Constitution of the hottest laser scanning bar cod

2022-08-11
  • Detail

Composition 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, laser full angle scanning reader is widely used in various fields with high degree of automation and large amount of logistics because it can scan and read 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 (Figure 1). 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 an axisymmetric elliptical beam (Fig. 2). The divergence angle V ⊥ 30 ° of the outgoing beam perpendicular to the direction of the P-W junction plane, and the divergence angle V ∥ 10 ° parallel to the direction of 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 only have a small scanning depth of field. Jay stman et al. Proposed to use the beam collimation technology shown in Figure 3 to overcome this exchange phenomenon and greatly improve the scanning depth of field range. This scanning direction is vertical. For the single line scanning reader, this elliptical spot is insensitive to 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 and the light of the light spot 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. This beam characteristic can be well approximated by the Fresnel diffraction characteristic of the aperture. With this scheme, the depth of field of standard UPC bar code can reach about 250mm to 300mm. This is enough for general commercial POS systems. But for occasions requiring large depth of field, such as airport baggage conveyor lines, it is not enough. 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 special optical collimating elements, so that the light field passing through it has a special distribution, thus having a small beam divergence angle, and obtaining a larger depth of field B3 (flammable building materials): no flame retardant effect

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. Holographic scanning system has many remarkable advantages, such as compact structure, high reliability and low cost. Since IBM first applied the 3687 scanner, it has been widely used, and continues to innovate. 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 optical path to achieve multi-directional scanning light (Fig. 4). At present, ms-700 and other scanners, which are widely used, also make the wedge angles of different surfaces of the rotating prism different, forming a scanning direction with several scanning lines. A high-density scanning pattern is composed of multidirectional and multiline scanning rays. Another advantage that this method may bring 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 barcode was designed. For UPC code, the "X" scanning pattern in two scanning directions can realize full angle scanning. With the development of scanning technology, the expansion of the application field of bar code and the urgent need to improve the degree of automation, the concept of full angle scanning is now being extended to other coding systems, such as 39 code, 25 code, etc. The height and width of these bar codes are relatively small. In order to achieve full angle scanning, much more scanning directions will be required. To this end, in addition to rotating the prism, another moving element will need to be added, such as rotating the folded plane mirror group in Figure 4

due to low scanning speed and small scanning angle, there are many schemes that can be used to realize beam scanning. In addition to rotating prisms and pendulum mirrors, beam scanning can also be achieved through many components of the moving optical system. For example, beam scanning is realized by moving semiconductor laser (Fig. 5), moving collimating lens, etc. In addition to DC motors, the power components that produce these movements can also be piezoelectric ceramics and electromagnetic coils. These power components have the advantages of not easy to damage, long service life and convenient use, and it is estimated that they will also be applied to a certain extent

light receiving system

the scanning beam is scattered after hitting 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 (Fig. 4). In this structure, the main optical axis of the receiving beam is the outgoing light axis. In this way, the scattered spot is always located on the axis of the receiving system due to the deformation caused by the straightening of the crimped high molecular chain. The instantaneous field of view of this structure is very small, which can greatly improve the signal-to-noise ratio, improve the ability to suppress the specular reflection of bar code symbols, and have low requirements for the receiving lens. In addition, it can make the sensitive surface of the receiver smaller. The sensitive area of high-speed photoelectric receivers is generally small, and the cost of receivers with small sensitive areas is also low, so this is also very important. Its disadvantage is that vignetting will occur when the scanning beam is located at the edge of each element of the scanning system (Fig. 6). In addition to taking structural measures to minimize vignetting, the scanning angle with poor characteristics should also be discarded

optical automatic gain control system is also widely used in all angle scanning readers, so that the received signal light intensity does not change with the distance of bar code symbols. This can reduce the dynamic range of the signal and is conducive to 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 this low-frequency system is also 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 semiconductor lasers. Then, in the electrical signal processing, the synchronous receiving and amplification technology is used to take out the bar code signal. As long as the modulation frequency is much greater than the barcode signal frequency, the barcode width error caused by it will be negligible. Synchronous receiving technology has a very high ability to suppress noise, 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, and it is required to have a certain aperture size to ensure that enough signal light is received. However, if the moving element only plays the role of scanning the outgoing beam, it can be made very small. Obviously, small moving elements are extremely beneficial to the selection of power elements and the improvement of service life and reliability

photoelectric conversion, signal amplification and shaping

the received optical signal needs to be converted into electrical signal by photoelectric converter. The bar code signal frequency in the full angle scanning reader ranges from a few 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. Therefore, the last received shipping activities in Southeast Asia are cold and energy 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 scanning reader is tens of kHz to hundreds of kHz. Generally, silicon photocell, photodiode and photoelectric triode are used as photoelectric converter devices. The light energy emitted by the handheld gun scanning reader is relatively strong, and the signal frequency is low. 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. Moreover, due to the low signal frequency, the automatic gain control circuit can be easily realized as pointed out in the report

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 what is usually called "digital signal". Similarly, due to the low signal frequency of the handheld gun scanner, there will be more room to choose the shaping scheme

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 symbols 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, and for the situation that each scanning line is evenly distributed α= π/2n, such as when n=2 α From this formula, we can estimate that for UPC code, if the scheme of scanning the left half and the right half and splicing is adopted, the data rate will be the lowest when n is 3, and for the scheme of reading completely through the whole barcode, the data rate will be the lowest when n is 5. This should be considered when designing the scanning system

in addition, low-speed scanning modules can also be combined into an array to achieve the performance of full angle high-speed scanning barcode. Obviously, this scheme is more suitable for assembly line 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 barcode signals and incomplete barcode signals, the decoder needs to have the ability to automatically identify effective barcode signals. Therefore, it has much higher requirements for the decoding unit, which requires the decoding unit to have extremely 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 number of scanning directions of the scanner increases and the scanning speed increases,

Copyright © 2011 JIN SHI