How to Start Designing a Wireless Sensor Network System

1. Wireless Sensor Network technology is widely used and flourishing Wireless sensors and sensor networks are new technologies with very broad market prospects and will have a profound impact on all areas of human life and production. When discussing the top ten emerging technologies in the future, the American magazine "Technology Review" listed wireless sensor networks as the first emerging technology in the future. Wireless sensor networks are also included in the four new technologies predicted by "Business Week" in the future. Wireless sensor networks have a very broad application prospect and have great application value in traditional and emerging fields such as industry, agriculture, military, environment, medical care, digital home, green energy saving, and smart transportation. Wireless sensor networks will be everywhere and will be fully integrated into our lives. Figure 1 is a schematic diagram of wireless sensor applications.

Figure 1 Some examples of wireless sensor applications

Due to the huge market and application prospects of wireless sensors and wireless sensor networks, many companies around the world have launched their own wireless sensor networks. These technologies are flourishing and each has its own advantages, but they are almost incompatible and interoperable with each other.

Figure 2 shows the various wireless sensor technologies currently under development. From this figure, we can see that different wireless sensor networks ultimately hope to achieve communication with the Internet, which may be the channel for these sensor networks to finally converge.

Figure 2 Some of the wireless sensor network technologies currently under development worldwide

2. How to choose the right wireless sensor technology

The basic architecture of a wireless sensor network system consists of three parts. The first part is a wireless transceiver chip, which is responsible for converting digital information into high-frequency wireless signals for transmission and restoring received high-frequency wireless signals into digital information. For wireless sensor transceiver chips, IEEE 802.15.4 can provide the best solution for wireless sensor applications, because the IEEE 802.15.4 specification may be the main and possibly the only practical standard. Currently, there are many companies around the world that provide transceiver chips in this area. Chips such as TI's CC2420/CC2520 are particularly suitable for low power consumption characteristics of button batteries and low-power applications.

To implement a typical wireless sensor network node and router, a multi-chip solution can be used, as shown in Figure 3, which consists of a wireless transceiver chip and a microcontroller (single-chip microcomputer). The microprocessor can use the low-power MSP430, and the wireless chip can use CC2520/CC2420, etc.

Figure 3 Typical wireless sensor network node or router

With the continuous development of technology, more and more companies have made wireless transceiver chips, microcontrollers and wireless transceivers into a system on chip (SoC). For example, TI uses ZigBee wireless microcontrollers such as CC2430/CC2431 with 8051 core. As wireless sensor networks require higher computing power, Freescale has recently launched 32-bit ZigBee wireless microcontrollers with ARM core. Using these SoC wireless microcontrollers to design wireless sensor networks will make wireless sensor nodes smaller, lower power consumption and lower price; TI's domestic technical partner Wireless Dragon Technology Co., Ltd. also provides these chips and related technical support for development tools. The

second part of the wireless sensor network architecture is the embedded software running inside the microcontroller or wireless microcontroller, also known as the software protocol stack (network stack). The network stack has two responsibilities. First of all, it must handle the frequent changes in the quality of wireless link communication between nodes and the interference caused by environmental factors on wireless communication, and have the ability to self-organize and self-recover the network; the second function of the network stack is to have a strong routing algorithm capability to ensure that information is reliably and efficiently sent from the source node (if it exists, it can be sent through hundreds or thousands of routing nodes) to the target node through various network topologies (star/mesh, etc.). Ensure the real-time requirements of communication.

The ZigBee Alliance is an independent standards organization composed of many technology suppliers and developers. It is also the world's largest alliance providing network software protocol stack standards based on the IEEE 802.15.4 platform.

The organization has been continuously developing from ZigBee2004, ZigBee2006, and ZigBee2007. The two network stacks currently provided are: ZigBee and ZigBee PRO. From the perspective of use, the ZigBee stack is very suitable for small networks that generally contain ten to hundreds of nodes. ZigBee PRO is a superset of ZigBee. It adds some functions to expand the network and better deal with wireless interference from other technologies. It can adapt to larger networks and has more reliable routing communication algorithms and wireless communication reliability. The third part of the wireless

sensor network architecture is application software. This part includes various software codes developed according to the user's existing development. Most of these codes are currently developed in C language, and can call the functions of the software protocol stack in the form of interfaces and APIs.

Among the various wireless sensor network technologies, we believe that using the 802.15.4 international standard and ZigBee technology as the starting point for our design of wireless sensor systems has the following advantages:

compatible with a global and reliable international standard.

Large chip suppliers such as TI and Freescale can be used to obtain stable sources of wireless transceiver chips and wireless microcontrollers, and free ZigBee protocol stacks and related source codes can also be obtained to lower the threshold for opening.

The ability to use high-performance software compilation and debugging environments such as KEIL and IAR can greatly speed up the opening speed and shorten the listing cycle.

What should I prepare for the design of a wireless sensor network system?

First of all, we need to prepare some knowledge and the technology and knowledge required for wireless sensor networks. Although we may be familiar with microcontrollers and related software development technologies, wireless transceivers and wireless SoCs (wireless microcontrollers) still have their own unique features, and IEEE802.15.4 and ZigBee protocol stacks are also knowledge areas with certain difficulties. Fortunately, there are already a large number of technical books available for reference in wireless sensor networks and wireless microcontrollers. Figure 4 shows some technical books related to wireless sensors, which may be helpful for getting started with wireless sensor networks.

Secondly, we still need an easy-to-use wireless sensor network (WSN) development system, because:

we need a complete software compilation and development platform, including IAR and KEIL compilation and debugging environments, online simulators and other necessary development tools.

The difficulty of our entry into wireless sensor system design is to master the use of wireless sensor network protocol stack software as soon as possible, and enter the development of related application software as soon as possible, so we need a set of wireless nodes, gateways, routers and wireless modules that have completed high-frequency testing to conduct hardware evaluation and run our embedded application software and protocol stack software.

We need to interface related temperature, pressure, acceleration, light, humidity and other sensors to this system to facilitate our system design.

When we enter an unfamiliar technical field, we often encounter many difficulties, and we need corresponding technical support and knowledge support.

At present, many domestic companies have launched various wireless sensor network development tools. Figure 5 is a new product of Chengdu Wireless Dragon Communication Technology Co., Ltd., a domestic company, which supports the American Texas Instruments TI CC2520 wireless transceiver and TI's ZigBee2007/PRO protocol stack wireless sensor network (WSN) development system. It includes an online simulator, PC GUI network monitoring and control software, related source code wireless sensor network demonstration code packaging, etc. It is one of the optional domestic tools for quickly entering the wireless sensor network system design.

Wireless Sensor Network Development System

4. Specific process of designing wireless sensor system

After we have completed the above knowledge preparation and relevant development tool preparation, we can start a wireless sensor design process. Next, we take a home energy-saving wireless sensor network system as an example to see the actual process of a wireless sensor network. The block diagram of the home energy-saving system is shown in Figure 6.

Figure 6 Home energy saving system using ZigBee network technology

From the figure, we can see that the electrical appliances in the home, including air conditioners, washing machines, refrigerators, etc., constitute a typical wireless sensor network. Through the energy management gateway and the wireless forwarding router installed outdoors, the data is transmitted to the energy company's computerized management network and database in real time to achieve the management of home energy.

The design tasks include designing wireless sensor network units (wireless nodes) embedded in home appliances, home wireless display units and home energy control units (wireless nodes or wireless routers), energy management gateways (wireless gateways), etc.

First, we use multiple wireless SoC modules (including wireless microcontrollers such as CC2430) provided by the Wireless Dragon WSN wireless sensor development system platform, download TI ZigBee protocol stack and smart energy related codes, and conduct wireless communication effect and reliability evaluation. This software package is provided free of charge by TI (after actual testing and evaluation by Wireless Dragon engineers) and can be used for technology development free of charge. Figure 7 is the software structure.

Figure 7 ZigBee Smart Energy Software Package

Using the powerful KEIL and IAR compiler development environment, online emulator and other debugging tools, and the completely open source ZigBee protocol stack software, we should be able to quickly complete the performance evaluation of wireless nodes, routers, and gateways, communication reliability evaluation, and ZigBee wireless communication code development.

Then we can still directly use these wireless SoC modules to add our application design, such as LED and LCD display parts, which can display the real-time electricity bill and whether there is energy waste at home. At the same time, we design the relevant control circuits we need, such as switch control and motor control, so as to perform energy-saving control on appliances such as air conditioners when necessary.

Since there is a set of network protocol analyzers and monitors in the development tools provided by Wireless Dragon, we can clearly monitor the various wireless packaging contents transmitted in the air during the test process, so as to quickly determine whether our wireless communication software code is correct.

Due to the use of ready-made high-frequency modules, we avoid the difficulties of high-frequency technology involved in getting started, but focus our main energy on the most important direction, our application code and the hardware design required by our application, which greatly speeds up the progress of system productization; for the wireless modules used in the products in customer orders, we can easily find OEM professional manufacturers to produce them. If everything goes well, this ZigBee home energy-saving system will hopefully be completed in 3-4 weeks.