Laser barcode scanner: It is widely used because of its unique outstanding advantages such as large depth of field, fast scanning speed and wide scanning range. In addition, laser omni-angle laser bar code scanner is widely used in various fields with high automation and large logistics volume because it can scan and read bar code symbols passing in any direction at high speed.
Laser barcode scanner consists of laser light source, optical scanning, optical receiving, photoelectric conversion, signal amplification, shaping, quantization and decoding. These components will be discussed in detail below.
(1) laser source
The visible semiconductor laser manufactured by MOVPE (metal oxide vapor phase epitaxy) technology has the advantages of low power consumption, direct modulation, small volume, light weight, curing, high reliability and high efficiency. As soon as it appeared, it quickly replaced the original He-Ne laser.
The beam emitted by a semiconductor laser is a non-axisymmetric elliptical beam. The divergence angle V ⊥≈ 30 of the outgoing beam perpendicular to the P-W junction direction and V ≈10 parallel to the junction direction. If the traditional beam collimation technology is adopted, the long and short axis directions of elliptical spots on both sides of the beam convergence point will be interchanged. Obviously, this will make the scanner only have a small scanning depth of field. Jay M.Eastman and others proposed to adopt the beam collimation technology shown in Figure 3, which overcame this exchange phenomenon and greatly improved the scanning depth of field. This elliptical beam can only be applied to single-line laser scanners. When arranging the light path, the long axis direction of the spot ellipse should be perpendicular to the scanning direction of light. For a single-line laser bar code scanner, due to its insensitivity to printing noise, the elliptical spot will have better characteristics than the circular spot described below. For the full-angle bar code laser bar code scanner, when scanning and reading the bar code, the beam sometimes sweeps the bar code with a large inclination. Therefore, the beam spot should not be oval. It is usually round. The common shaping scheme is to add a small round hole in front of the collimating lens. This kind of beam characteristics can be well approximated by Fresnel diffraction characteristics of small holes. With this scheme, the depth of field can be about 250 mm to 300 mm for standard UPC bar codes. This is enough for the general commercial POS system. However, it is not enough for occasions that need large depth of field, such as airport baggage conveyor lines. At present, the commonly used scheme is to increase the size of bar code symbols or make different scanning rays forming scanning patterns converge in different areas to form a "multi-focal plane". However, a more attractive scheme is to use a special optical collimating element, so that the light field passing through it has a special distribution, a very small beam divergence angle and a large depth of field.
(2) Optical scanning system
The laser beam emitted from the laser source also needs to pass through the scanning system to form a scanning line or a scanning pattern. Full-angle barcode laser barcode scanner generally adopts two schemes: rotating prism scanning and holographic scanning. Holographic scanning system has obvious advantages such as compact structure, high reliability and low cost. Since the first application of IBM in 3687 scanner, it has been widely used and has been constantly innovating. It can be expected that its market share will be bigger and bigger.
Rotating prism scanning technology has a long history and mature technology. It scans the light beam with a rotating prism, and changes the light path with a set of folded plane mirrors to realize multi-directional scanning of light. At present, widely used scanner products, such as MS-700, also make the wedge angles of different faces of the rotating prism different, thus forming multiple scanning lines in one scanning direction. The high-density scanning pattern consists of multi-directional and multi-line scanning light. Another possible benefit of this method is that it can reduce the harm of laser radiation. In order to improve the circulation speed of supermarkets, the concept of full-angle scanning is put forward for the first time, and the corresponding UPC bar code is designed. For the "X" scanning pattern in two scanning directions of UPC code, all-angle scanning can be realized. 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 all-angle scanning is now being extended to other code systems, such as 39-code and 25-code interleaving. The aspect ratio of these barcodes is very small, so more scanning directions will be needed in order to realize full-angle scanning. For this reason, in addition to rotating the prism, it is necessary to add another moving element, such as rotating the folded plane mirror group in Figure 4.
Because of the low scanning speed and small scanning angle, many schemes can be used to realize beam scanning. In addition to rotating prism and pendulum mirror, many components in moving optical system can realize beam scanning. Such as a moving semiconductor laser, a moving collimating lens, etc. In addition to DC motors, the power elements that generate these movements can also be piezoelectric ceramics and electromagnetic coils. These power components have the advantages of being difficult to be damaged, long in service life and convenient to use, and it is estimated that they will also have certain applications.