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1 Introduction
The current situation of warehouse management in our country is not optimistic. Most warehouses are still in the primitive manual management stage, and a lot of labor costs need to be invested to register and manage the information of the goods in the warehouse and the vehicles and containers entering and leaving the warehouse. This will not only cause human resources Waste, and there is a higher error rate. Warehouse management is a very important link in logistics management.
In recent years, some advanced modern management technologies have been introduced into the country, such as the establishment of a database to store cargo information, and passive electronic tags have also been tried to be used in cargo information registration. However, the above technology still requires more manual operations, especially the information collection process, which requires some handheld devices to register the information of goods and vehicles.
Radio Frequency Identification (Radio Frequency Identification, commonly known as electronic tags) is the latest application technology in the global logistics field. Combining the technical advantages of RFID itself with the needs of warehouse management can well solve the current problems in warehouse management and improve work efficient. In this paper, combined with active RFID technology, through on-site inspection of the warehouse environment, an architecture system from hardware to software is designed. Using RFID and wireless networks, a set of practical value, efficient and low-cost warehouse management system is realized.
2 Overview of RFID technology
RFID technology uses wireless radio frequency to carry out non-contact two-way data transmission between radio frequency reader and radio frequency tag to achieve the purpose of target identification and data exchange. The identification work does not require manual intervention, has fast response speed, strong anti-interference ability, can identify high-speed moving objects and can identify multiple tags at the same time. It has the characteristics of fast automatic scanning, small size, large information capacity, strong durability, reusable, high security and confidentiality, and easy to carry.
According to different implementation methods, RFID can be divided into two categories: active RFID and passive RFID. There is no battery on the electronic tag of passive RFID, and all the power required for its work is obtained by converting the electromagnetic wave sent by the received reader, so the transmitting power of the reader is generally larger. On the contrary, the electronic tag of active RFID has its own battery, which can provide power for all devices to work, so the transmitting power requirement of the corresponding reader is not high, and the effective reading distance is also increased compared with the former. The warehouse management system proposed in this paper takes into account the above-mentioned advantages of active RFID and is realized based on this technology.
3 Warehouse Management System
3.1 Overview of Warehouse Management
The current warehouse management is mainly divided into three parts: one is the recording and management of incoming and outgoing cargo information; the second is the recording of related information about incoming and outgoing vehicles (containers); the third is the positioning of containers in the warehouse. For the requirements of the first two parts, the vast majority of warehouses still use manual recording methods, causing problems such as high labor costs and high error rates. The active RFID-based warehouse management system proposed in this paper is mainly used to meet the requirements of the first and second parts, which can effectively improve the efficiency of warehouse management.
3.2 System Architecture
The entire warehouse management system is shown in Figure 11. The goods in the warehouse are loaded by trucks and transported into the warehouse through the warehouse gate. Goods are sometimes placed in a container. Therefore, the RFID mobile node in this system, that is, a tag, is usually installed in a truck (container). Since any goods entering and leaving the warehouse must pass through the gate, the RFID fixed node, that is, the card reader, is installed on the warehouse gate, and it is connected to the client PC via a USB cable. At the same time, the PC client is connected to the PC as the server through a 2.4G wireless local area network covering the entire warehouse, and the PC as the server is the general console of the entire system. The information received by the card reader is finally returned to the PC console through the PC client.
3.3 hardware design
The hardware of the node. Among them, the control chip and the radio frequency chip are respectively Silicon C8051 F31 0 and Chipcon CCI 1 O0. Among them, the RFID mobile node is equipped with a FreescaleMMA726O acceleration sensor to obtain the current movement state of the truck (or container). The mobile node also uses the 16KB Flash memory inside the MCU to store cargo information. At the same time, the RFID fixed node is equipped with an FT232 serial-to-USB chip, which is used to communicate with the console through the USB interface. In terms of power supply, mobile nodes are powered by 1 800mA mobile phone batteries, and fixed nodes are powered by external power sources.
The communication frequency between the RFID card reader and the tag in this system is set at 433MHz, but the common 2.4GHz. The use of 433MHz as the communication frequency of the radio frequency chip is based on the following considerations: 433MHz frequency has lower requirements for the transmission environment. Since this system is used in a warehouse, considering that there are a large number of goods stacked in the warehouse, and mobile nodes may be installed in vehicles (or containers) ), so the transmission penetrability requirements are higher. In contrast, 2. 4GHz is more suitable for transmission in open locations. In addition, this system does not have high requirements for the transmission distance (about 100 meters) and speed. After actual measurement, the 433MHz frequency is fully competent.
4 Software design
4.1 Software Architecture
The software structure of RFID node is shown as in Fig. 3. The introduction of the concept of physical layer and media access control ~ J (MAC) layer in wireless sensor network (wSN) here is a fusion of WSN technology and RFID technology. By constructing these two layers of protocol stack, the RFID system can have a clearer software architecture.
Above the hardware, it is the driver of each hardware module, including SPI, serial port, on-chip Flash, acceleration sensor and radio frequency chip CC11 OO driver. Among them, the CC1100 driver is a part of the physical layer, which provides parameter settings such as the frequency of the radio frequency chip and wireless transceiver functions. Above the physical layer is the MAC layer protocol stack designed by this system.
The MAC layer protocol stack includes the definition of RFID data packet format, the allocation of node addresses, and the sending and receiving of data packets. A total of 12 types of RFID data packets are defined, 4 types are used for access control management, and 8 types are used for cargo management; the network address of RFID is represented by one byte; and 4 functions are designed for data packet sending and take over. Above the driver and physical layer, in addition to the MAC layer, some system calls are defined to provide functions such as MCU and RF chip state switching, mobile node motion state detection, and cargo information recording on Flash in cargo management. The detailed structure of each protocol layer introduced above is shown in Figure 4.
With the support of the underlying software structure, the uppermost software architecture corresponds to the functions of the application layer and the network layer in the OSI model, and realizes functions such as access control management and cargo management. This layer is divided into upper and lower layers. The lower layer corresponds to the functions of the network layer. A set of communication protocols with low-power algorithms are designed for the communication between RFID readers and tags (see section 4.2). Based on the communication protocol of the network layer, the two functions of access control and cargo management can be realized at the application layer.
4.2 RFID communication protocol
As mentioned in section 4.1, the communication protocol between the RFID reader and the mobile node constitutes the software foundation of the two main functions of the warehouse management system. The design of the communication protocol is mainly divided into two parts, one part is used for data communication during positioning of mobile nodes, and the other part is used for data communication during cargo information inquiry and management.
First of all, the RSSI algorithm is used in this system to locate the mobile node. The RSSI algorithm uses the strength of the signal during communication between nodes as the positioning parameter. However, because the signal strength and distance are not completely monotonous, according to the actual measured signal strength data, the positioning accuracy is reduced, and the signal strength within 5 meters is averaged in units of 5 meters to obtain the signal strength. Monotonically decreasing relationship between distance and distance. Based on this, the mobile node can be located according to the RSSI algorithm.
The specific positioning process is as follows: the two-way handshake protocol similar to the TCP/IP protocol is adopted to ensure the robustness of the communication. The card reader broadcasts a location query request, which contains N thresholds representing different ranges. When the mobile node receives this request, it detects the current signal strength value, compares it with these N thresholds, determines its current location, and responds. When the reader receives a response from a tag, it sends a second confirmation request to the tag, and the tag replies to this request. At this time, the card reader thinks that the positioning of the tag is successful, and returns the result to the console through the serial port.
For the communication of the second type of cargo query management, the specific process is as follows: the PC console sends a cargo management command to the card reader. Cargo management commands include querying, modifying, adding and deleting cargo information in a truck (or a container). When the card reader receives the instruction, it forwards the instruction to the mobile node, and the mobile node performs corresponding operations in the internal Flash according to the content of the instruction, such as query and modification, and returns the result to the card reader. After receiving the result, the card reader returns to the console, and the console updates the database.
4.3 Low-power algorithms
In this system, the mobile node is designed as an active RFID, so low power consumption becomes a problem that must be solved. The following methods are applied in this system to realize low power consumption algorithms. First, the mobile node is in a sleep state when it is not working, and wakes up once every 3 seconds on average, and wakes up about 4ms each time. Since the power consumption in the sleep state is negligible, the actual power consumption is about 1/1000 of the wake-up state. Secondly, the mobile node can use the acceleration sensor to detect the current state of its own movement. If the node is moving, it means that the goods are likely to be passing through the warehouse gate. At this time, in order to monitor the correctness, the wake-up time is shortened to 0.5 seconds: as usual When the node is stationary, the time interval for waking up can be increased. This method can get the power?
Good allocation can further reduce power consumption. Third, when positioning, the mobile node will record where it was last time. If there is no change in its position in a new query, it will not communicate with the card reader for the next step and go to sleep directly. state.
4.4 Data encryption
In addition, because RFID transmits data through radio frequency chips, the data in the transmission process may be intercepted, resulting in the leakage of warehouse information or malicious changes. Therefore, in the design of the system application layer protocol stack, data encryption is added. This ensures that the data processed below the application layer are all encrypted data, that is to say, the data packets in the transmission process are all encrypted. When the receiving node receives the data packet and parses it layer by layer to its application layer, the encrypted information will be decrypted by the console after it is returned to the console, which can ensure the security of the warehouse data and make this system applicable Used in military warehouses with high security requirements.
5 Summary
This paper proposes a warehouse management system based on active RFID. It not only gives the hardware design scheme and the design of the communication protocol between nodes, but also adds low-power algorithms, so that the system can be practically applied to warehouse management, which is useful for improving warehouse vehicles. It has important practical significance to manage the entry and exit of containers and record cargo information, which can greatly reduce labor costs and improve efficiency.
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