Application of RFID ZigBee system in third-party logistics

This article uses the flat warehouse of the growing third-party logistics company in Tianjin Binhai New Area as the application environment to establish a set of intelligent warehousing solutions based on the concept of the Internet of Things and with scalable value. In the RFID system of smart warehousing, each or several RFID electronic tags need to have its corresponding reader/writer for information sensing, and each group of RFID Card readers also needs to exchange data with the server. However, in warehousing operations, the road surface for lifting vehicles needs to be smooth to ensure smooth operation, and the warehouse space is tall and has a wide span, so wired layout is inconvenient. Wireless networks must be used to achieve wireless data interaction. Therefore, the increasingly mature ZigBee technology is used to wirelessly network RFID card Readers to achieve dynamic data exchange.

1 ZigBee ad hoc network solution demonstration

Based on the operating characteristics of third-party logistics warehousing and the actual transmission distance requirements, after horizontally comparing several wireless transmission methods, a wireless ad hoc network solution based on ZigBee technology was selected. ZigBee ad hoc network is a high-reliability wireless data transmission network platform based on IEEE802.1 5.4 wireless communication technology and composed of up to 65,535 wireless data transmission modules. Within the entire network, each ZigBee data transmission module can communicate with each other, and the distance between nodes can be infinitely expanded from the standard 75 m. It has the characteristics of easy use, reliable operation, and low price.

Compared with other networking methods, ZigBee technology first adopts an ad hoc network communication method. Each ZigBee node can work independently. Once a problem occurs on a node, data can be transmitted through other nodes, and new RFID can be transmitted anytime and anywhere. When the reader joins the network, it will not affect the use of the entire network; if Wifi technology is used, a failure of an AP will cause all RFID Readers in the coverage area to be unable to exchange data. Secondly, the ZigBee protocol stack is simple and relatively easy to implement. Running ZigBee requires a system The resource is about 28 Kb, while the Bluetooth protocol stack is relatively complex and requires about 250 Kb of system resources. In addition, ZigBee is more flexible than Bluetooth and is more conducive to controlling system costs.

In warehousing operations, according to the operation process, a large number of data transmission nodes are required, and the cost of a huge number of communication equipment and communication costs when running on the network directly affect the system cost; ZigBee technology will not generate more besides the initial investment cost. Daily usage charges. Although the transmission rate of ZigBee is not fast (the 2.4 GHz frequency band is only 250 Kb/s), considering the characteristics of warehousing operations, the electronic tag on the cargo pallet only writes the ID number of the cargo, and the byte length is usually within 32 B, so It will not have too much impact on the transmission rate and meets normal working conditions. Moreover, ZigBee has low power consumption. Under the same power supply environment, the continuous working time of Bluetooth and WIFI is much shorter than ZigBee.

RFID related protocols only stipulate communication interfaces, while ZigBee has a relatively complete communication networking protocol. ZigBee can choose the 2.4 GHz ISM frequency band (global common frequency band) in the working frequency band, while RFID can work in 915 MHz or other frequency bands. Both Do not interfere with each other on the communication frequency. Considering that the warehouse is an indoor environment and the distance between nodes is relatively close, the ZigBee module can penetrate obstacles of a certain thickness during operation, so the signal attenuation is negligible. Through self-organizing network protocols, devices in the network can communicate directly or indirectly wirelessly. The reliability and frequency utilization of the network are very high, and ZigBee has a relatively complete security authentication mode.

In summary, it is most appropriate to choose ZigBee technology as the data transmission network for the RFID system in third-party logistics smart warehousing.

2 Network principles and structures

A ZigBee network needs to be composed of a central coordinator (Coordinator) and a router (Router). Each ZigBee network requires and only needs one central coordinator to create the network. When a node joins, it allocates addresses to child nodes; the router is responsible for sending, receiving and forwarding data, and finding the most suitable routing path. When a node joins, Nodes are assigned addresses when joining, so a ZigBee network may require multiple routers. When a network consists of a central coordinator and N routers, the network is a real MESH network, and all data sent by each node is automatically routed to the target node.

Application of ZigBee-based RFID system in third-party logistics smart warehousing

The RFID system based on ZigBee self-organizing network technology adopts MESH network structure and consists of a main control node and several sub-nodes (the number of sub-nodes depends on the number of RFID readers in the warehouse), as shown in Figure 1. The main control node is composed of the server and the central coordinator through the serial port; the sub-node is composed of the card reader and the router through the serial port. The serial port selects RS 232 two-way communication mode.

After all ZigBee devices are started, the master node will start to build a ZigBee network, add all sub-nodes to the network, assign network addresses to each sub-node, and Store the information in the database. After the card reader collects the tag data, it first sends the data to the router connected to it. The router then sends the tag data together with the card reader information to the master control node through the multi-hop ZigBee network for storage. The master control node sends the reader to the card reader. The parameter configuration command is sent to the reader through communication with the router, and then the location node device enters the power saving mode; when the main control node issues a command to the child node, the location node device can be awakened at any time, by looking for the network of the child node address, and then convey the command to the router through the multi-hop network according to the network address, and then pass it to the corresponding card reader through the router. Finally, the router sends a confirmation notification of receipt of the command to the master node.

3. Network hardware layout

Taking a third-party logistics flat warehouse as an example, card readers are installed in each cargo space and at the entrance and exit of the warehouse. The goods enter and leave the warehouse with the pallet. The electronic label is affixed to the pallet, and the label stores the ID number of the goods. The ZigBee equipment used for networking is based on the TI company's CC2530F256 chip, runs the ZigBee2007/PRO protocol, and the circuit has been integrated into the ZigBee module. It has all the characteristics of the ZigBee protocol. The advantage of using the module is that users do not need to understand the complex ZigBee protocol. All ZigBee protocol processing is automatically completed within the ZigBee module, and the node programs are written into the module in an embedded manner. The user only needs to transmit data through the serial port.

Among them, the ZigBee module, the card reader and the server follow the RS 232 asynchronous serial two-way communication format; all RFID card readers and their indicator signals are connected to the microcontroller through the hub device, and are uniformly controlled by the microcontroller. The card reader It is also connected to the microcontroller through the RS 232 serial port. The power supplies of the ZigBee module, RFID card reader and microcontroller are all between 5 and 12 V, using standard TTL levels.

The server and main control ZigBee module are installed in the general dispatch room of the warehouse and are used for sending and receiving instructions and normal dispatching of warehousing operations; all card readers and ZigBee devices are installed in the actual warehouse locations and warehouse entrances and exits to become a single network node. All nodes are composed of RFID system based on ZigBee wireless transmission. The cargo space status indicator light controlled by the microcontroller is installed above the cargo space to remind the staff of normal warehousing operations; the visual interface converted after processing by the database is displayed on the host computer in the dispatch room on the one hand, and on the other hand displayed in the warehouse A large screen is provided for staff to browse.

Application of RFID (ZigBee) system in third-party logistics

When other workflows and emergencies occur during warehousing operations, such as inventory counting, transfer of goods to warehouse areas, misplaced goods or damaged labels, etc., the RFID reader wakes up the ZigBee router in time through the signal sensed in real time, and then transmits the information in a timely manner. It is handed over to the central coordinator to the backend database for storage management. Almost all processes liberate workers from traditional operating methods. The accurate information collection mode improves the reliability of data management of the system and effectively prevents human errors.

4 Conclusion

After actual operation testing, ZigBee modules are separated by about 3 m from each other in an indoor environment, and the signal is good. Under normal warehousing operating conditions, the time it takes for the server to receive data packets is about 20 to 40 ms, which meets normal working conditions.

An ordinary third-party warehouse with about 500 cargo spaces requires about 500 ZigBee modules. The unit price of ZigBee modules on the market is basically RMB 40 to RMB 60. Therefore, the cost of installing a ZigBee self-organizing network system in the warehouse is between 20,000 and 30,000 yuan. For large and medium-sized warehousing companies, it is cost-effective. However, for some small warehousing companies that value profits, it is still difficult to improve their business immediately. Therefore, if costs cannot be effectively controlled, the plan will not be generalizable. According to different enterprise sizes and different warehouse characteristics, the number of ZigBee modules can be appropriately reduced during the implementation of the actual solution. For example, several adjacent RFID card readers can share a ZigBee module through a hub device, thus effectively reducing costs. However, the ensuing problem is that the amount of data transmitted by each ZigBee module at a time is very limited. If the problem of data sequential transmission method and transmission interval cannot be solved well, the response speed and work efficiency of the system will be greatly affected. However, I believe that these problems will be solved in the near future with the continuous development of ZigBee technology.