Low-power wireless sensor

The Internet of Things revolution is within easy reach. By 2020, there will be more than 30 billion connected objects in the world. As the world’s population grows and resources become more valuable, this kind of interconnection is expected to provide real-world data to promote efficiency and simplify business practices.

With the widespread adoption of Internet Protocol (IP), it has become easier to process data and make full use of information. Fortune 500 companies provide enterprise and database solutions for data storage and provide software tools to streamline business processes such as asset tracking, process control systems, and building management systems. Smartphones and tablets provide people with very useful information, such as providing real-time parking information, or providing real-time monitoring information about the health of the machine to make maintenance plans. Many wireless sensors have been deployed so far, but in order to measure and optimize processes that were not previously involved, more sensors are still urgently needed to provide data.

In order to further expand the scale of sensor deployment, the development of IP standards is under way. The goal is to make small wireless sensors as easy to use as web servers. There are two driving forces behind the development of standards: First, the high reliability of low-power, time-synchronized mesh networks is recognized, and the second is that ongoing IP standard-setting work enables wireless sensor networks and Seamless integration of the Internet. The combination of these two forces will effectively promote the reliable communication of small, low-power sensors and enable such sensors to eventually support IP.

Wireless Sensor Network Challenges

Wireless is inherently unreliable, so it is important to understand the sources of unreliability in order to solve unreliability problems in communication systems. In low-power wireless networks, the main sources of unreliability are external interference and multipath fading. Interference occurs when an external signal, such as a WiFi signal, temporarily interferes with the two radio systems. This requires them to resend signals and therefore consume more power. Multipath fading occurs when a wireless signal rebounds upon encountering an object in a nearby transmitter, and various echoes can cause destructive interference to the receiver antenna. This phenomenon is a function of the location of the device, the frequency used, and the surrounding environment. Because the environment around any wireless system changes over time, any RF channel will experience problems during the lifetime of the wireless system. However, multi-fading is affected by frequency. Therefore, although a certain frequency may be experiencing problems, there are still other RF channels working. Due to interference and multipath fading, the key to establishing a reliable wireless system is to diversify channels and paths without sacrificing the benefits of low-power operation. Dust Networks (now a business unit of Linear Technology) is the first to provide such a system, which uses time synchronization, channel frequency hopping mesh network technology.

Time synchronization, channel frequency hopping network

In a time-synchronized channel frequency-hopping mesh network, all wireless nodes on a multi-hop network are synchronized to within tens of microseconds, and time is divided into time slots. Communication is coordinated through a schedule that indicates what each node should do in each time slot (send, receive, sleep). Because they are all synchronized, these nodes only turn on their radios when communicating, which greatly reduces the radio duty cycle (common duty cycles < 1%) and extends battery life. In addition, because the schedule can be flexibly set, the network is always available for applications. Unlike other "sleep" network architectures, it takes a long time to completely shut down the network. All packets sent between two nodes are transmitted on the frequency calculated in pseudo-random frequency hopping. The resulting frequency diversity is a way to effectively resist interference and multipath fading. Time-synchronized mesh networks enable up to 10 years of battery life and >99.999% end-to-end reliability.

Time synchronization grid network has been successful

In recent years, time-synchronized channel frequency-hopping mesh networks have been widely used. In 2004, Dust Networks introduced the SmartMesh® system for the first time, and industrial processes were among the first to adopt this system.

Some industrial applications have the harshest operating environment and the most stringent requirements for data integrity. If such high data integrity can be ensured, the efficiency, productivity, and safety of industrial equipment can be greatly improved. Because traditional wired industrial sensors are expensive to install, there are generally only a few possible measurement points in the factory. Although this creates a great demand for the use of wireless sensors in industrial applications, traditional point-to-point wireless systems lack the required reliability and are difficult to install, thus limiting the use of wireless systems in small and isolated applications.

With the introduction of time-synchronized mesh networks, wireless systems can provide reliability that is typically only available on line systems, so the use of low-power wireless systems has become a reality. Low-power wireless systems are standardized through the industry standard IEC62591 (also known as WirelessHART), which enables interoperability of devices in the industrial process market. Most large industrial manufacturers (such as Emerson Process, Siemens, ABB, Endress & Hauser, Pepperl & Fuchs, and Phoenix Contact) are delivering Wireless HART products. Today, the SmartMesh network has been widely used, deploying more than 30,000 networks in more than 120 countries around the world, improving safety and efficiency in a variety of locations, including steel and oil refineries, remote oil fields and offshore drilling platforms, and Food and beverage factory.

In addition to industrial process areas, SmartMesh systems have also been successfully deployed in data centers and commercial office buildings to optimize air conditioning costs. Streetline Networks is an intelligent parking service provider that monitors the availability of parking spaces in urban areas in real time. The vehicle detector is installed below the parking space and on the road surface of the lane. This poses a challenge because the antenna of the sensor device is located underground, and when the parking space is occupied, it is covered by a metal body. Such applications were previously considered to be impossible or impractical and can now be implemented with time-synchronized channel-hopping mesh networks.