
between nozzles 10 that are adjacent to each other.;
Be~ause the liquid jet printer is used for fran~ing
and value-stamping machines, special measures are needed to
ensure a consistent print image on various receiving surfaces
30, for such receiving surfaces 30 can be of different
thicknesses. As an example, letters can be of different
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thicknesses as a result of the number of pages that they contain
and/or because of the thickness of the paper used for the
envelopes. Pre-printed cheques that pass through value imprinting
machines can be of paper of different thicknesses. A consistent
print image cannot be achieved with a fi~ed arrangement of the
print head and the surface for the receiving surface, relative
to each other.
For this reason, the printer system according to the
present invention has an associated distance measuring system 60,
70. This distance measuring system 60, 70 incorporates an
associated control system that ensures a uniform distance
between the guide plate 20 and the receiving surface 30 by
raising or lowering either the print head or the base for the
receiving surface.
In order to determine the distance, a process that
permits a high level of resolution in the range of tenths of a
millimeter is needed. The distance measuring system 60, 70 is
arranged within the print head and is preferably integrated into
the guide plate 20. Distance sensors that operate on the echo-
sounding principle, as well as those that are based onreflection of optical signals can be used as measuring processes.
A preferred embodiment is described in greater detail
below. A laser 60, e.g., a CO2 laser, which is directed onto the
receiving surface 30 at an acute angle, generates a focussed
laser beam that strikes the surface of the receiving surface 30
at angle a and generates a patch of light A. Some of the
scattered laser beam passes along the line AL. An image B of the
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light patch A is formed within the plane of focus oE the imaging
lens (L)80, which is of focal length f. If the scattering
surface of the receiving surface 30 is displaced vertically,
for example by a thinner envelope, this will generate the point
of impact A' and the image point B'. The distance BB' in the
focal plane of the imaging lens 80 is a measurement for the
vertical displacement d of the surface of the receiving surface
30:
f*d
: BB' = ----------------
(AL - f) cos (~)
A system of several opto-electronic detectors is
arranged in the focal plane of the imaging lens 80 in order to
record the distance BB'. Monolithic integrated CCD (charge
coupled device) line sensors can be:used advantageously for
this purpose.
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