Composition of the hottest laser scanning bar code

2022-07-25
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Constitution of laser scanning barcode reader

scanning barcode reader has been widely used because of its unique advantages such as large depth of field, high scanning speed, wide scanning range and so on. In addition, the laser full angle scanning reader is widely used in various fields of automation, timely reinforcement and large logistics volume because it can scan and read the bar code symbols passing in any direction at high speed

the laser scanning barcode reader is composed of laser source, optical scanning, optical receiving, photoelectric conversion, signal amplification, shaping, quantization and decoding. These components are discussed in detail below

laser source

the visible light semiconductor laser manufactured by MOVPE (metal oxide vapor phase epitaxy) technology has the advantages of low power consumption, direct modulation, small size, light weight, solid state, high reliability and high efficiency. As soon as it appeared, it quickly replaced the original He Ne laser

the beam emitted by the semiconductor laser is a non axisymmetric elliptical beam. The divergence angle V ⊥ 30 ° of the outgoing beam perpendicular to the P-W junction plane, and the divergence angle V ∥ 10 ° parallel to the junction plane. If the traditional beam alignment technology is adopted, the long and short axis directions of the elliptical facula on both sides of the beam convergence point will be exchanged. Obviously this will make the scanner have only a small scanning depth of field. Jay stman et al. Proposed the use of beam collimation technology to overcome this exchange phenomenon and greatly improve the scanning depth of field. This scanning direction is vertical. For the single line scanning reader, the elliptical spot is insensitive to the printing noise, so the elliptical beam can only be applied to the single line laser scanner. When arranging the light path, the long axis direction of the ellipse of the light spot and the light should have better characteristics than the circular light spot mentioned below

for the full angle barcode scanning reader, the beam sometimes scans the barcode with a large inclination angle when scanning and reading the barcode. Therefore, the beam spot should not be made into an ellipse. It is usually rounded. At present, the commonly used shaping scheme is to add a small circular aperture in front of the collimating lens. The beam characteristics can be well approximated by the Fresnel diffraction characteristics of the aperture. With this scheme, for standard UPC bar codes, the depth of field can reach about 250mm to 300mm. This is sufficient for general commercial POS systems. However, it is not enough for occasions requiring large depth of field, such as airport baggage conveyor lines. At present, the commonly used scheme is to increase the size of the bar code symbol or make the different scanning rays constituting the scanning pattern converge in different areas to form a "multifocal". However, the more attractive scheme is to use a special optical collimating element, so that the light field passing through it has a special distribution, which has a small beam divergence angle and a large depth of field

optical scanning system

the laser beam emitted from the laser source also needs to form scanning lines or scanning patterns through the scanning system. Generally, two schemes, rotating prism scanning and holographic scanning, are adopted for all angle barcode scanning reader. The holographic scanning system has the advantages of compact structure, high reliability and low cost. Since IBM first applied the 3687 scanner, it has been widely used, and it continues to bring forth the new. It can be expected that its market share will become larger and larger

rotating prism scanning technology has a long history and is technically mature. It uses a rotating prism to scan the beam, and a group of folded planar mirrors to change the light path to achieve multi-directional scanning light. At present, ms-700 and other scanners, which are widely used, also make the wedge angles of different surfaces of the rotating prism different to form several scanning lines in one scanning direction. A high-density scanning pattern is composed of multi-directional and multi-line scanning rays. Another possible benefit of this method is that it can reduce the harm of laser radiation

the concept of full angle scanning was first proposed to improve the circulation speed of supermarkets, and the corresponding UPC bar code was designed. The "X" scanning pattern in two scanning directions of UPC code can realize full angle scanning. With the development of scanning technology, the expansion of bar code application field and the urgent need to improve the degree of automation, the concept of full angle scanning is now being extended to other code systems, such as 39 code, 25 code, etc. The height width ratio of these bar codes is small. In order to achieve full angle scanning, much more scanning directions will be required. For this reason, in addition to rotating the prism, another moving element will be added, such as folding the plane mirror group

due to the low scanning speed and small scanning angle, there are many schemes that can be used to realize beam scanning. In addition to rotating prism and pendulum mirror, beam scanning can also be achieved through many components of the moving optical system. For example, beam scanning is realized by moving semiconductor laser, moving collimating lens, etc. In addition to DC motor, piezoelectric ceramics and electromagnetic coils can also be the power components that generate these movements. These power components have the advantages of not easy to damage, long service life and convenient use. It is estimated that they will also be applied to a certain extent

light receiving system

the scanning beam is scattered after it is shot on the bar code symbol, and the receiving system receives enough scattered light. In the laser full angle scanning reader, the return receiving system is widely used. In this structure, the main optical axis of the receiving beam is the outgoing beam axis. In this way, the scattered spot is always on the axis of the receiving system. The instantaneous field of view of this structure is very small, which can greatly improve the signal-to-noise ratio and the ability to suppress the specular reflection of bar code symbols. The mold needs to be cleared in time, and the requirements for the receiving lens are also very low. In addition, it can make the sensitive surface of the receiver smaller. The sensitive area of high-speed photoelectric receiver is generally small, and the cost of receiver with small sensitive area is also low, so this is also very important. Its disadvantage is that when the scanning beam is located at the edge of each component of the scanning system, it will produce vignetting phenomenon. In addition to taking structural measures to minimize vignetting, the scanning angle with poor characteristics should also be discarded

the optical automatic gain control system is also widely used in the full angle scanning reader, so that the received signal light intensity does not change with the distance between the bar code symbols. This can reduce the dynamic range of the signal and facilitate subsequent processing

handheld gun scanning reader has the characteristics of slow scanning speed and low signal frequency. The receiver with low response frequency, such as silicon photocell, has a large sensitive area, and the low-frequency system is easy to achieve a high signal-to-noise ratio. Therefore, in addition to the above back receiving scheme, other schemes can be adopted. For example, the outgoing laser beam can be modulated at a higher frequency by using the easy modulation of the semiconductor laser. Then, the bar code signal is taken out by the synchronous receiving and amplification technology during the electrical signal processing. As long as the modulation frequency is much greater than the bar code signal frequency, the bar code width error will be negligible. Synchronous receiving technology has very high noise suppression ability, so it is not necessary to use the return receiving structure. This will bring considerable flexibility to the arrangement of the optical receiving system. Using this flexibility can improve the performance of some aspects of the reader. For example, in the return receiving scheme, the moving element is also an integral part of the receiving system. It is required to have a certain aperture size to ensure that enough signal light is received. However, if the moving element only scans the outgoing beam, it can be made very small. Obviously, small moving elements are extremely beneficial to the selection of power elements or to the improvement of service life and reliability

photoelectric conversion, signal amplification and shaping

the received optical signal needs to be converted into electrical signal through photoelectric converter. The bar code signal frequency in the full angle scanning reader ranges from several megahertz to tens of megahertz. Such a high signal frequency requires the photoelectric converter to use avalanche photodiode (apo) or PIN photodiode with high frequency response. The full angle scanning reader is generally used continuously for a long time. For the safety of users, the output energy of the laser source is required to be small. So the energy received in the end is very weak. In order to obtain a high signal-to-noise ratio (which is determined by the bit error rate), the preamplifier circuit composed of low-noise discrete components is usually used to amplify the signal with low noise

the signal frequency of the handheld gun type scanning reader ranges from tens of kHz to hundreds of kHz. Generally, silicon photocell, photodiode and photoelectric triode are used as photoelectric converter devices. The handheld gun type scanning reader emits relatively strong light energy and low signal frequency. In addition, as mentioned above, synchronous amplification technology can also be used. Therefore, its requirements for the characteristics of electronic components are not very high. And because the signal frequency is low, the AGC circuit can be easily realized

due to the edge fuzziness of bar code printing, especially the limited size of scanning spot and the low-pass characteristics of electronic circuit, the edge of the obtained signal will be blurred, which is usually called "analog electrical signal". This kind of signal also needs to recover the edge as accurately as possible through the shaping circuit to become the so-called "digital signal". Similarly, the hand-held gun scanner will have more room to choose the shaping scheme due to its low signal frequency

from the above situation, we can see that high signal frequency brings great technical difficulties and cost improvements. For a full angle scanning reader with certain reading ability, its data rate R is proportional to n/(H × Cos α- W × sin α)。 Where, n is the number of scanning directions, h and W are the height and width of bar code symbol respectively, α It refers to the angle value when the bar code symbol is in the most unfavorable position for scanning and reading relative to the scanning pattern. For the situation that each scanning line is evenly distributed α= π/2n, e.g. when n=2 α From this formula, we can estimate that for UPC code, if the scheme of scanning the left and right halves and splicing is adopted, the data rate is the lowest when n is 3. For the scheme that can only be read through the whole barcode, the market share of the data rate is still very small when n is 5. This should be taken into account when designing the scanning system

in addition, low-speed scanning modules can be combined into an array to achieve the performance of full angle high-speed scanning of bar codes. Obviously, this scheme is more suitable for pipeline applications

decoding

after quantizing the shaped electrical signal, the decoding unit translates the information contained therein. Due to the high data rate of full angle scanning reader, and the vast majority of non bar code signals and incomplete bar code signals, the decoder needs to have the ability to automatically identify effective bar code signals. Therefore, it requires much higher requirements for the decoding unit, and requires the decoding unit to have very high data processing capacity and great data throughput. At present, the close combination of software and hardware is widely used. For UPC and ean codes, the decoder also has the function of automatic splicing of left and right code segments. However, this splicing may splice the left half and the half from two different barcodes. Parity and parity do not guarantee that this will not happen. With the development of scanning technology, the increase of scanning direction and scanning speed, this code segment splicing function is not very necessary. Most of the bosses, senior engineers and sales managers of many civil operated experimental machine enterprises have gone through this position and provided products

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