Design of Industrial Wireless Gateway Based on ZigBee

introduction

Based on the requirements for the real-time nature of monitoring data transmission, the openness of data interfaces, and the security of data links, many companies and research institutions at home and abroad began to study and establish industrial wireless measurement and control systems. ZigBee short-range wireless network communication technology has become one of the research hotspots in the industry in recent years due to its advantages of safe and reliable data transmission, simple and flexible networking, low equipment cost, and long battery life. The American Institute of Instrumentation and Automation has established the Industrial Wireless Standards Committee (ISA1OO). The committee is dedicated to the standardization process of industrial wireless technology, and it is expected that official standards will be issued by the end of 2008. The purpose of the ISA100 standard is to enable industrial wireless devices to support industrial field applications with low complexity, reasonable cost, low power consumption, and appropriate data communication rates. The main contents of the standard include industrial wireless network architecture, coexistence, robustness, interoperability with wired field networks, etc. This design project is an industrial wireless gateway, which is mainly used to solve the interoperability problem between industrial wireless network and wired network.

1 Introduction to ZigBee wireless communication technology

ZigBee technology is a short-range wireless communication technology with a unified technical standard. The standard takes low power consumption, low rate transmission, low cost, and low complexity as important goals. The physical layer (PHY) and media access control layer (MAC) protocols are based on the IEEE 802.15.4 protocol standard, and the network layer (NWK) and application layer (APS) are formulated by the ZigBee Alliance. ZigBee technology defines its work in the * fund project: the National 863 Plan project, based on the smart meter of 802.15.1 (number 2006AA040302-2). 2. On the 4 GHz ISM frequency band, the transmission rate is 20 kb / s to 250 kb / s. Communication equipment based on ZigBee technology has a coverage of tens of meters, and can also increase the routing node to expand the coverage, so it is more suitable for industrial control, remote control and other related fields. ZigBee communication equipment can join and leave the network freely and flexibly. Its low power consumption and low cost extend the working time of the equipment and reduce the overall cost of the system.

2 Industrial wireless gateway model design

2.1 Demand analysis

At present, the wireless gateways used in industry mainly use communication technologies such as 802.11b, GPRS, and Bluetooth. The high cost, high power consumption, poor mobility, and poor security limit the application of such traditional gateways. The use of ZigBee wireless communication technology can be larger To some extent improve these properties. ZigBee wireless gateway can gradually replace the traditional gateway in industrial applications due to its technical advantages. At present, it has begun to build an industrial measurement and control system in a small area for test application. In terms of function, the traditional gateway mainly completes protocol conversion and part of the system management functions, and the ZigBee wireless gateway, as an access component for the wireless network to access the wired network, should have some unique functions in addition to the functions of the traditional gateway: Wireless network management functions, including wireless network formation, wireless device joining and disconnecting management functions; in terms of protocol conversion, wireless and wired protocols must be developed at the same time, data packets are converted between wireless and wired; in order to achieve monitoring of wireless networks And control, in the development of wireless and wired protocols, we must consider providing corresponding services and service interfaces for the upper layer.

2.2 Design of communication model

Based on the above analysis, a wireless gateway for industrial Ethernet is designed for industrial applications. The communication model of the wireless gateway is shown in Figure 1.

The communication model mainly includes the following three aspects:

â‘ Wireless communication mechanism. The data communication between the field device and the wireless gateway uses ZigBee wireless communication technology. ZigBee wireless communication technology adopts CSMA-CA access mode, which effectively avoids conflicts between radio carriers and ensures the reliability of data transmission. The MAC layer and PHY layer are formulated by the IEEE 802.15.4 working group, NwK and APS are formulated by the ZigBee Alliance, and the other parts-ZDO (ZigBee device object) and ZAO (ZigBee application object), are determined by users according to different applications carry out.

â‘¡ Ethernet protocol conversion. The access function of the wireless gateway is mainly reflected in the protocol conversion, that is, the ZigBee wireless communication protocol is converted into the Ethernet wired protocol, and the control network is accessed through the Ethernet. IEEE802.3 PHY and IEEE802.3 MAC are standard Ethernet physical layer and medium access layer, IEEE802. 2 LLC provides Ethernet frame and IP layer interface, and the transport layer is standard TCP. / UDP protocol.

â‘¢ upper layer service interface (high layer service inteRFace). For industrial applications, wireless gateways are required to provide upper-layer services and interfaces so that users can configure and adjust field devices through the wireless gateway. The upper service interface is located between the ZigBee APS layer and the TCP / IP layer, and provides a common interface for the system to implement various services.

3 Research on key technologies of industrial wireless gateways

3.1 Network management function

(1) Network formation

After the wireless gateway is powered on, each layer of the wireless protocol stack is initialized first, and then a new network is started through the network request primitive. Only when the gateway device that has the coordinator capability and is not currently connected to the network can a new one be established The internet. Figure 2 shows the network formation process. At the beginning of networking, the network layer first requests the MAC layer to allocate the channel specified by the protocol, or the PHY layer performs effective channel scanning. The network layer management entity waits for the channel scanning result, and then selects a channel with an allowable energy level according to the scanning result. After finding a suitable channel, choose a personal area network identifier (PANID) for this new network. The PANID can be specified when the network forms the request, or a PANID can be randomly selected (except for the broadcast PANID is fixed at 0xFFFF). The PANID should be unique in the selected channel. Once PANID is selected, the wireless gateway will select the 16-bit network address 0x0000 as its short address, and make related settings at the same time. After completing the settings, send a network start request through the MAC layer and return to the network formation state.

(2) Network maintenance Network maintenance mainly includes the process of equipment joining and leaving the network. After the network is formed, the MAC layer connection permission flag is set by the network management entity to determine whether to allow other devices and devices to initialize as the coordinator to enter the network. There are joint methods and direct methods for joining. In the protocol implementation, direct network joining is adopted. In this way, the device to be added sends a request to join the beacon frame. After the gateway receives it, the network management entity first determines whether the device already exists on the network. If it exists, it is added to the network; if it does not exist, it sends a beacon frame to the device and assigns this device a unique 16-bit short address in the network. The beacon frame here is generated by the gateway wireless protocol MAC layer as the PHY layer payload, and it contains information such as PANID and time slot assignment. Devices in the network can also request to disconnect from the network. When the gateway receives the device disconnection request, the MAC layer sends a report to the network layer, starts the disconnection process, and deletes the device-related information from the device list.

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