The decoding principle of laser barcode scanner
Dec 11, 2018

Laser barcode scanner is usually composed of a light source, an optical lens, a scanning module, an analog digital conversion circuit and a plastic outer casing. When scanning a group of bar codes, the light source is irradiated onto the bar code, and the reflected light is collected through the lens to the scanning module. The scanning module converts the optical signal into an analog digital signal, which can be transmitted to the computer, which is what we want. Bar code content.

 

Common laser barcode scanners are usually composed of a light source, an optical lens, a scanning module, an analog digital conversion circuit, and a plastic housing. It uses a photoelectric element to convert the detected optical signal into an electrical signal, and then converts the electrical signal into a digital signal through an analog digital converter for transmission to a computer for processing. The scanning engine is a very important part of the internal structure of the scanner. The barcode scanner that are commonly seen on the market actually have different scanning engines, but the general components are light sources and optical lenses.

(1) Laser source

 

The visible light semiconductor laser fabricated by MOVPE (Metal Oxide Vapor Phase Epitaxy) technology has the advantages of low power consumption, direct modulation, small size, light weight, solidification, high reliability, and high efficiency. It quickly replaced the original He-Ne laser when it appeared.

 

The beam emitted by the semiconductor laser is a non-axisymmetric elliptical beam. The divergence angle of the exiting beam perpendicular to the P-W junction plane is V⊥≈30°, and the divergence angle V‖≈10° parallel to the junction plane direction. If the conventional beam collimation technique is used, the long and short axes of the elliptical spot on both sides of the beam convergence point will be exchanged. Obviously this will give the scanner only a small scan depth of field. Jay M. Eastman et al. proposed using the beam collimation technique shown in Figure 3 to overcome this switching phenomenon and greatly improve the range of scanning depth of field. This elliptical beam can only be applied to a single-line laser scanner. When arranging the optical path, the direction of the long axis of the ellipse of the spot should be perpendicular to the direction in which the light is scanned. For single-line laser bar code scanners, such elliptical spots will be better than the circular spot characteristics described below due to their insensitivity to printing noise.

 

For a full-angle laser barcode scanner, the bar code is sometimes swept at a large tilt angle as the beam scans the barcode. Therefore, the beam spot should not be made elliptical. It is usually rounded. A commonly used shaping solution is to add a small circular aperture stop in front of the collimating lens. This beam characteristic can be well approximated by the Fresnel diffraction characteristics of the small holes. With this solution, the depth of field can be approximately 250mm to 300mm for standard size UPC barcodes. This is enough for a general commercial POS system. However, it is not enough for places such as the airport baggage conveyor line that require a large depth of field. At present, the commonly used scheme is to increase the size of the bar code symbol or to make different scanning rays constituting the scanning pattern converge in different regions to form a "multi-focal plane". However, a more attractive solution is to use a special optical collimating element to make the light field passing through it have a special distribution and thus have a very small beam divergence angle, resulting in a large depth of field.

 

(2) Optical scanning system

 

The laser beam emitted from the laser source also needs to form a scan line or scan pattern through the scanning system. Full-angle bar code laser bar code scanners generally use two methods of rotating prism scanning and holographic scanning. Holographic scanning systems have significant advantages such as compact structure, high reliability and low cost. Since IBM's first application on the 3687 scanner, it has been widely used and is constantly being updated. It can be expected that its market share will increase.

 

Rotating prism scanning technology has a long history and is relatively mature in technology. It uses a rotating prism to scan the beam, and a set of folded plane mirrors to change the light path to achieve multi-directional scanning light. At present, the scanner products such as the MS-700 are used to make the wedge angles of different faces of the rotating prism different to form a plurality of scanning lines in one scanning direction. A multi-directional multi-line scanning light constitutes a high-density scanning pattern. Another benefit that this approach may bring is the reduction in laser radiation hazards.

 

The concept of full-angle scanning was first proposed to improve the speed of the supermarket, and the corresponding UPC barcode was designed. The full-angle scan is already possible for the "X" scan pattern of the two scan directions of the UPC code. With the development of scanning technology, the broadening of bar code application fields 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 codes and 25 codes. The bar code width and width of these codes are relatively small, and much more scanning direction numbers are required in order to achieve full-angle scanning. For this purpose, in addition to the rotating prism, it is necessary to add another moving element, such as rotating the folded plane mirror set in Fig. 4 and the like.

 

Handheld single-line scanners have many options for beam scanning due to low scanning speed and small scanning angle. In addition to rotating prisms and mirrors, beam scanning is achieved by many components in the moving optics. Beam scanning is achieved by moving a semiconductor laser, a moving collimating lens, or the like. The power components that generate these movements may be piezoelectric ceramics and electromagnetic coils in addition to the DC motor. These power components have the advantages of being less susceptible to damage, long life and ease of use, and are expected to be applied.

 

(3) Light receiving system

 

The scanning beam is scattered after being incident on the bar code symbol, and the receiving system receives enough scattered light. In laser full-angle laser bar code scanners, a return-to-receive system is commonly used. In this configuration, the main optical axis of the received beam is the exit ray axis. Thus, the scattered spot is always on the axis of the receiving system. The instantaneous field of view of this structure is extremely small, which can greatly improve the signal-to-noise ratio, and also improve the suppression of specular reflection of bar code symbols, and the requirements for the receiving lens are also low. In addition, it also makes the sensitive surface of the receiver smaller. High-speed optoelectronic receivers are generally not sensitive, and receivers with small sensitive areas are also less expensive, so this is also important. Its disadvantage is that it produces a vignetting phenomenon when the scanning beam is located at the edge of each component of the scanning system. In addition to taking structural measures to minimize vignetting, the scanning angle of poor characteristics should also be discarded.

 

An optical automatic gain control system is also commonly used in full-angle laser bar code scanners, so that the received signal light intensity does not change with the distance of the bar code symbol. This can reduce the dynamic range of the signal and facilitate subsequent processing.

 

The handheld gun-type laser barcode scanner has the characteristics of slow scanning speed and low signal frequency. A receiver with a low response frequency, such as a 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-described return reception scheme, other schemes can be adopted. For example, the outgoing laser beam can be modulated at a relatively high frequency by utilizing the easy modulation of the semiconductor laser. Then, in the electrical signal processing, the synchronous receiving amplification technique is used to take out the barcode signal. As long as the modulation frequency is much larger than the bar code signal frequency, the bar code width error it brings will be negligible. Synchronous reception technology has a very high noise suppression capability, so it is not necessary to use a return-to-receive structure. This will give considerable flexibility to the arrangement of the optical receiving system. This flexibility allows for improved performance in some aspects of the reader. For example, in a return-to-receive scheme, the moving component is also an integral part of the receiving system, requiring it to have a certain aperture size to ensure that sufficient signal light is received. However, if the moving element only plays the role of scanning the outgoing beam, it can be made small. Obviously, small moving elements are extremely advantageous for selecting power components, improving life and reliability.

 

(4) Photoelectric conversion, signal amplification and shaping

 

The received optical signal needs to be converted into an electrical signal by a photoelectric converter. The bar code signal frequency in a full-angle laser bar code scanner ranges from a few megahertz to several tens of megahertz. Such high signal frequencies require the opto-electrical converter to use an avalanche photodiode (APO) or PIN photodiode with high frequency response capability. Full-angle laser bar code scanners are generally used continuously for a long time. For the safety of users, the laser source is required to emit less energy. Therefore, the energy received last is extremely weak. In order to obtain a higher signal-to-noise ratio (which is determined by the bit error rate), a low-noise discrete component is usually used to form a preamplifier circuit to amplify the signal with low noise.

 

The signal frequency of the handheld gun-type laser barcode scanner is several tens of kilohertz to several hundred kilohertz. Silicon photo cells, photodiodes and phototransistors are generally used as photoelectric conversion devices. The handheld gun-type laser bar code scanner emits relatively high light energy and has a low signal frequency. In addition, as described above, synchronous amplification technology can also be used. Therefore, its requirements for the characteristics of electronic components are not very high. Moreover, since the signal frequency is low, the automatic gain control circuit can be realized more conveniently.

 

Due to the edge ambiguity in bar code printing, more importantly because of the limited size of the scanning spot and the low-pass characteristics of the electronic circuit, the resulting signal edges will be blurred, commonly referred to as "analog electrical signals." This signal must also be restored to the edge as accurately as possible by the shaping circuit to become what is commonly referred to as a "digital signal." Similarly, handheld gun scanners have more room for choice of shaping schemes due to the lower signal frequency.

 

From the above situation, we can see that the high signal frequency brings about technical difficulties and cost increases. For a full-angle laser bar code scanner with a 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 respectively the height and width of the bar code symbol, and α is the angle value at which the bar code symbol is most unfavorable for scanning and scanning, and is uniformly distributed for each scanning line α=π /2n, if n=2, α is 45°. From this formula, we can estimate the scheme for scanning the left half and the right half and splicing for the UPC code. When n is 3, the data rate is the lowest. The entire barcode is read, and the data rate will be the lowest when n is 5. This needs to be considered when designing the scanning system.

 

In addition, low-speed scanning modules can also be combined into one array to achieve full-angle high-speed scanning of bar code performance. Obviously, this solution is more suitable for use in pipeline applications.

 

(5) Decoding

 

After the shaped electrical signal is quantized, the information contained therein is decoded by the decoding unit. Since the full-angle laser bar code scanner has a high data rate and most of the obtained are non-barcode signals and incomplete bar code signals, the decoder needs to have the ability to automatically recognize valid bar code signals. Therefore, it has much higher requirements on the decoding unit, requiring the decoding unit to have extremely high data processing capability and great data throughput. At present, the combination of software and hardware is widely adopted. For UPC and EAN codes, the decoder also has automatic stitching function for the left and right code segments. However, this stitching may stitch the left half and the half from two different bar codes. Parity and check digits do not guarantee that this will not happen. With the development of scanning technology, the number of scanner scanning directions and the scanning speed are increased, this code segment splicing function is not very necessary. Many companies' products provide a switch that allows users to choose this feature.

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