JPH0567227A - Optical mark reader - Google Patents

Abstract

PURPOSE:To provide a bar code reader which can easily correspond to the change of a read condition. CONSTITUTION:A control part 3 operates a lens system 2 so that the focus of a laser beam 7 is made in a position indicated by a focus position information 13. A height measurement part 6 detects the position of a bar code 15. A distance conversion part 5 converts the position of the bar code 15 into focus position information 13. At the time of conversion, a value adjusted to the read condition (the size of the bar code, for example) is added (or subtracted) and it is transmitted as focus position information 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical symbol reader, and more particularly to an autofocus type bar code reader using a laser beam.

[0002]

2. Description of the Related Art A bar code reader scans a bar code with a laser beam to determine the density of the bar code (presence or absence of bar).
It is a device for reading a barcode by detecting scattered reflection laser light modulated in accordance with. Therefore, the recognition performance of the bar code reader is greatly affected by the relationship between the spot diameter of the laser beam on the bar code and the bar thickness or the bar interval of the bar code. In other words, if the spot diameter is large, a bar adjacent to it will be detected when a certain bar should be detected. Also, if the spot diameter is too small, the resolution will be too good, and stains on the barcode and even small print omissions will be detected as information.

Therefore, the output system of the laser beam is designed so that an appropriate spot diameter can be obtained based on the size of the bar code to be detected.

[0004]

However, in recent years, there have been various types of barcodes, and barcodes of different types having different basic widths (width of the thinnest bar) have come out. The conventional bar code reader described above has a poor recognition rate for a bar code having a basic width different from the designed value.

Further, in the bar code reader for physical distribution, it is necessary to recognize the bar codes attached to the articles of various sizes during the transportation of the articles. In this case, there is a variable focus type bar code reader that measures the height of the bar code adhering surface during transportation and reads by focusing on that position (O pl.
USE E June 1990 issue, p.120-124 "Bar code reader" (Hiroyuki Miyazaki)).

With this bar code reader, there is no problem if the distance to the bar code can be accurately recognized and the focus can be focused on the bar code, but accurate distance measurement causes an increase in cost. Therefore, a simple distance detecting means is used. A simple detecting means generally obtains discrete distance information, and accurate distance recognition is impossible. An appropriate spot diameter cannot be obtained by focusing with the distance information including this error and irradiating the laser beam.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a bar code reader which can cope with the optimization of the spot diameter and a slight distance deviation so as to solve the above-mentioned problems.

Therefore, the bar code reader of the present invention is provided with means for optimizing the spot diameter on the bar code by focusing at a position farther or closer to the bar code to be read.

[0009]

The function of the laser beam is that the beam radius is the smallest at the focal point and spreads on both sides. The beam divergence is given by the following equation.

Ω (Z) = ω ₀ [1+ (λZ / πω ₀ ² ) ² ] ^1/2 (1) where ω (Z) represents a beam radius taken in a plane perpendicular to the optical axis. , Z denote coordinates taken in the optical axis direction. The origin is set at the beam waist (focus) position, and the beam radius at that position is ω ₀ . λ is the wavelength of the laser light. FIG. 2 shows three types of ω (Z) including ω ₀ .

Ω ₀ is a value set when the optical system is designed. If ω ₀ is set and λ is fixed, the beam divergence is determined to be one. When ω ₀ is decreased, the beam divergence, that is, the slope of the curve is increased, and when ω ₀ is increased, the beam divergence is decreased. Even if Z changes, if the change in beam diameter is small, stable reading is possible in a wide range, but the diameter of the beam waist must be increased in order to reduce the inclination of the curve. But,
Since it is necessary to determine an appropriate value for the beam diameter according to the basic width of the barcode to be read, it is necessary to set the slope of the curve in consideration of the diameter of the beam waist.

The optical system is designed with the beam diameter of the beam waist thus set. FIG. 3 shows the laser beam thus determined. The optimum ω ₀ is set so that the laser beam irradiates the bar code with the beam diameter 2ω ₀ at the focal point. If a barcode with a larger basic width is read with this laser beam, the resolution will be too high at the focal position. The beam spot diameter suitable for reading this large barcode is 2ω.
_If it is ₁ , this spot diameter is formed at a position away from the focal point by Z ₁ . The relationship between this ω ₀ and Z ₁ is given by the following equation.

Ω ₁ = ω ₀ [1+ (λZ ₁ / πω ₀ ² ) ² ] ^1/2 (2) Therefore, when reading this large bar code, the focus is
A laser beam having an appropriate spot diameter can be obtained by irradiating the laser beam with a shift of Z _{1 with} respect to the distance to the barcode.

Next, the case where the distance to the bar code is measured and the focus is determined according to the distance, particularly the case where the distance information is discrete information will be described.

It is assumed that the distance information is output according to FIG. In this case, the actual distance l is nS ≦ l <(n + 1)
In the case of S, the measured value is output as nS. That is, the distance that is actually continuous is output as a representative value with S as a unit. Therefore, if the focus is set based on this distance information, the optimum spot diameter is given when the actual distance is n.
Only in the case of S, the optimum spot diameter is not formed between (n + 1) S and nS. This (n + 1) S
So that even if the optimum spot diameter is not formed between n and nS, it does not affect the reading of the bar code so much that the beam diameter spreads so that S is included in the allowable range of the beam diameter, or Set the resolution for distance measurement. That is,
If the allowable range of the beam diameter is constant, the resolution of the distance measurement may be low when the spread of the beam diameter is small, and the resolution of the distance measurement must be high when the spread of the beam diameter is large. Further, as described above, the beam diameter at the beam waist is restricted by the basic width of the smallest bar code to be read. In addition, the above consideration must be added in order to deal with the above-mentioned bar codes having different basic widths.

An example will be described in which two sizes (small bar code and large bar code) of bar codes are recognized by determining the focal point by discrete distance measurement. The measurement system is shown in FIG.
And Small barcode recognizable spot radius ω _S
Is ω ₀ <ω <ω ₁ and the large barcode is ω ₂ <ω _l <ω ₃ , and when this is associated with the ω (Z) curve, it becomes as shown in FIG. 4. Therefore, the small barcode is ΔZ _S and the large barcode is ΔS
Defocus is allowed up to Z _l . That is, a beam in this range may be used. Next, how to use the beam in this range will be described with reference to FIG.

For large bar codes, the measured distance is nS.
, The actual distance is nS to (n + 1) S. Therefore, if the focus is on the point P, the beam l ₂ of ΔZ _l in FIG. 4 can be used in nS to (n + 1) S. That is, it is understood that the focus should be set at a position where ΔS is subtracted from the measured value.

On the other hand, in the case of a small bar code, the same consideration is made, and when the measured value is (n-1) S, the point (n-1) S is focused and used. (N-1) S for small barcodes
~ NS, the beam diameter is not used from ω ₀ to ω ₁ ,
This is because the expansion of the beam diameter gives priority to the large bar code and S.

[0019]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 shows an embodiment in which the present invention is applied to a bar code reader for reading a bar code attached to an article to be conveyed during conveyance.

The height measuring unit 6 measures the height of the bar code sticking surface of the article 10a carried on the conveyor 8 from the conveyor 8 and outputs height information 11. The distance conversion unit 5 inputs the height information 11 and the information 12 corresponding to the basic width of the barcode to be read, and outputs the information 13 indicating the focus position. The control unit 3 obtains the information 13 indicating the focal position and outputs the control signal 14 for controlling the lens system 2 so as to focus on the focal position.

The laser beam 7 output from the laser light source 1 passes through the lens system 2, the focal position is determined, the direction is changed by the polygon mirror 4, and the bar code affixed to the conveyed article 10 is attached. Irradiate 16. The polygon mirror 4 rotates in the direction of the arrow, which causes the laser beam 7 to rotate.
Scan over the bar code 16. The reflected and scattered light from the bar code is detected by the detecting means, but is omitted because it is not directly related to the present invention.

Next, the operation of the distance converter 5 will be described in detail. First, an instruction for the height measuring unit 6 to output continuous position information will be described. It is assumed that the barcode is read in two sizes, large and small, and the optimum spot diameters are ω ₀ and ω ₁ in FIG. 3 described in the section of action.

The optical system is designed for a small bar code. Therefore, when the information indicating the small barcode is given as the barcode information 12, the distance conversion unit 5 directly converts the height information 11 into the focus position information 13 and sends it to the control unit 3. Since the control unit 3 forms a focus at the position given by this focus position information 13, the bar code 16 is irradiated at the focus of the beam.

Next, when information indicating a large bar code is given as the bar code information 12, the distance conversion unit 5 adds Z ₁ given by the equation (2) to the height information 11 and then the focus position information 13 is added. And sends it to the control unit 3. As a result, the control unit 3 focuses the light beam at a position displaced from the surface of the bar code 16 by Z _1, so that the bar code 16 is irradiated where the beam spreads.

Next, a case where discrete information is obtained as the height information 11 will be described. Discrete information is obtained by the mechanism shown in FIG. 6, for example. The article 34 having the barcode 35 attached to the upper surface is conveyed in the arrow direction on the conveyor belt 33. The light emitting unit 31 is sandwiched by the conveyor belt 33.
And the light receiving unit 32 face each other. In the light emitting unit 31, light emitting elements 41 to 47 are arranged side by side at equal intervals perpendicular to the surface of the conveyor belt. On the other hand, in the light receiving section 32, light receiving elements (not shown) are similarly arranged so as to face the respective light emitting elements 41 to 47. The light emitting section 31 and the light receiving section 3
When the article passes between the two, the light to the light receiving element corresponding to the height is blocked. The height of the article can be obtained from the information on the intercepted light receiving element and the distance between the light receiving elements. The article 34 whose height has been measured is next read by the barcode detection unit 36 as the barcode 3
5 is detected.

It is assumed that the discrete height information thus obtained is as shown in FIG. 5 (a) described in the section of action, and the set beam is as shown in FIG. 5 (b). Therefore, the value and logic are the same as those of the action column.

First, the resolution of the height detector, that is, the distance S between the light receiving elements will be described. Using l ₂ in FIG. 5B, S
Is determined. This is calculated from the equation (1) as follows: S = (n + 1) S−nS = (ω ₃ ² −ω ₀ ² ) ^1/2 × (πω ₀ / λ) − (ω ₂ ² −ω ₀ ² ) ^1/2 × ( πω ₀ / λ) = (πω ₀ / λ) ((ω ₃ ² −ω ₀ ² ) ^1/2 − (ω ₂ ² −ω ₀ ² ) ^1/2 ). Therefore, the resolution needs to be S or less.

Next, the operation of the distance converter 5 will be described.
It The height information 11 is the discrete information of FIG. bar
If the code information 12 indicates a small bar code, the height information 1
1 is converted into focus information 13 as it is and sent to the control unit. one
On the other hand, when the bar code information 12 indicates a large bar code,
The value obtained by subtracting ΔS from the information 11 is converted into focus information 13.
To the control unit 3. This ΔS is ω in equation (1)₃give
Z and ω₀Ω because it is the difference from Z₃= Ω₀[1+ (λΔZ / πω₀ ²)²]^1/2  ΔZ that satisfies

[0030]

As described above, according to the present invention, the bar code for reading the focal point of the laser beam is displaced by a predetermined amount on the optical axis and the irradiation is performed. This has an effect that it is possible to easily cope with changing reading conditions.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a graph showing a beam waist and a beam divergence of a laser beam.

FIG. 3 is a graph illustrating a beam diameter of a laser beam.

FIG. 4 is a graph illustrating a beam diameter of a laser beam.

5A is a graph showing discrete information from a distance measuring unit, and FIG. 5B is a graph showing distance and beam diameter according to an embodiment of the present invention.

FIG. 6 is a perspective view of an apparatus according to an embodiment of the present invention.

[Explanation of symbols]

 1 laser light source 2 lens system 3 control unit 4 multi-faceted rotary mirror 5 distance conversion unit 6 height measurement unit

Claims (1)

[Claims] 1. An optical symbol reader for irradiating an optical symbol with a laser beam and recognizing the optical symbol by detecting scattered reflected light of the laser beam from the optical symbol. The beam spot is a predetermined distance from the focus of this laser beam,
An optical symbol reader characterized in that the spots are shifted in the optical axis direction of the laser beam.