Note: Descriptions are shown in the official language in which they were submitted.
CA 02256237 1998-12-16
ASCOP037US
Be it known that we, Manfred Ludi and Christian Moy, have invented an
improvement in
Measurement of Paper Speed Using Laser Speckle Detection
The invention relates generally to printing on mail pieces, and relates more
particularly to
noncontact printing on moving mail pieces, in which the motion is detected in
a noncontact
fashion, preferably by detecting the speckle pattern on a mail piece when
illuminated by a
coherent, monochromatic beam of light such as a laser.
Background of the invention
Many decades of experience have permitted development of highly reliable,
sturdy, and
inexpensive postage meters (franking machines). These include the many models
of postage
meter developed by the same assignee as the assignee of the present invention,
such as the
Smile series of meters.
Notwithstanding the high reliability, sturdiness, and inexpensiveness of such
postage meters,
some postal authorities have suggested that such meters be discontinued from
use and that
postage indicia be printed instead by means of digitally contro(led printers,
typically employing
off-the-shelf inks and printing devices. While at first blush it might seem
that such printing
methods would be subject to fraud of many different types, it has been
suggested that this
great risk of fraud might be protected against by placing a machine-readable
entity such as a
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CA 02256237 1998-12-16
high-resolution two-dimensional bar code in the digitally printed indicium.
The bar code
would contain information that has been cryptographically processed to permit
authentication
of the bar code, and thus of the legitimacy of the postal indicium.
A critical service decision of the postal authority is what to do about mail
pieces that have
postal indicia for which the bar code cannot be read completely enough to
pernut its
authentication. One choice is to mark all such mail "return to sender", a
choice which would
be extremely undesirable if the result were the return of an appreciable
number of mail pieces
which were, in fact, authentic despite having bar codes that, for some reason,
cannot be read
in sufficient detail to permit authentication.
The other possibility is to deliver mail pieces for which the bar code is not
sufficiently
readable, rather than to return them to the sender. If such a policy were
adopted and if it were
to become generally known, then many parties would be tempted to attempt to
obtain free
postal services by using conventional printers to print plausible-looking
postage indicia that
contained bar codes with intentionally unreadable bar codes.
In the face of these concerns, it is immediately apparent to those skilled in
the art that the bar
code must indeed be of high resolution, with very little dimensional
distortion. For the
designer of a postage printer, however, this is not an easy goal to achieve.
Even the slightest
inaccuracy in the measured position (and velocity) of a mail piece will result
in a printed
indicium that is stretched, or compressed, or otherwise distorted in the
dimension through
which the mail piece moves during printing.
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Those who are skilled in the art of postage printing will immediately
appreciate that printing
on mail pieces is more difficult than other printing tasks, for example
because mail pieces are
of varying thickness, are made of varying materials, and each piece tends to
be irregular in
thickness. It is, by comparison, quite easy to print on sheets of uniform
thickness, dimensions,
and materials. A further difficulty with printing on mail pieces is that they
often move with
nonuniform velocity and are introduced into the postage printer at irregular
and unpredictable
times.
A typical prior art way of measuring movement and position of a mail piece is
by means of
opposing resilient rollers in gripping contact with the mail piece; a resolver
or other position
transducer generates a signal indicative of movement of the mail piece. Such a
measurement
method has numerous drawbacks. For example, the pressured contact of the
rollers with the
mail piece can cause the mail piece to flex or otherwise move relative to the
paper path, thus
disturbing its position relative to the print head which is typically an ink-
jet print head.
Because an ink-jet print head requires precise positioning relative to its
target (here, a mail
piece), then the flexing and other movement caused by opposed rollers may
disturb the
printing.
Many other problems present themselves with contact-type movement sensing. For
example,
a mail piece of irregular thickness will give rise to inaccuracies in the
measured movement,
because the rollers ride up and down the irregularities of the mail piece.
For all these reasons, it is desirable to provide a reliable means of
measurement of movement
of a mail piece, preferably a means that does not require contact with the
mail piece.
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Summary of the invention
In a postage printing device according to the invention, a printer is employed
to print postage
indicia on mail pieces. The printer is preferably a noncontact printer such as
an ink-jet printer.
Printing occurs as the mail piece moves relative to the print head of the
printer, which requires
that reliable motion information (e.g. a print clock signal) be made available
to the electronics
driving the print head. The reliable motion is provided in a noncontact way,
preferably by
directing a laser beam toward the mail piece and detecting a moving speckle
patterli in the
light scattered from the mail piece. In this way, a precise measurement of
mail piece
movement is made which permits printing an accurate printed indicium on the
mail piece.
Description of the drawing
The invention will be described with respect to a drawing in several figures,
of which:
Fig. 1 shows a side view of a generalized paper path;
Fig. 2 shows a perspective view of a generalized paper path;
Fig. 3 shows a side view of a paper path according to one embodiment of the
invention; and
Fig. 4 shows a typical speckle pattern as seen at a detector.
Detailed description
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Fig. I shows a side view of a generalized paper path. A mail piece 30 moves
laterally (to the
right in Fig. 1) past a print head 31, typically an ink-jet print head. (Other
print technologies
may be employed without departing in any way from the invention.) Resilient
rollers 35, 36
are positioned opposite each other relative to the paper path of the mail
piece 30, and are
biased toward each other. A resolver or other position transducer, omitted for
clarity from
Fig. 1, may be mechanically coupled with one of the rollers so as to provide
an electrical signal
indicative of the lateral movement of the mail piece 30.
In some prior-art printing systems the print head 31 moves perpendicularly to
the movement
of the target 30, that is, in and out of the page in Fig. 1. With such a
printer, the usual design
decision is to hold the target 30 motionless when printing is happening, and
then to refrain
from printing during times when the target 30 is moving laterally, for example
to the right in
Fig. 1. Such a system leads to jerky movement of the target 30, rather than
continuous
movement thereof For liglitweight individual sheets of paper this is not
unacceptable and
indeed many commercially successful ink-jet printers employ just such a jerky
motion of paper
through the printer. Because the paper is motionless at the time of printing,
it is a relatively
easy niatter to determine when drops of ink should be fired to the paper,
because it is only
necessary to monitor the position of the print head as it moves across the
paper, and such
monitoring is not mechanically difficult. For example, a plastic tape may be
held along the
path of the print head, with an optosensor detecting stripes in the tape. The
detected stripes
are used to clock data to the firing actuators of the print head.
In some other prior-art printing systems the print head 31 is motionless and
the target 30
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moves relative to the print head 31 at the time that the print head 31 is
printing: In such a
case, a resolver or encoder is used to pick up and measure rotation of one of
the rollers 35,
36, and a signal derived from the resolver or encoder is used to clock data
to.the firing
actuators of the print head.
Fig. 2 shows a perspective view of a generalized paper path of a prior-art
printing system of
the type in which the print head moves perpendicularly to the movement of the
target. The
target 30 moves along its paper path as shown by arrow 33. The print head
moves along a
guide rail 32, for example in the direction indicated by arrow 34. The
movement of the target
30 stops and starts repeatedly, to permit printing by the print head 31 during
the stopped
intervals.
Fig. 3 shows a side view of a paper path according to one embodiment of the
invention. In
this embodiment, an ink-jet print head 31 is positioned relative to the paper
path of the mail
piece 30. Laser 37 is positioned relative to the paper path 38 and directs a
monochromatic,
coherent beam of light 39 toward the mail piece 30. Scattered light goes in
many directions
and gives rise to constructive and destructive interference, sensed in
direction 40 by sensor 41.
The assumption is that the mail piece has a surface that is not shiny, that
is, its roughness is
greater than the illuminating wavelength. In this case, the phenomenon of
speckles is
observed. The monochromatic laser light is diffusely reflected in all
directions in a random
way. This diffuse reflection causes phase shifts in the reflected light that
are highly dependent
on the surface structure; the result is a surface phenomenon.
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When an imaging device is used, it is seen that there are regions of
constructive and
destructive interference formed on the device. When a retina of a human eye is
employed as
the imaging device, the regions of constructive and destructive interference
are perceived as a
pattern of speckles. It is also noted that the speckle pattern moves if the
reflection surface
(here, a mail piece) moves. Movement of the speckle pattern is directly
correlated with
movement of the mail piece. Fig. 4 shows a typical speckle pattern as seen at
a detector.
In the system according to the invention, the speckle detection may be in a
single
photodetector, such as a photodiode or a phototransistor, or in any arbitrary
array thereof.
The photodetector used is not limited to a specific shape or size. CCD-type
sensors of any
shape may also be employed. With any of these detectors, a signal may be
derived that is
indicative of motion of the mail piece, and may be used to clock data to the
firing actuators of
the print head 31.
Another detection approach is simply to sense the light intensity at a
detection point. As
regions of constructive and destructive interference pass by the point, and
the detected signal
is a fast sequence of bright and dark events. This sequence may be evaluated
as a frequency.
A calibration may be performed that is a function of the geometrical setup of
the apparatus
and of the wavelength of light employed.
The laser 37 is preferably a laser diode. The laser diode may emit visible or
infrared light, as
desired.
While the invention has been described with respect to particular embodiments,
it is not limited
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CA 02256237 1998-12-16
thereto. For exarnple, the light source is shown as a laser but the benefits
of the invention
present themselves with any source of a monochromatic, coherent light beam.
Those skilled in
the art will have no difficulty appreciating that there are numerous obvious
variations which
nonetheless fall within the scope of the invention, as defined by the claims
that follow.
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