Autofocus bar code reader

Details Autofocus bar code reader Autofocus bar code reader

1998-08-25

2001-11-13

Hajec, Donald (Department: 2876)

Registers

Coded record sensors

Particular sensor structure

C235S462330

Reexamination Certificate

active

06315203

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a bar code reader, and more particularly to an autofocus bar code reader.
BACKGROUND OF THE INVENTION
Referring to
FIGS. 3 and 4
, a less preferred embodiment of an autofocus bar code reader, generally designated by the reference numeral
30
, is explained. A conveyer
31
transports an article
32
having attached thereto a bar code label
33
. A distance measuring section
9
measures distance
34
to the bar code label
33
and generates a distance information pulse signal
17
indicative of the measured distance. The distance information pulse signal
17
is fed to a is focal position controlling section
8
. A laser beam generating section
35
generates circular shape laser beam
16
. In response to the distance information pulse signal
17
, the focal position controlling section
8
conducts a focal position control so that the circular shape laser beam
16
will have its focal position on the bar code label
33
. The circular shape laser beam that has been subjected to the focal position control is used as a scan laser beam
18
upon lighting the bar code label
33
. Reflected light
19
by the bar code label
33
is intercepted and then decoded to obtain information on the bar code label
33
. The distance measuring section
9
determines the distance
34
to the bar code label
33
by calculation after inputting result of detection of height of the article
32
by a multi-optical-axis sensor. The multi-optical-axis sensor includes a light emitter
36
and a light interceptor
37
between which a number of parallel optical beams extend. The light interceptor
37
detects the optical beams, in number, which are interrupted by the article
32
and provides the output indicative of the detected result to the distance measuring section
9
.
In the bar code reader of the above kind, the diameter of the scan laser beam
18
is determined in accordance with the width of the narrowest bar of the bar code label
33
. If the laser beam diameter is too large as compared to the width of the narrowest bar, a reduction in resolution occurs, making it impossible to detect the width of each bar with good accuracy, causing a drop in read performance. If the laser beam diameter is too small as compared to the width of the the narrowest bar, there is the tendency to detect fine defects and/or small dirty spots on a bar code label, causing a drop in read performance. It is therefore generally known to adjust the diameter of the scan laser beam upon being intercepted by a bar code label on its face as large as the width of the narrowest bar of the bar code label. Read operation under this condition may be called read operation with the optimized beam diameter. As shown in
FIG. 2
, the diameter
40
of a laser beam
41
is the smallest at its focal position
42
. Therefore, it is the common practice to adjust the beam diameter equal to the above-mentioned optimized beam diameter at the focal position
42
that lies on the face of a bar code label to be read.
The preceding description explains why the distance to a bar code label is measured in laying the focus of laser beam on the face of the bar code label upon reading the bar code label in the case where the distance is subject to variations.
In order to maintain high read performance of a bar code reader, it is demanded to eliminate or at least reduce deviation from the optimized beam diameter.
Referring to
FIG. 4
, the laser beam generating section
35
is further described to clarify what causes the deviation from the optimized beam diameter. The laser beam generating section
35
includes a semiconductor laser diode
1
, a collimating lens
2
, a cylindrical lens a
3
, and a cylindrical lens b
4
, and generates a shaped laser means in the form of a circular shape laser beam
16
. It has been found that variation in the ambient temperature of the laser beam generating section
35
causes variation in the focal position of the circular shape laser beam
16
, thus inducing occurrence of the above-mentioned deviation. As is readily seen from
FIG. 2
, any deviation from the focal position
42
causes the laser beam diameter
40
to increase, causing a drop in read performance.
This phenomenon is observed in a bar code reader that employs a semiconductor laser diode as a source of laser beam when the ambient temperature is subjected to variation. As shown in
FIG. 4
, the semiconductor laser diode
1
generates an elliptical shape divergent laser beam
14
having different angles of divergence
50
in different directions. The maximum value of the angles of divergence
50
is around 60 degrees. The collimating lens
2
converts the elliptical shape divergent laser beam
14
to a collimating laser beam
15
.
Distance a between the semiconductor laser diode
1
and the collimating lens
2
may be increased by using, as the collimating lens
2
, a lens with increased aperture. The use of such lens result in an increase in accommodation space for the collimating lens
2
and an increase in manufacturing cost. The semiconductor laser diode
1
generates the elliptical divergent laser beam at a low output level. Thus, it is necessary to collect the entire laser beam within the angles of divergent
50
to provide the shaped laser beam at a sufficiently high output. This is the reason why the lens with a large aperture is required to collect all of the beams emitted by the semiconductor laser diode
1
. The collimating laser beam
15
still has an elliptical cross sectional profile. Thus, two cylindrical lenses
3
and
4
are provided to convert the collimating laser beam
15
to the circular shape laser beam
16
with a circular cross sectional profile. The two cylindrical lenses
3
and
4
have a short focal distance. Two cylindrical lenses with a long focal distance may be used. In this case, an increase in accommodation space for the cylindrical lenses
3
and
4
and an increase in manufacturing cost result.
The semiconductor laser diode
1
and the lenses
2
,
3
and
4
are mounted to and assembled with a casing made of aluminum. An increase in the ambient temperature causes thermal expansion of the casing. This expansion causes a change in distance between the semiconductor laser diode
1
and the collimating lens
2
, a change in distance between the collimating lens
2
and the cylindrical lens a
3
, and a change in distance between the cylindrical lens a
3
and the cylindrical lens b
4
. In the case where the lenses with a short focal distance are used, these changes cause a substantial deviation of the focal distance of the circular shape laser beam
16
from the distance to the bar code label.
This phenomenon may be explained by the fact that, in a formula for a single lens with a focal distance f, (1/a)+(1/b)=1/f, a small change in the variable a causes a great change in the variable b. In
FIG. 4
, the collimating lens
2
collimates the elliptical shape divergent laser beam
14
. This may be expressed by the formula after substituting the variable a with the distance a and the variable b with infinite. The collimating laser beam has its focal position spaced by infinite distance. Thus, the variable b in the formula is infinite. In this case, the term 1/b in the formula is zero to give the relation that a=f.
FIG. 5
shows the results of calculation of the variables f, a and b. As readily seen from
FIG. 5
, the shorter the focal distance f is, the higher is the rate of reduction in the variable b with respect to an increase in the variable a. In other words, with the same increase in the variable a, the rate at which collimating light converges increases as the focal distance f of a lens decreases. This means that the laser beam
15
, which is to be collimated, tends to converge in response to an increase in the distance between the semiconductor laser diode
1
and the collimating lens
2
. Further, the shorter the focal distance of the collimating lens
2
is, the higher is the rate at which the focal position of the collimating laser beam
15

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