Design of wireless sensor networks for industrial use

1. Wireless sensor networks are a new hotspot in industrial automation

Wireless sensor network technology has entered the field of industrial automation and industrial measurement and control. Most industrial instruments and automation products will soon be embedded with wireless transmission functions, completing the transition from wired to wireless.

Figure 1 is a typical schematic diagram of an industrial wireless sensor network. The core components are low-power sensor nodes (which can be powered by batteries for a long time, solar cells, wind energy, mechanical vibration power generation, etc.), network routers (with mesh network routing functions) and wireless gateways (which transmit information to industrial Ethernet and control centers, or transmit information through the Internet).

Figure 1 Typical industrial wireless sensor network

Due to the huge market, many established players in the field of industrial automation, such as GE and Honeywell, have launched various industrial wireless sensor network products and systems. There are also many research institutes and large companies in China that are conducting related research. However, the technologies involved in wireless sensor networks are highly confidential. It is difficult for ordinary engineers like us to understand the details and have the opportunity to participate in any design work.

So, as ordinary engineers engaged in automation and industrial control, can we have the opportunity to design industrial wireless sensor network products that suit our own application needs? To develop the wireless industrial automation projects we need?

The development of wireless SoC technology will make our dreams become reality, and now should be an obvious turning point and intersection.

The answer is yes: it is entirely possible for us to design an industrial wireless sensor network that suits our application characteristics.

2. Choose the right microcontroller and development platform

The radio frequency communication conditions in industrial environments are relatively poor. The multipath effect caused by the reflection and scattering of signals by various large-scale equipment and metal pipes in the factory, as well as the electromagnetic noise generated by the operation of motors and equipment, will interfere with the correct reception of wireless signals. At the same time, the strong electromagnetic interference in the industrial environment also poses new challenges to the core microprocessors used in industrial wireless sensor networks. To design an industrial network system that runs in such an environment, we first need to select a suitable microprocessor and high-frequency circuit.

FIG2 is a schematic diagram of a typical hardware structure of an industrial wireless sensor network node.

At present, TI and FREESCALE have launched three sets of the latest wireless microcontroller solutions: MC13224, CC2530, MSP430F5437+CC2520, which are all good SoC microcontroller solutions (see Table 1). These solutions are characterized by highly integrated design, microprocessor and wireless transceiver inside the same chip, requiring a circuit board area of ​​less than 2 square centimeters, and only a few peripheral parts, which have strong anti-interference capabilities. The gateway, router and sensor node of the industrial wireless sensor network can all be designed using the same microprocessor.

The above scheme, under the premise of ensuring system reliability, has the biggest features of economy and convenience, because the price of wireless MCU chips is very low, even lower than many types of ordinary MCUs, designers can design and debug without worrying about chip damage, etc. In addition, the knowledge of embedded design in China is already quite popular. The core technology of designing industrial wireless sensor network gateways, routers, nodes and designing ordinary MCU systems we are familiar with is no different. Moreover, IAR compilation and debugging system is the most powerful commercial embedded C language software design tool in the world. With the wireless MCU development platform, sample engineering design, and JTAG online emulator provided by Chengdu Wireless Dragon Communication, you can accurately locate the fault to each line of instruction, realize slow-motion replay of wireless networking and communication, and capture wireless data packaging in the air at any time. With the help of these advanced debugging and development tools, the entire process of wireless communication software and hardware design is completely transparent and controllable, allowing you to complete the design task quickly and easily like developing your other MCU systems.

3. ZigBee PRO meets the requirements of industrial wireless network design

Different from home-oriented wireless network technologies (ZigBee 2004 to ZigBee 2006 belong to this category of home-oriented technologies), wireless network technologies for industrial automation applications need to meet the following five requirements.

High reliability: Most industrial control applications require a reliable data transmission rate of more than 95%. In order to achieve high-reliability transmission using wireless communication in industrial sites, the following challenges are faced. The ZigBee PRO protocol stack uses a 2.4 GHz physical layer based on DSSS (Direct Sequence Spread Spectrum) technology (including data modulation, activation and sleep RF transceiver, channel energy detection, link quality indication of channel receiving data packets, idle channel assessment, data transmission and reception, etc.) with strong anti-interference ability, and the MAC layer and application layer (APS part) have response retransmission function. In addition, the CSMA mechanism of the MAC layer enables the node to monitor the channel before sending, which can also avoid interference. The network layer adopts a mesh network networking mode. There can be multiple paths from the source node to the destination node. The redundancy of the path enhances the robustness of the network. If there is a problem with the original path, such as interference, or one of the intermediate nodes fails, ZigBee PRO can perform routing repair and select another suitable path to maintain communication. At the same time, the frequency agility newly added by ZigBee PRO also greatly enhances its reliability as an industrial network. When the ZigBee PRO network is interfered by external factors, such as wireless interference from various industrial sites, and cannot work normally, the entire ZigBee PRO network can automatically and dynamically switch to a clean working channel among all 16 channels (to achieve FHSS frequency hopping function). Compared with other industrial wireless network protocols currently using DSSS+FHSS, ZigBee PRO has better reliability and anti-interference. The wireless microcontrollers in Table 1 can all support the ZigBee PRO wireless network protocol stack.

Strict real-time performance: For industrial closed-loop control applications, data transmission delay should be less than 1.5 times the sensor sampling time. ZigBee PRO network is optimized for industrial communication applications that are sensitive to delays. The communication delay and the delay from sleep state activation are very short. The typical value of device search delay is milliseconds, the typical value of sleep activation delay is 15ms, and the active device channel access delay is 15ms. Combined with ZIGBEEPRO's new routing algorithm, the network routing efficiency is greatly improved. The delay in transmission through multi-hop relay is greatly reduced, which can fully guarantee the real-time performance of end-to-end communication.

Low energy consumption: Due to cost and installation restrictions, wireless sensor network nodes used for ubiquitous sensing of the entire industrial process usually do not use external power supplies, but rely on their own batteries. Due to the revolutionary new designs of the new wireless microcontrollers and ZIGBEEPRO wireless front-ends listed in Table 1, the battery life of the nodes should reach 3 to 10 years. Industrial wireless sensor networks that use the least energy can be realized.

Security: With the advancement of industrial control system networking, network security and data security issues are becoming increasingly prominent, and some security vulnerabilities will cause huge losses to industrial control applications. Wireless communication is more vulnerable to attacks due to the open nature of the channel, and its security mechanism will be more complex.

A high security mode is designed for industrial network applications. When a node joins the network, the Trust Center (TC) will first assign a master key to the newly joined node. Then, the newly joined node uses this master key to establish a link key with any node in the network through the SKKE process. Finally, the link key is encrypted to generate a network key shared by the network. The network key (NWK Key) is placed in the application layer payload and sent to the other party, and then the encrypted data is transmitted through the network. ZigBee PRO's security design can fully meet the main requirements of industrial wireless networks for secure communications.

Moreover, as shown in Table 1, the new 16-bit and 32-bit wireless MCUs have powerful data processing capabilities and are fully capable of implementing complex security algorithms to meet the challenges posed by industrial wireless sensor networks.

Compatibility: In order to protect the original investment of users, the industrial wireless sensor network must have the ability to interconnect and interoperate with the original wired control system of the factory. The wireless gateway designed with ZigBee PRO can achieve seamless connection with the current industrial Ethernet, CAN bus, various industrial control buses, and IP communication with the Internet. ZigBee is also an important standard for global wireless sensor networks and is a network protocol software for industrial wireless sensor networks with good compatibility.

In summary, the ZigBee PRO wireless network, which targets sensing and control, has an enhanced commercial-grade and industrial protocol stack, which can fully meet the requirements in the above five aspects. Using the ZigBee PRO protocol stack, you can design industrial wireless sensor network projects and products that meet your own special application requirements, as shown in Figure 2.

4. From Wired to Wireless, We Welcome New Technical Challenges

This article focuses on the implementation of the industrial wireless sensor network. In fact, under the premise of realizing the two-way communication between the industrial wireless sensor network and the nodes, it is very easy to realize the wireless control of industrial equipment, including relays, I/O, switch control, and motor control. It is a natural thing that only requires some small expansions in software and hardware.