Comparing Wireless Protocols for Industrial Automation
Original Title：Comparing Wireless Protocols for Industrial Automation
The fourth industrial revolution (Industry 4.0) has imparted machines with more intelligence and automated facilities with more efficiency and flexibility. These increasing complex systems have driven adoption of wireless communications in industrial settings. After all, Industry 4.0 smart machines and modular automation are defined by:
Secure and adaptable control connectivity
Collection and continual adjustment of production-process values
Machinery-condition monitoring for predictive maintenance routines
Networking for big-data analytics capabilities
Wireless technologies that support these functions are based on cellular, Wi-Fi, Bluetooth, and IEEE 802.15.4 standards and protocols. That’s in part because design engineers expect compatibility of components from different vendors — which by definition necessitates connectivity through industry-standard interfaces and not proprietary interfaces. In fact, interoperability is just one aspect of Industry 4.0.
Figure 1: Wireless connectivity is key to coordinating material handling and collaborative robotic tasks. (Image source: Getty Images)
Individual devices incorporating wireless communication are typically costlier than wired networks. However, this increased upfront cost offset in several ways … and wireless devices often prove to be the most cost-effective option over the long run. That’s because the cost of running cabling through a production area can be significant. It takes effort to plan the routing of cables and their connectors. Plus cables require protection and physical support from cable trays or carriers … and they necessitate junction boxes and other accessories. Planning, ordering, and installing all of this cable-related hardware lengthens the time taken to implement a network.
Wi-Fi-based standards for automation
The Institute of Electrical and Electronics Engineers (IEEE) released 802.11 in 1997 defining the standard wireless implementation of local area networks (LANs). To ensure the market fully leveraged this standard, the industry consortium Wi-Fi Alliance soon followed — led by wireless-device companies interested in establishing testing and certification programs to maintain cross-supplier product interoperability. Today the Wi-Fi standard as defined by IEEE 802.11 is complemented by additional Wi-Fi Alliance standardization for exceptionally reliable compatibility of devices adherent to the requirements.
Figure 2: Industry 4.0 (also called the industrial Internet of Things or IIoT) is inextricably tied to the adoption of wireless technologies. Employing standardized interfaces to allow connectivity between various devices and computing systems, these wireless technologies include mobile devices used as HMIs (as shown here) as well as countless other wireless field components communicating machine status. (Image source: Getty Images)
While Wi-Fi is quite useful for monitoring applications and connecting machines to enterprise-level systems, Wi-Fi’s speed, latency, and connection-stability issues have limited its application in demanding industrial automation applications related to machine controls. That’s meant that Wi-Fi in industrial applications today is mostly restricted to uses having fairly forgiving requirements. These include:
Barcode scanners that communicate data to manufacturing execution systems (MESs) forgiving of a second or two of delay
Motion sensors uninvolved with real-time control functions
Long-term machine condition monitoring with sensors such as accelerometers (to track vibration generation over time) as well as temperature, pressure, humidity, and gas-concentration sensors for monitoring equipment efficiency and health
Figure 3: Though unsuitable for machine controls, Wi-Fi is useful for machine-monitoring applications and connecting factory floors to enterprise-level systems. (Image source: The Wi-Fi Alliance)
There have been several attempts to adapt Wi-Fi to industrial control applications, but these have had limited success. One exception protocol having some successful IIoT adoption is the Wireless Network for Industrial Automation and Process Automation (WIA-PA) — a Chinese industrial wireless communication standard.
Wi-Fi of course operates at either 2.4 or 5 GHz, with higher frequencies enabling faster data transfer but reduced range due to the way in which higher frequencies are more readily dissipated when passing through walls and other solid objects. Specialized standards use other frequency bands. For example, IEEE 802.11ah low-data Wi-Fi (HaLow Wi-Fi) operates around 900 MHz — and is usually employed in sensors needing extended ranges and very low power consumption. At the other extreme, IEEE 802.11ad Wi-Fi (WiGig) operates at around 60 GHz to provide very fast data transfer.
IEEE 802.15.4-based wireless standards
Other wireless options are low-rate wireless personal area networks or LR-WPANs as defined by the IEEE 802.15.4 standard. LR-WPAN technologies prioritize low cost and low power over speed and range. With the basic specification allowing data-transfer rates to 250 kbit/sec and ranges to 10 m, technologies employing LR-WPAN communications are intended to allow communication between low-cost devices without any additional communications infrastructure. Protocols based on the IEEE 802.15.4 standard such as 6LoWPAN, WirelessHART, and ZigBee are fast becoming preferred IIoT protocols.
1. WirelessHART: One 802.15.4-based protocol supported by the HART Communications Foundation, ABB, Siemens, and others is called WirelessHART. This is a well-supported and robust standard for industrial automation applications. Network reliability is maintained using a frequency-hopping meshed network with time synchronization. In contrast, most wireless communication protocols based on Wi-Fi and cellular technologies use a less robust star network topology that requires all devices to connect to a central device. All communications are encrypted using 128-bit AES, and user access can be tightly controlled.
Figure 4: The LTP5903-WHR SmartMesh network manager supports line-powered WirelessHART gateways to let engineers integrate a standards-based wireless sensor network for scalable bidirectional communications. (Image source: Analog Devices)
Because WirelessHART uses a meshed topology, data can be routed directly between devices. This can extend network range and form redundant communication paths. That way if one path fails, the sender automatically switches to a redundant path. Frequency hopping also allows WirelessHART to avoid issues with interference.
2. 6LoWPAN: IPv6 over Low-Power Wireless Personal Area Networks (commonly called 6LoWPAN) is a protocol that allows IPv6 packets to transmit over an IEEE 802.15.4 based network. This means that very low power devices can connect to the internet making it well suited to IoT sensors and other low-powered devices.
3. ZigBee: Maintained by the Zigbee Alliance and most widely used in smart-home and building-automation applications, ZigBee is perhaps the most established IEEE 802.15.4-based protocol. It lets nodes remain in sleep mode most of the time to greatly extend battery life. ZigBee typically operates in the 2.4 GHz band and has a fixed data transfer rate of 250 kbit/sec. It can support various network topologies including star, tree, and mesh. Tree and mesh topologies extend the network range.
Figure 5: Zigbee is useful for (among other applications) motion, vibration, humidity, temperature, and presence sensors in industrial settings. (Image source: Zigbee Alliance)
Bluetooth LE and cellular IoT in industrial automation
Bluetooth Low Energy (BLE) is an alternative to IEEE 802.15.4 where low cost and low power are top priorities, and speed, as well as range, can be sacrificed. It operates at the same 2.4-GHz frequency as standard Bluetooth. The top benefit of Bluetooth LE is how it’s natively supported by mobile operating systems such as Android of the Open Handset Alliance, iOS of Apple, and various permutations of Microsoft’s Windows. This plus the fact that large electronics suppliers such as Logitech Corp. have invested the most R&D makes it no wonder Bluetooth LE is still primarily a wireless-connectivity option for consumer devices. This is in contrast to WirelessHART, which has been and remains primarily focused on IIoT applications.
Figure 6: The Bluetooth Low Energy (BLE) standard has a serial port profile that systems recognize as a full serial interface — useful for replacing wired devices with upgrades connected by BLE. (Image source: Bluetooth Special Interest Group)
All this said, the last few years have seen a smattering of sensors, remote controls, locks, and handheld devices employing Bluetooth LE for industrial-automation tasks. That trend will likely increase in coming years.
In contrast with BLE and IEEE 802.15.4-based protocols for low-power short-range communications, cellular technologies are long-range wireless communications. The 2G GSM cellular protocol has been mostly superseded by 3G and 4G high-speed cellular protocols so common in cellphones and IoT devices. The catch is that cellular communications consume significant power, so in industrial applications (especially for such connectivity on machines) the system is connected to a permanently wired power supply. Cellular LTE categories indicate maximum data-transfer rates — though at the cost of higher power consumption. LTE Cat-0 and Cat-1 connectivity are suitable for IoT devices. In contrast, LTE-M is a low-power cellular protocol designed specifically for machine-to-machine and IoT applications.
In contrast with its relatively widespread use in cellphones, industrial 5G applications are less mature. That’s because consumers prioritize download speeds (so have been quick to adopt introductory 5G devices) and engineers of IIoT systems prioritize low latency and ubiquitous coverage. In fact, low latency is of top importance in industrial automation. It’s true that the first 5G networks hold latency to under 30 msec, but there are efforts to bring latency down further to just 1 msec. That’s fast enough for demanding real-time industrial control (not just monitoring) applications — such as transmitting feedback signals in machine tools, for example.
One way 5G reduces latency is with network slicing. This networking technique divides a network’s bandwidth into different virtual lanes that are then individually managed. Some lanes are reserved for low-latency transmissions — with most traffic forbidden from using those lanes. Then only industrial-control applications needing the quickest transmission are allowed to use these reserved fast lanes.
The rise of the LoRA wireless protocol
Long-range wide-area network modulation (LoRA) is the low-cost wireless protocol of choice for remote and offshore applications in renewable energy, mining, and logistics industries. It is a low-power wireless technology that can communicate over very long ranges — even to beyond 10 km — on one battery for up to 10 years. In short, LoRA is a non-cellular technology operating in license-free frequency bands. It employs sub-gigahertz frequency bands such as 433 and 915 MHz and spread-spectrum modulation based on chirp spread spectrum (CSS) modulation. This makes it very well suited to IoT devices set in remote locations that only need modest data transfer rates. LoRA also features 128-bit encryption and authentication controls. Another useful feature (especially for sensors in IIoT applications) is geolocation using trilateration between devices.
LoRA uses proprietary technologies developed by Semtech Corp. but has a vast array of open-source elements. It’s supported (and device interoperability ensured) by the LoRa Alliance — a large association which includes IBM, Cisco, TATA, Bosch, Swisscom, and Semtech.
Wireless protocols for industrial automation abound. Each is suited to certain applications. Uses that demand low power consumption and accept short-range transmissions often benefit from inclusion of ZigBee and Bluetooth LE connectivity. More demanding industrial applications needing communications robustness may necessitate devices having WirelessHART wireless connections. Uses that demand long-range transmission high data transfer rates necessitate cellular. Here, 5G is poised to transform wireless communications. Communicating data over very long ranges (and consuming minimal power) is often best through LoRa.
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