Research And Design Of The Metal Detector For Doorway In EAS

EAS and metal detector have been existed a long time from appearance to large scale application. But there aren’t EAS products that have the function of the metal detection. As the popularizing of the EAS, the defect which can’t detect the metal is more and more apparent and causes that the thieves make use of it to escape the detection of the EAS easily. Based on the common radio frequency EAS for specific application environment, we research and design a set of doorways detector with metal detection function, which is expected to strengthen the ability of the supermarket security.The paper first introduces the present situation of the technology of EAS and metal detector. It points out that the significance of the issue is to strengthen the RF EAS performance and to improve the profits of large retail store. Then we make a deep and detailed analysis of the physics theory basis of the metal detection. The metal detector mainly makes use of the principle that metal objects will produce eddy current in alternating electromagnetic field to detect metal. Using characteristics which different metal induct differently in alternating electromagnetic field distinguish the size and types of metal object. Finally, we analyze the characteristics and principle of several metal detectors and compare to the requirement of the issue then we get the best metal detection scheme in EAS.After determining principle project, we put forward the ensemble framework of scheme and the composition of it. The design of detecting coil is an important part in the scheme. The receiving coil of it is differential connection and has a strong anti-interference ability to the interference which is from the RF EAS. The sine signal is produced by DDS chip which is pure digital. The frequency and phase of the sine single is controlled through the Single Chip Micyoco. The function and design process of the filter and power amplifier circuit is described detailedly in the process of hardware design. The filter in fore-end is Butterworth Low-passing Filter which is used to filter high frequency interference which is form DDS chip. The filter in rear-end is RF interference filter which is used to filter RF interference which is form EAS. The signal is sampled and converted into digital then transmitted to the DSP to be processed and judge whether need issued a warning signal. In the process of software design, we first give the ensemble framework of software process, then give the illustration of subroutines module and key program code. Finally, we make a detailed instruction of the man-machine interactive program module which is an important part in software design.The metal detector which is designed in paper uses the digital technology and detecting coil of it is differential connection. Because of that, it has a very good inhibitory ability to restrain high frequency interference from RF EAS.