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United States Patent |
5,202,569
|
Nakazawa
|
April 13, 1993
|
Symbol reading device for varying the focal point of a scanning laser
beam through variance of scanning laser beam optical path length
Abstract
In a symbol reading device, at least two laser beams having different
optical path lengths, and, therefore, different focal positions are used
to scan a symbol surface. The two laser beams are produced from a single
beam issuing unit, and only one beam at a time scans the symbol surface.
Furthermore, stationary reflecting mirrors are used to create the two
laser beams of different focal length, reducing the number of moving
mechanisms and the overall complexity of the device.
Inventors:
|
Nakazawa; Atsushi (Osaka, JP)
|
Assignee:
|
Sumitomo Electric Industries, Ltd. (Osaka, JP)
|
Appl. No.:
|
884140 |
Filed:
|
May 18, 1992 |
Intern'l Class: |
G06K 007/10 |
Field of Search: |
250/234,235,236,566
358/206
235/467
359/216,217,218,219
|
References Cited
U.S. Patent Documents
4101193 | Jul., 1978 | Waterworth et al. | 359/217.
|
Foreign Patent Documents |
59-038721 | Mar., 1984 | JP | 359/218.
|
1-304589 | Dec., 1989 | JP.
| |
2-7182 | Jan., 1990 | JP.
| |
2133891 | May., 1990 | JP.
| |
Primary Examiner: Nelms; David C.
Assistant Examiner: Shami; K.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
I claim:
1. A symbol reading device for reading symbols on a target surface
comprising:
a housing having an aperture;
a laser beam issuing means for emitting a reading laser beam;
scanning means for scanning a laser beam through a range of angles wider
than those encompassing said aperture;
optical means disposed between said laser beam issuing means and said
scanning means for splitting said reading laser beam into at least a first
and second laser beam having first and second optical paths respectively,
said first and second laser beams being incident on said scanning means
from different directions to prevent simultaneous emergence of said first
and second laser beams through said aperture, said first and second
optical paths having different lengths as measured from said laser beam
issuing means to said scanning means;
light receiving means for receiving a laser beam which has emerged through
said aperture and been reflected off of said target surface; and
processing means for identifying symbols on said target surface based on
output from said light receiving means.
2. A symbol reading device which has a housing with an aperture, a laser
beam issuing means for emitting a reading laser beam which emerges through
said aperture, a light receiving means for receiving a laser beam which
has emerged through said aperture and been reflected off of a target
surface, and a processing means for identifying symbols on said target
surface based on output from said light receiving means for reading
symbols on said target surface, said symbol reading device comprising:
scanning means for scanning a laser beam through a range of angles wider
than those encompassing said aperture; and
optical means disposed between said laser beam issuing means and said
scanning means for splitting said reading laser beam into at least a first
and second laser beams having first and second optical paths respectively,
said first and second laser beams being incident on said scanning means
from different directions to prevent simultaneous emergence of said first
and second laser beams through said aperture, said first and second
optical paths having different lengths as measured from said laser beam
issuing means to said scanning means.
3. A symbol reading device as in claim 2, wherein said laser beam issuing
means comprises a semiconductor laser.
4. A symbol reading device as in claim 2, wherein said scanning means
comprises a polygonal mirror.
5. A symbol reading device as in claim 2, wherein said optical means
comprises a half mirror for splitting said laser beam into said first and
second laser beams.
6. A symbol reading device as in claim 5, wherein said first optical path
leads directly from said half mirror to said scanning means.
7. A symbol reading device as in claim 5, wherein said optical means
further comprises at least two reflecting mirrors for directing said
second laser beam along said second optical path.
8. A symbol reading device as in claim 4, wherein said polygonal mirror has
at least two reflecting faces that are inclined at different angles with
respect to a longitudinal axis of said polygonal mirror for allowing a
laser beam to be scanned in at least two parallel directions.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a symbol reading device such as a bar code
reader or an optical character reader (OCR) which optically reads bar
codes, characters and other symbols formed on the surface of various
objects.
2. Description of the Related Art
Because of their ability to read characters and symbols on the surface of
various objects without actual physical contact with those objects, bar
code and optical character readers are enjoying extensive use. These
symbol reading devices are used as both stationary and hand held symbol
readers. It is desirable that the distance between a symbol or bar code
carrying surface and the symbol reading device over which symbols or bar
codes can be read (i.e., the reading distance) have a fairly broad range
as defined by upper and lower limits (i.e., the reading range).
Conventional symbol reading devices scan symbols such as bar codes using a
laser beam and receive reflected light from the symbol faces with a light
receiving device. An output of the light receiving device represents the
intensity of the reflected light. For example, a bar code consisting of
black bars and white spaces will reflect light which causes the light
receiving device to produce a small signal for bars and a large signal for
spaces. Therefore, after suitable processing (i.e., amplification), the
output of the light receiving device may be discriminated with reference
to a proper slice level, thereby producing a binary signal in association
with the scanned bar code. The symbol reading apparatus then identifies
the scanned bar code based on the binary signal.
An He-Ne laser has commonly been used as a light source for generating a
laser beam, but in recent symbol reading devices, the use of a
semi-conductor laser has increased with a view to reducing the overall
size and weight of the symbol reading device. However, laser light issuing
from a semi-conductor laser is diffusive and usually requires focusing
with a lens to produce the nearly parallel rays of a laser beam. Because a
bar width of 0.2 mm and less is by no means rare in bar codes, the
scanning laser beam must be focused to a spot diameter of 0.2 mm and less
in order to enable identification of such small bar widths. Due to these
circumstances, semiconductor laser beams are not completely collimated,
but are convergent with a focal point at a specific distance.
High resolution symbol reading is possible near focal points since the
diameter of the beam spot is quite small. However, at positions remote
from the focal point only low resolution symbol reading can be achieved.
Thus, conventional symbol reading devices employing a semi-conductive
laser have unacceptably narrow reading ranges.
A first prior art technique directed to this problem is described in
Unexamined Published Japanese Patent Applications No. 304589/1989 and No.
7182/1990. These documents propose the expansion of the reading range by
mechanically moving the laser beam light source, lenses and other optical
components to vary the optical path length of a scanning laser beam. Thus,
the distance between the symbol reading device and the focal point
location is variable.
The basic layout of the device disclosed in Unexamined Published Japanese
Patent Application No. 7182/1990 is shown in FIGS. 1A and 1B. As shown,
the light from a semi-conductor laser light source 201 is condensed by a
condensing lens 202 to form a laser beam 203. The laser beam 203 has a
beam waist BW in the focal position at distance FL which is determined by
the relative positions of the semiconductor laser light source 201 and the
condenser lens 202. If a symbol such as a bar code is read out at the
position of the beam waist BW, reading with maximum resolution can be
accomplished.
In the technique depicted in FIGS. 1A and 1B, the condenser lens 202 is
displaceable in direction 204 along an optical axis of laser beam 203;
wherein the distance FL from the semiconductor laser light source 1 to the
focal position, beam waist BW, can be shortened. Since the position of
beam waist BW can be varied, symbols can be read with high resolution over
a broad range of reading distances.
The complexity of the structural mechanisms necessary to perform the
mechanical movement of the laser light source, lenses and other optical
components in this first technique causes an increase in manufacturing
cost. Additionally, the increase in the number of moving parts leads to
lower reliability. Specifically, with reference to the device shown in
FIGS. 1A and 1B, when the condenser lens 202 is brought close to
semi-conductor laser light source 1 as shown in FIG. 1B, the angle .alpha.
subtended at a semiconductor light source 1 by the condenser lens 202 will
increase. This increase causes the spot diameter at beam waist BW to also
increase. In other words, shown in FIG. 2, the spot diameter at the beam
waist BW increases in substantial proportion to the distance FL. As a
result, symbols which are a remote distance from the symbol reading
apparatus cannot be read with high resolution.
A second prior art technique that also successfully solved the
aforementioned problem is described in Unexamined Published Japanese
Patent Application No. 133891/1990. The apparatus incorporating this prior
art technique has a plurality of beam issuing units. Each beam issuing
unit is composed of a semiconductor laser and a lens, and are set to have
focal points at different reading distances. The beam issuing units are
selectively operated in accordance with a particular reading distance or a
selected reading distance so that symbols can be read with high resolution
over a broad reading range.
This second prior art technique involves the use of multiple semiconductor
laser light sources and lenses which causes the cost of the symbol reading
apparatus to increase. Additionally, there is the problem that a
complicated optical layout is required to achieve registry among the
optical paths of the laser beams issued from the plurality of beam issuing
units.
A third prior art technique that has successfully solved the aforementioned
problem is shown schematically in FIG. 3. In FIG. 3, a beam issuing unit
211 emits a laser beam 212, which has its optical path switched back by a
reflector mirror 213A or 213B. The reflected laser beam 212 is again
reflected by a polygonal mirror 215. Polygonal mirror 215 is rotated at a
constant speed in the direction of arrow 214, and guides laser beam 212
towards a symbol face 216 carrying a bar code to be read. As the polygonal
mirror 215 rotates, the direction in which the laser beam 212 travels will
change, scanning the symbol face 216 automatically.
The reflector mirror 213B, which is closer to the beam issuing unit 211, is
inserted into or retracted from the optical path of laser beam 212 by a
drive mechanism (not shown). When inserted, the laser beam 212 is
reflected by the mirror 213B, and when retracted laser beam 212 is
reflected by mirror 213A. As a result, the optical path length of laser
beam 212 is changed, and the distance from the symbol reading device to
the focal position where the beam waist of the laser beam is formed, can
be varied between two values. Possible modifications include the addition
of reflector mirrors between beam issuing unit 211 and reflector mirror
213A. Alternatively, a single mirror can be displaced between reflector
mirror 213A and the position of reflector mirror 213B. This symbol reading
device permits the beam waist of the laser beam to be formed at varying
positions thereby accomplishing high resolution reading of symbols at
various distances from the symbol reading device.
The problem with the third prior art technique is that a driving mechanism
is required for driving each reflector mirror positioned between beam
issuing unit 211 and the reflector mirror 213A. As a result, the number of
moving parts increases as the reading range of the symbol reading device
increases which causes both an increase in cost and a decrease in
reliability. In the modified symbol reading device using a single
reflector mirror displaced continuously along the optical path of laser
beam 212 between the beam issuing unit 211 and reflector mirror 213A, the
long operating distance of the single reflector mirror impairs response of
the system.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a symbol reading device
with a broad reading range which has simple structure, high reliability,
and low manufacturing costs.
In a first embodiment of the present invention, at least two laser beams
having different optical path lengths; and, therefore, different focal
positions are used to scan a symbol surface. The two laser beams are
produced from a single beam issuing unit, and only one beam at a time
scans the symbol surface. Furthermore, stationary reflecting mirrors are
used to create the two laser beams of different focal length, reducing the
number of moving mechanisms and the overall complexity of the device.
Specifically, the symbol reading device of this embodiment has a housing
with an aperture; laser beam issuing unit for emitting a reading laser
beam; a scanning device (generally a polygonal mirror) for scanning a
laser beam through a range of angles wider than those encompassing the
aperture; optical components disposed between the laser beam issuing unit
and the scanning device for splitting the reading laser beam into at least
a first and second laser beam having first and second optical paths,
respectively, the first and second laser beams incident on the scanning
device from different directions to prevent simultaneous emergence of the
first and second laser beams through the aperture, the first and second
optical paths having different lengths as measured from the laser beam
issuing unit to the scanning device; light receiving device for receiving
a laser beam which has emerged through the aperture and been reflected off
of a target surface; and, a processing unit for identifying symbols on the
target surface based on output from the light receiving device.
Other objects, features, and characteristics of the present invention;
methods, operation, and functions of the related elements of the
structure; combination of parts; and economies of manufacture will become
apparent from the following detailed description of the preferred
embodiments and accompanying drawings, all of which form a part of this
specification, wherein like reference numerals designate corresponding
parts in the various figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B show the basic layout of a prior art symbol reading device.
FIG. 2 shows the relationship between the distance to the focal point of a
laser beam and the spot diameter of the beam waist of the laser beam, as
observed with the symbol reading device of FIGS. 1A and 1B.
FIG. 3 is a schematic diagram of a prior art symbol reading device.
FIG. 4 is a plan view showing the basic layout of a bar code reader which
is an embodiment of the symbol reading device of the present invention.
FIG. 5 is a diagram showing schematically the relationship between the
scanning angle and the width of the aperture in the housing.
FIGS. 6-8 are plan views schematically showing the sequence of operations
of the bar code reader shown in FIG. 4.
FIGS. 9A and 9B show the focal positions of laser beams in two different
optical paths.
FIGS. 10A and 10B are side views of a polygonal mirror for modifying the
embodiments of the present invention.
FIG. 11 is a diagram showing how a bar code is scanned with laser beams
reflected from the polygonal mirror of FIGS. 10A and 10B.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Several embodiments of the present invention are described below with
reference to the accompanying drawings.
FIG. 4 is a plan view showing the basic layout of a bar code reader which
is an embodiment of the symbol reading device of the present invention.
With reference to FIG. 4, the bar code reader comprises a housing 1 that
has an aperture 11, and a laser beam issuing unit 2 that issues
semiconductor laser light and is accommodated in the housing 1, a
polygonal mirror 3 that provides a rotary laser beam scanning device for
scanning a target symbol surface, and an optical unit 4 interposed between
the laser beam issuing unit 2 and the polygonal mirror 3 for splitting the
laser beam from the laser beam issuing unit 2 into a first and second
laser beam having a first and second optical path, L1 and L2,
respectively. The optical unit 4 guides the first and second laser beams
so that they are incident on the polygonal mirror 3 via different optical
path lengths and different directions.
The housing 1 also accommodates a motor (not shown) for rotating the
polygonal mirror 3 clockwise as shown by the arrow in FIG. 4; a condenser
lens 7 for condensing light reflected by code carrying surface 5 which
passes through light-receiving aperture 12; a light-receiving device 8
such as a photodiode that receives the reflected light condensed by the
condenser lens 7; and a control unit 9 that performs a waveform shaping
and binarizing operation on an output signal from the light-receiving
device 8 to obtain a binary signal corresponding to the code carrying
surface 5, and that identifies the contents of a bar code 6 on the basis
of that signal.
The housing 1 may be formed of a metal or a resin. The housing 1 may be
either a fixed or a hand-held type.
Aperture 11 is provided for permitting laser beams to emerge out of the
housing 1. The aperture 11 may be in a grid pattern for restricting the
emergence of laser beams out of the housing 1. The aperture 11 only needs
to allow the emergence of laser beams; therefore, the aperture 11 may be
covered with glass or a filter that attenuates the transmission of light
at wavelengths other than that of desired laser beams.
The polygonal mirror 3 has six reflecting faces 3a on its periphery that
extend parallel to the longitudinal axis of rotation 31. The reflecting
faces 3a are equidistant from the longitudinal axis of rotation 31 and any
two adjacent reflecting faces 3a form an angle of 120.degree.. Hence, a
laser beam incident on the polygonal mirror 3 is scanned 120.degree. as
the polygonal mirror 3 rotates. More specifically, with reference to FIGS.
4 and 5, an incident laser beam is scanned through 60.degree. to both the
left and right symmetrically with respect to a centerline m connecting the
center of the aperture 11 and the longitudinal axis of rotation 31.
The distance between the aperture 11 and the polygonal mirror 3 and the
width of the aperture 11 are set so that a laser beam emerging through the
aperture 11 will spread through angles that assume the central 40.degree.
range of the above-defined scanning angle of 120.degree.. More
specifically, with reference to FIG. 5, the emerging laser beam emerges
through the aperture 11 over the angular range of 20.degree. to both the
left and right symmetrically with respect to the centerline m.
The laser beam issuing unit 2 comprises a laser beam oscillating device 21
that outputs semiconductor laser light or helium-neon laser light, a lens
22 that converges the output laser light from the laser beam oscillating
device 21, and a diaphragm stop (not shown). The settings of the lens 22
and the diaphragm stop are such that the spot diameter of the laser beam
will be the smallest at a predetermined distance (typically ca. 200-300
mm) from the bar code reader.
The optical unit 4 comprises mirrors 41, 42, 43 and 44. Mirror 41 is a
half-mirror by which the laser beam from the laser beam issuing unit 2 is
split into the first and second laser beams having optical path L1 (as
indicated by a solid line in FIGS. 4 and 6-8) and an optical path L2 (as
indicated by a dashed line in FIGS. 4 and 6-8, respectively). The other
mirrors 42, 43 and 44 are disposed sequentially in the optical path L2.
Optical path L1 is such that the first laser beam is incident on the
polygonal mirror 3 through the half-mirror 41 in a direction that forms an
angle of 30.degree. to the right with respect to the centerline m as
viewed in FIG. 4. Optical path L2 is such that the second laser beam is
incident on the polygonal mirror 3 via the mirrors 41-44 in a direction
that forms an angle of 30.degree. to the left with respect to the
centerline m as viewed in FIG. 4 (i.e., symmetrical to optical path L1
with respect to the centerline m).
The operation of the bar code reader under discussion is described below
with reference to FIGS. 4, 6 and 7.
FIG. 6 shows the case where the reflecting face 3a of the polygonal mirror
3 is inclined so that the line p, normal to the reflecting face 3a which
passes through the longitudinal axis of rotation 31, forms of an angle
.theta. of -25.degree. with respect to the centerline m, the minus sign
indicating a direction reverse to the direction of the rotation of the
polygonal mirror 3 (whereas an angle, etc. in the polygonal mirror 3 the
direction of rotation is indicated by a plus sign). In FIG. 6 the second
laser beam travelling in the optical path L2 is incident on the reflecting
face 3a at an angle .beta. of -5.degree. with respect to the line r normal
to reflecting face 3a and, hence, will be reflected at the same angle of
+5.degree. with respect to the normal r. In other words, the second laser
beam will be reflected from the polygonal mirror 3 at an angle of
-20.degree. with respect to the centerline m, and will emerge out of the
housing 1 through the aperture 11 at left end 11a to reach the code
carrying face 5. Also in FIG. 6, the first laser beam travelling in the
optical path L1 is incident on the reflecting face 3a at an angle of
55.degree. with respect to the normal r and, hence, will be reflected at
an angle of -80.degree. with respect to the centerline m and not emerge
out of the housing 1.
As the polygonal mirror 3 shown in FIG. 6 rotates clockwise, the second
laser beam in the optical path L2 is scanned across the aperture 11 from
the left end 11a towards the right end 11b. As the polygonal mirror 3
further rotates, it passes through the stage shown in FIG. 7, and then the
angle .theta. that normal p forms with the centerline m becomes -5.degree.
as shown in FIG. 7. With .theta. equal to -5.degree., the angle .beta.
becomes -25.degree., causing the second laser beam in the optical path L2
to emerge out of the housing 1 through the aperture 11 in the neighborhood
of the right end 11b. The first laser beam in the optical path L1 is
reflected at an angle of -40.degree. with respect to the centerline m and
will not emerge out of the housing 1.
As the polygonal mirror 3 shown in FIG. 7 further rotates, the second laser
beam in the optical path L2 will no longer emerge out of the housing 1
through the aperture 11. When the polygonal mirror 3 rotates so that the
angle .theta. that normal p forms with the centerline m becomes
+10.degree., as shown in FIG. 8, the first laser beam in the optical path
L1 will be incident on the reflecting face 3a at an angle n of +25.degree.
with respect to the normal r. The reflected first laser beam will emerge
out of the housing 1 through the aperture 11 in the neighborhood of the
left end 11a. As the polygonal mirror 3 further rotates, the first laser
beam will be scanned towards the right end 11b of the aperture 11.
The above-described procedure is repeated for each of the reflecting faces
3a on the rotating polygonal mirror 3. It should be noted here that the
optical path length from the laser beam issuing unit 2 to the polygonal
mirror 3 differs between optical paths L1 and L2. The different optical
path lengths occur within the housing 1 and, hence, the distance between
the focal point e of the first laser beam in the optical path L1 and the
housing 1 will differ from the distance between the focal point f of the
second laser beam in the optical path L2 and the housing 1 as shown in
FIGS. 9A and 9B. Stated more specifically, the focal point e for the case
shown in FIG. 9B, which is associated with the optical path L1, is more
remote from the housing 1 than the focal point f for the case shown in
FIG. 9A, which is associated with the optical path L2. Since the overall
reading range Q of the bar code reader is the sum of the reading range Q1
provided by the first laser beam in the optical path L1 and the reading
range Q2 provided by the second laser beam in the optical path L2, the
symbol reading apparatus of the present invention will have a broad
reading range. It should also be noted that the distance between the focal
positions e and f corresponds to the optical path length difference
between the two optical paths L1 and L2.
As described on the foregoing pages, the first and second laser beams in
the respective optical paths L1 and L2 as split by the optical unit 4 are
made to differ in optical path length, so that they can be focused at
different positions in the direction of the depth of reading. Thus, the
region that can be scanned with a small beam spot is increased to provide
a broader reading range. In addition, the length of optical paths L1 and
L2 are changed by the optical unit 4, and not by any mechanical movement
of a laser beam issuing unit and/or a laser beam scanning device. This
obviates the need to provide mechanical structure for moving a laser beam
issuing unit and scanning device relative to each other. As a result, the
overall construction of the reading apparatus is simplified, improving the
reliability of the symbol reading device and reducing its manufacturing
cost.
If desired, the optical unit 4 may include two or more half-mirrors so as
to split the laser beam into three or more laser beams having different
length optical paths. It is also possible to adopt other structures such
as one that uses a prism mechanism as an optical unit.
For example, in FIGS. 10A and 10B, the polygonal mirror 3 is provided with
at least two reflecting faces 3a and 3b that are inclined at different
angles with respect to the longitudinal axis of rotation 31. Therefore, a
laser beam is scanned in two parallel directions C and D. An advantage of
this embodiment is that if the bar code 6 contains an illegible portion
6a, due to a smudge that occurred during printing (see FIG. 11), reading
performance can be improved because one of the scanning directions C and D
is generally scanned over the legible areas.
The foregoing embodiments assume that the symbol reading device of the
present invention is applied to a bar code reader. It should be noted that
the present invention is also applicable to other optical symbol readers
such as an optical character reader.
While the invention has been described in connection with what is presently
considered the most practical and preferred embodiments, it is to be
understood that the invention is not limited to the disclosed embodiments;
but, on the contrary, is intended to cover various modifications and
equivalent arrangements included within the spirit and scope of the
appended claims.
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