Bar code scanning system composition structure principle

Bar code scanning system composition structure principle, the main body of the scanning system is the optical structure, the structure should have the following two properties:

1. It has a scanning light path to produce a light spot, which can move in a straight line along a certain track under manual or automatic control and pass through the left blank area, start character, data character, termination character and right blank area of a bar code symbol.

2, with a bar code symbol reflected light receiving system, it should be able to and good at receiving scanning spot from the bar code symbol reflected back on the diffuse reflection light.

The barcode scanning system can take different light sources, scanning forms, and optical path designs to achieve the above functions.

(1) The light source of the scanning gun

There are many kinds of light sources used in bar code scanning gun, mainly including semiconductor light source, laser light source, incandescent lamp, flash lamp and other light sources. Here mainly introduces the semiconductor light-emitting tube and laser.

① Semiconductor light-emitting tube

Semiconductor light-emitting diodes, also known as light-emitting diodes, are actually a combination of P-type semiconductors and N-type semiconductors. When a forward voltage is applied to the P-N junction, the light-emitting diode emits light, as shown in Figure 2-29(a) and Figure 2-29 (B).

Figure 2-29 Semiconductor Light Emitting Diode

② Laser

The development of laser technology has a history of more than 30 years, and it is now widely used in various fields. There are many types of lasers. Early bar code scanning guns generally used He-Ne lasers as scanning light sources. In the 1980 s, semiconductor lasers have developed rapidly. Because of their low power consumption, small size, low working voltage, high reliability and low price, the original He-Ne lasers were quickly replaced.

Our common semiconductor lasers are small power, the volume is as large as an ordinary triode, so the semiconductor laser is also called laser diode. Because the bar code scanning gun generally uses laser diodes, its volume and cost are greatly reduced. At the beginning, only laser diodes that can only emit infrared laser, and red light laser diodes (now there are blue laser diodes) appeared after the 1990 s. The development of laser diodes has become an important aspect of the development of bar code technology in recent years.

Compared with other light sources, laser has its unique properties:

1, there is a strong direction.

2. Monochromativity and excellent coherence. Other light sources do not get monochromatic light as emitted by lasers, regardless of the filtering technique used.

3, can obtain extremely high light intensity. The bar code scanning system uses low-power laser diodes, which will not cause harm to the human eye when it is briefly injected into the human eye, but it still has light intensity that other light sources are difficult to achieve.

(2) Scanning mode

The main ways of scanning and reading bar code symbols are manual scanning, automatic scanning and CCD scanning.

① Manual scanning

Manual scanning is relatively simple. There is no scanning device inside the scanning gun. The position of the emitted lighting beam is fixed relative to the scanning gun. To complete the scanning process, a handheld bar code scanning gun is required to sweep the bar code symbol. This bar code scanning gun belongs to a fixed beam scanning gun. The light pen and most card bar code readers use this scanning method. See Figure 2-30 for the scanning diagram of the light pen.

② Automatic scanning

Automatic scanning means that the bar code scanning gun contains a device that makes the scanning beam scan, such as a rotating mirror group, a swing mirror, etc. The scanning light source for automatic scanning is a laser, and the basic schematic diagram of automatic scanning is shown in Figure 2-31.

In the figure, the scanning beam is emitted from the laser (LS), passes through the half-mirror lens surface (BSP), and then passes through the mirror surfaces of the periodically rotating prism (PG) to form the scanning movement of the laser beam. At the same time, the reflected light of the illumination spot on the bar code symbol passes through the mirror surface of the rotating prism, is reflected by the half-mirror lens surface (BSP), and passes through the converging lens on the photodetector. In this scanning structure, the scanning beam of the laser does not pass through the lens system for receiving light, maintaining the characteristics of narrow laser beam and concentrated light energy. However, outside the lens system, the optical axis of the laser beam and the receiving system remain coincident, thus ensuring that the illumination point of the laser is the receiving point of the detector.

The scanning device of the hand-held laser bar code scanning gun generally uses a pendulum mirror instead of a prism, which swings under the drive of an internal vibration coil to realize scanning. The omnidirectional laser scanning gun in supermarkets generally uses rotating prism scanning and holographic scanning. Both of these scanning methods can produce scanning in multiple directions and multiple positions.

Figure 2-32 Schematic diagram of rotating prism scanning structure

In the scanning optical path in the figure, each plane mirror and a plurality of mirror surfaces (six in the figure) of the prismatic rotating mirror are combined to form a plurality of scanning lines in a certain direction. The combination of the three plane mirrors and the rotating mirror surface constitutes a multi-line scan in three directions. The scanning light spot returns to the original path through the plane mirror and the rotating mirror, and then converges on the photodetector by the composite objective lens.

The scanning principle of holographic scanning is more novel, and its scanning device is a rotating holographic disk, which replaces the rotating prism above. Figure 2-33 shows a structural diagram of a reflective holographic scan.

Figure 2-33 Schematic diagram of reflective holographic scanning structure

The basic process of holographic scanning is as follows: the laser beam ① hits the holographic disc ②, the rotating holographic disc realizes beam scanning, then the scanning line sweeps the bar code symbol ③, finally the reflected light signal returns to the holographic disc ②, and converges to the photoelectric detector ④ through the holographic lens.

On the holographic disk, there are up to 20 scanning lenses, which, together with the pairing of a plurality of plane mirrors, can sweep up to 100 scanning rays. After design, it is particularly good at realizing multi-directional multi-line scanning at multiple distances. In addition, the holographic scanning device has significant advantages such as compact structure, high reliability and low cost.

③ CCD scanning

CCD (Charge Coupled Device)-Charge Coupled Device is an electronic automatic scanning photoelectric detector. The bar code scanning gun composed of photodiodes first presents the entire image of the bar code symbol on the CCD of the linear array, and then the CCD performs photoelectric conversion on the optical signal on it and performs automatic scanning without adding any movement mechanism, so it is a special automatic scanning.

Shenzhen Hongxing Yongli Electronic Technology Co., Ltd. [www.xingyongli.com] is a high-tech enterprise specializing in the research and application of bar code automatic identification technology. It integrates bar code hardware, bar code consumables, bar code automatic identification technology and bar code application system to provide customers with high-quality bar code related products and system integration professional services.