Remote Monitoring with Wireless Sensors Employing Wi-Fi
Original Title：Remote Monitoring with Wireless Sensors Employing Wi-Fi
Modern factories have increasingly complex digital systems, with interconnections between devices and software from many different vendors. This complexity has led to a move away from proprietary interfaces, replacing them with common standards such as Ethernet and Wi-Fi®. Standardization of digital communications can be seen as part of the fourth industrial revolution (Industry 4.0) in which Internet of Things (IoT) technologies greatly simplify the connection of different devices (Figure 1). This article reviews the most common forms of Wi-Fi-based sensor networking and their typical applications.
Figure 1: Wi-Fi-enabled sensing is increasingly common in industrial settings.
Wi-Fi history and versions
Wi-Fi is a wireless networking protocol based on IEEE 802.11, but has been further standardized to ensure interoperability of devices. The Wi-Fi standard is maintained by the Wi-Fi® Alliance and only products which have been certified to meet this standard may carry the trademark.
The 802.11 standard is well-established for wireless local area network (LAN) applications. It was released by the Institute of Electrical and Electronics Engineers’ (IEEE) in 1997, as 802.11-1997. Subsequent major releases have included, in chronological order, 802.11b, 802.11a, 802.11g, 802.11n, and 802.11ac. Although IEEE 802.11 provides the technical basis for Wi-Fi, IEEE did not have any certification or testing, which led to issues with interoperability in early devices.
In 1999 the Wi-Fi Alliance was formed by some of the first companies to adopt IEEE 802.11. The aim of this alliance was to improve interoperability between the devices the member companies were producing. The founding companies included 3Com and Nokia. Wi-Fi generations correspond to major releases of the IEEE 802.11 standard, as shown in Table1.
Table 1: Wi-Fi standards through the years.
Range, speed, and frequency
Wi-Fi can operate at different frequencies and devices can often be configured to use different frequencies. The most common frequencies are 2.4 GHz and 5 GHz.
In general, higher frequencies provide higher speeds of data transfer. However, higher frequencies are also more easily dissipated, especially when passing through solid objects. Lower frequencies will, therefore, usually provide greater range.
When operating in the same frequency range as other devices, Wi-Fi is also more susceptible to interference. For example, at 2.4 GHz, Wi-Fi interference may occur with microwave ovens, cordless phones and Bluetooth devices. This can mean that in certain environments, 5 GHz may actually give better range than 2.4 GHz. If issues are encountered at a particular frequency it can often be easiest to simply try a different channel or even band.
Frequency ranges are bands within which specific channels are defined. For example, 2.4 GHz is divided into 14 channels. Channel 1 is from 2401 to 2423 MHz, channel 2 is from 2406 to 2428 MHz, etc. Considerably more channels are available in the 5 GHz band.
IEEE 802.11ah, known as Wi-Fi HaLow or extended-range, operates at the lower frequency band around 900 MHz, combined with narrow 1 MHz RF channels. These narrow, low-frequency channels, combined with protocol changes mean a much lower power consumption, even lower than Bluetooth Low Energy. Range should be about double that of 2.4 GHz — more than 40 meters at 150 kbps for a single stream or over 80 meters using a more complex dual-stream chip. Although the IEEE has already released the 802.11ah standard, the Wi-Fi Alliance has not yet started certifying devices.
At the other end of the spectrum, IEEE 802.11ad, or WiGig, operates at a higher frequency band around 60 GHz to enable high data transfer rates of typically around 7 Gbit/sec.
Wi-Fi network topology
A network’s topology is the basic structure of the connections between devices (Figure 2). For example, in a star topology, one device is a hub and all the other devices connect to the hub. In a fully connected topology, each device is connected to every other device. A mesh topology is similar to a fully connected one in that the connections are decentralized, but there may not be connections between every pair of devices, it may also be referred to as a partially connected mesh. In a bus topology every device is connected to a cable, known as the bus.
Figure 2: Network topologies abound, but most Wi-Fi networks are star or mesh. (Image source: Design World)
Wi-Fi networks are typically either star or mesh. Mesh topologies are robust and secure, it reduces power consumption and improves range since individual links can be shorter. For large IoT networks with lots of low-power sensors, these are important advantages. However, star networks can also offer advantages in this respect. In a start network, it is possible for individual devices to transmit intermittently and only the hub requires continuous power for the Wi-Fi signal.
Specialized Wi-Fi implementations for industry
As mentioned above, Wi-Fi HaLow uses a lower frequency to achieve greater range and reduced power consumption. This can be useful for small battery operated devices. For control and industrial automation applications, where real-time communication is required, Wi-Fi has struggled to provide a sufficiently high-speed, low-latency and stable connection. Although there has been interest in real-time Wi-Fi for at least a decade, this technology has not been widely adopted. Perhaps the most successful implementation of real-time Wi-Fi is WIA-PA, a Chinese industrial wireless communication standard for process automation.
Industrial use of Wi-Fi is more typical in less demanding applications such as motion sensors and barcode scanners. Condition monitoring of machinery is becoming very common. For rotating machinery, accelerometers are used to monitor vibrations. Environmental monitoring is also an important aspect of condition monitoring, with small temperature, pressure, humidity and gas concentration sensors often deployed.
Condition monitoring sensors are deployed in many different environments. These include the obvious factory and warehouse machinery, as well as high-value commercial vehicles including trucks, earth moving equipment and aircraft. Condition monitoring is also very well established and critical within power generation, mining and drilling operations.
Monitoring of traffic, pollution levels and weather are some more examples of applications where wireless sensors are deployed.
Wi-Fi is not the only standard that enables wireless communication between industrial devices. For short-range and low-power applications, Wi-Fi competes with Bluetooth and Zigbee. For long-range applications. The main technologies that compete against Wi-Fi are cellular technologies — 3G, 4G, and 5G.
Consider just one example of a low-power microcontroller unit (MCU) to help engineers setup communications via Bluetooth Low Energy (BLE) as well as Wi-Fi via an XBee Wi-Fi module:
Bluetooth is a well-established low-power form of communication. Zigbee is a newer technology based on IEEE 802.15.4, which is intended to use even lower cost hardware and power than even Bluetooth. Although Wi-Fi HaLow is intended to compete in this area, it doesn’t achieve the ultra-low cost and power of Zigbee. Complicating things even more, 5G has its own low-power technology — Low-Power Wide-Area (LPWA).
Complementing many of these low-power offerings are energy-harvesting capabilities:
Many industrial device manufacturers still utilize proprietary industrial wireless technologies. Although this makes interoperability more difficult, it means they can provide enhanced security and real-time communications. As Wi-Fi continues to improve in these areas, engineers can expect to see more devices adopting this open standard. On the other hand, 5G is showing a lot of potential for wireless IIoT applications. The next few years will bring more competition between the latest Wi-Fi 6 and 5G standards.
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