Industrial Wireless Part 1: Cutting the Cord in Industrial Automation
| By: Kristin Lewotsky, Contributing Editor
Editor’s Note: Part one of this three-part series focuses on industrial wireless technologies that take advantage of unlicensed spectrum, limiting cost. In part two, we will focus on the current state of 4G LTE and 5G cellular wireless and the technologies that make them effective for industrial applications. In part three, we will bring it all together to show how to choose the ideal industrial wireless solution and how to efficiently and effectively deploy it.
Wireless networking is becoming pervasive in the modern industrial environment. It controls the swarms of automated guided vehicles that speed around the modern warehouse. It lets sensors relay data used to detect problems before they become critical. It enables operators and engineers to walk around the factory floor with key performance indicators (KPIs) and machine documentation at their fingertips.
Fueled by the rise of Industry 4.0 and supercharged by COVID-19, the industrial wireless market in discrete industries is set to surge by $3.94 billion between 2020 and 2024, for a CAGR of 11% (source: Technavio). Leveraging a variety of technologies, facilities are using wireless for controls, communications, and data capture from the host of devices that make up the industrial Internet of things (IIoT). “We see a lot of vision applications being deployed, lots of quality control and predictive maintenance,” Sebastian Elmgren, portfolio manager for small manufacturing at Ericsson (Stockholm, Sweden). “Safety is also an important use case, as are energy monitoring and management. It’s a quite broad palette because there are so many different verticals now involved.”
To support the wide variety of use cases and their requirements, the industry has developed a range of networking technologies. Realizing the benefits of industrial wireless involves a number of choices driven by the application and the needs of the project.
Transmission frequency is a key characteristic for any wireless technology. Frequency affects data rate, transmission distance, ability to pass through objects. The lower the frequency, the greater the transmission distance and ability to penetrate objects but the lower the data rate. The higher the frequency, the greater the data rate but the shorter the distance. The operating spectrum needs to be selected to best the needs of the application project.
The RF band encompasses both licensed and unlicensed spectrum. Licensed spectrum is reserved for specific clients or applications, such as public services, critical infrastructure, vertical industries, or paid licensees like communications service providers (CSPs).
In addition to licensed spectrum, regulatory bodies around the world have reserved a number of frequency bands that do not require licenses to use. Known as the industrial, scientific, and medical (ISM) bands, these frequencies can be used without license or fee, although they are still subject to certain regulations such as power limitations. Although they were initially intended for non-communications use, several of the ISM bands have been adopted by limited-range wireless networking standards such as Bluetooth (IEEE 802.15.1), Wi-SUN (IEEE 802.15.4) and Wi-Fi (IEEE 802.11). Those bands include subset of 800 (EMEAR/India)/900 (America/APJC) MHz, 2.4 GHz, 5.8 GHz, and 6 GHz (see Table 1). The ISM bands are subdivided by wireless protocols into multiple channels, each of which consists of a subset of frequencies.
Many of the bands are globally harmonized, although several, including the 6 GHz band, vary by region and country.
Table 1: ISM frequency bands
|Frequency band||Spectrum||Region||Data rate||Distance|
|900 MHz||902-928 MHz||Region two (Americas)||low||high|
|2.4 GHz||2.400-2.500 GHz||Global||medium||medium|
|5.8 GHz||5.7255 5.875 GHz||Global||higher||medium|
|6 GHz||5.925-7.125 GHz||United States||highest||medium|
Spectrum limitations can lead to interference, not just among competing networks on a factory floor. Microwave ovens, for example, also operate at 2.4 GHz. More than one wireless user has reported a head scratching intermittent fault that has eventually been traced to a new microwave in the production floor breakroom.
High demand for bandwidth can also cause performance issues. The greater the number of devices on the network, the lower the capacity. The greater the number of networks using the same spectrum on a factory floor, for example, the higher the degree of interference. To address spectrum shortage amend the growing IIoT, several countries, including the United States, the United Kingdom, and South Korea have released portions of the 6 GHz band to the ISM band; the latest update to the Wi-Fi standard, 802.11ax, is specified for to work in this frequency band. It is expected to use the capacity crunch, at least in the immediate term.
There are several transmission technologies used for wireless networking. The ones most commonly applied to industrial wireless for the unlicensed band are:
Frequency-hopping spread spectrum (FHSS): The transmission hops from frequency to frequency throughout the transmission, in an established pseudorandom sequence known by transmitter and receiver. If two networks use different sequences, they will not interfere, making the network very effective in noisy environments.
The data stream is also difficult to decode without prior knowledge of the sequence, adding to security.
Direct-sequencing spread spectrum (DSSS): Each bit of data is encoded into a series of 8-11 bits, or chips, by processing it with a pseudo-random numerical sequence. Next, all chips are sent simultaneously across different frequencies within a single channel. Depending on the code length and the modulation type, DSSS can send up to eight bits per symbol. DSSS has a faster data rate than FHSS, although it is more vulnerable to interference.
Orthogonal frequency-division multiplexing (OFDM): The channel is divided into evenly-spaced subcarriers, each of which is orthogonal, or independent of the others so that they do not interfere. Data transmission is then multiplexed?the bit string is broken down into a set of sub strings, and then each is sent over a different subcarrier simultaneously. Transmitting over multiple subcarriers helps reduce inter-symbol interference, reducing bit-error rate. Each subcarrier transmits at a lower data rate but because there’s a large number, the aggregate data rate is higher. ODFM has a faster data rate than the previous two transmission technologies but is more vulnerable to interference.
Orthogonal frequency-division multiple access (OFDMA): Essentially, a multi-client version of OFDM in which each user is allocated subsets of subcarriers. It enables resources to be divided up among multiple devices simultaneously preventing contention.
The choice of transmission technology, or coding, is defined by the various wireless standards.
The optimal network topology depends on the needs of the application. The most common wireless topologies are (see Figure 1).
- Point to point (P2P): A dedicated wireless link between two nodes, a node and an access point (AP), or between two access points
P2P is limited in use but for the right application, such as PLC to PLC communications or connecting two machines together, it may be all that is necessary. On the downside, it is limited by the range of the wireless technology.
- Point to many (P2M): Multiple nodes connecting to a single hub
P2M topologies, also known as star topologies, are common solutions for IIoT deployments. The devices send their data to a central hub such as an edge device, where it can be processed and/or sent to an on-premises or public cloud for processing and dissemination. P2M networks are simple, inexpensive, and easier to secure, given that nodes connect only the central hub.
On the downside, if the hub goes down, it takes the entire network down with it. The range is also limited by transmission distance of the wireless technology. Depending upon the transmission frequency used, a P2M network may need a line-of-sight connections to prevent interference by physical obstructions.
- Mesh: A network of interconnected nodes that can serve as both endpoints and repeaters for other nodes
|Figure 1: The three most common wireless network topologies are (from left) point-to-point (P2P), point to many (P2M) or star, and mesh. (Courtesy of Cisco Systems)|
In a mesh network, the data “hops” from repeater to repeater until it reaches its destination. As a result, they are less affected by obstructions. Mesh networks can be implemented as full mesh topologies, with all nodes doubling as repeaters, or as partial mesh topologies, in which only central nodes function as repeaters and others are strictly endpoints. The topology is commonly used for both IIoT and controls applications. Because nodes connect as repeaters, mesh networks extend the range of a wireless technology. The networks are also easier to expand because new nodes can simply be added as needed.
On the downside, nodes that act as repeaters must be in “always on” mode, increasing power consumption and reducing charge lifetime in battery-operated devices.
A variety of wireless technologies based on unlicensed spectrum are available for the industrial automation sector. Some are carried over from the consumer space. Others have been developed specifically for the industrial sphere, although not all apply to discrete automation. Taken as a group, they give users a wide variety of options to serve the needs of the application (see Figure 2).
|Figure 2: Representation of wireless technologies using unlicensed spectrum presented by operating frequency is a function of speed show the breadth of choices available to industrial users. (Courtesy of Cisco Systems)|
Defined by the IEEE 802.11 family of standards, Wi-Fi is a half-duplex protocol, meaning that data can be transmitted between two connected devices in only one direction at a time. Wi-Fi primarily operates over the 2.4 GHz, 5.8 GHz, and, more recently, the 6 GHz ISM bands. Although the earliest releases used DSSS and FHSS coding, later versions changed to OFDM and, most recently, to OFDMA (802.11ax). Wi-Fi can be used with P2P, P2M, and mesh topologies.
With a combination of flexibility, performance, and interoperability, it’s a highly accessible networking technology. “Because it’s a public technology and it’s compatible with your third-party, it’s the wireless technology I see by far the most on the plant floor today,” says Justin Shade, senior product marketing specialist for wireless products at Phoenix Contact (Middletown, Pennsylvania).
The high data rates provided by Wi-Fi make it well suited for controls applications like automated guided vehicles (AGVs), robotics, and mobile access to assets like control panels and machine data, or connecting a control cabinet device to an existing infrastructure (see Figure 3). “Depending on what’s available to you from a spectrum point of view, you can make all of that reliably work long term, says Shade. “You can pick one technology to manage one centralized platform. You don’t have to worry about different frequencies and different vendors.”
|Figure 3: Wi-Fi can be used to link mobile and stationary devices to an industrial Ethernet link via a wireless access point. (Courtesy of Phoenix Contact)|
That said, just because the Wi-Fi network can do everything, doesn’t necessarily mean it should. It’s one thing to add a sensor or two to the system, say to monitor the temperature of the control. Wi-Fi probably wouldn’t be an ideal solution for IIoT deployments. “It’s not really meant right now to be a battery-powered device,” says Shade. “Can you power by battery? Sure. Will it work as efficiently and as long as low-power technologies like ZigBee? No, because they’re built for end-node sensors that are running off of AA batteries.” A wired sensor and a Wi-Fi device with an Ethernet jack in it would consolidate all activities into a single platform that went all the way across, but the trade-off would be that it would rise in cost because it is more feature rich. “Those features may not be necessary, but from the management standpoint, the network is now easier to manage because it’s potentially one vendor with one point of contact if anything is not working. It really depends what the end-user needs.”
A discussion of Wi-Fi would be incomplete without mention of the most recent version, IEEE 802.11ax. Although the release boosts transmission speed by around 39% compared to its predecessor, 802.11ac, 802.11ax really shines in terms of its ability to boost connection density by a factor of four, thanks to its use of OFDMA encoding. 802.11ax also takes advantage of the newly released 6 GHz spectrum, as well as the well-established 2.4 GHz and 5.8 GHz bands. The new encoding method, coupled with the additional spectrum, should dramatically decrease interference in the modern industrial environment.
Both 802.11ac and 802.11ax have promised truly blistering speeds (3.5 Gbps and 9.6 Gbps, respectively). It’s important to remember that these data rates are theoretical. In the real world, particularly in the industrial environment, data rates will be significantly lower. Roughly 50% of the data rate will be lost to overhead. The ability of a system to operate even at half theoretical data rate then depends on whether the rest of the components in the network are compatible.
That said, expectations for 802.11ax (now branded as Wi-Fi 6 for the GHz/5.8 GHz bands and Wi-Fi 6E for the 6 GHz band) are high. “802.11ax is going to be the next game-changing technology for the industrial environment,” says Shade. “The new spectrum addresses probably the biggest challenge that every end-user has: How do I make sure that I’m not interfering with one network over another?” He sees the biggest benefit in the new encoding scheme, which will dedicate subcarriers to different users to support simultaneous transmission. “The way Wi-Fi works today, if you have a busy channel and someone is trying to download a cat video on Facebook and you’re trying to send commands on the same channel to a robot arm that is moving a 2-ton piece of steel, they are both considered equal, so the packet going to the robot arm will have to wait until the spectrum is clear. With the parallel communication on 802.11ax, the robots can get the faster update rates that they need.”
To test performance of Wi-Fi 6 in an IIoT gateway, a team at Cisco acquired a pair of autonomous mobile robots (AMRs). The results were promising. “When we tested the AMR with Wi-Fi 6, we achieved latencies as good as 3-ms,” says Patrick Grossetete, is a distinguished engineer, technical marketing at Cisco Systems (San Jose, California). “We have been working to enhance the protocol to make sure that we keep the latency consistent.”
Bluetooth was originally part of the IEEE 802.15 family of standards (802.15.1) for wireless personal area networks (WPAN); it was subsequently taken over by the Bluetooth Alliance. With power limited to less than 100 mW, Bluetooth is a short-range standard design for data communications between fixed and mobile devices. Bluetooth operates in the 2.4 GHz band, which means that it competes with not just all other Bluetooth networks but with Wi-Fi 2.4 GHz networks, the aforementioned microwave ovens, and more. The technology uses FHSS encoding, specified in the US as 79 channels with a hop rate of 1.6 kHz, which can make it effective for time-sensitive, short-range applications like robotics.
The standard has been split into two parts. Classic Bluetooth only supports P2P topologies. It can be configured as a P2M topology in which the nodes exchange data with a central hub but it must be remembered that communication between hub and various endpoints takes place serially rather than in parallel (see Figure 4). The hub aggregates information before transferring it to some designated repository, typically using a higher-speed technology such as cellular.
Bluetooth Low Energy (BLE) was originally targeted at consumer use cases like fitness monitors, but it has found applications in the industrial sphere. BLE supports P2P, P2M, and mesh topologies, as well as broadcast applications that can be useful in inventory management and logistics.
|Figure 4: Bluetooth modules replace Ethernet cables between backbone and a node with a wireless connection. (Courtesy of Phoenix Contact)|
Bluetooth can be a good fit for many of the same use cases as Wi-Fi, including fixed and mobile robots, cranes and lifting equipment, and moving machine parts. In particular, it can be used for machine-to-machine communications, making it possible to rapidly reconfigure production lines. Bluetooth can also replace mechanical interfaces such as slip rings in rotating equipment and of course also provides a low-power solution for sensor networks.
Users should be aware that while industrial Wi-Fi is very similar in installation to home Wi-Fi, the industrial Bluetooth experience can be significantly different. “From an interface standpoint, industrial Bluetooth devices don’t pair the way your phone does to your car,” says Shade. “There are IP addresses and passwords that need to be entered.” Users may also run into random issues. “A lot of times, vendors put proprietary twists on their devices that won’t allow them to connect,” he adds. “That can be frustrating for users?they just want to plug it in and have it work. That’s why right now, Wi-Fi is more broadly used than Bluetooth.”
Low-Power Mesh Networks
A discussion of unlicensed wireless networking technology would be incomplete without at least a nod to low-rate wireless personal area networks (LR-WPANs; IEEE 802.15.4). Examples of these technologies include Zigbee, WirelessHART, Z-wave, ISA100, Wi-SUN and 6LoWPAN. Aimed at the IIoT, these networks consist of battery-operated devices designed to power up briefly at intervals, take a reading, send a small packet of data (a few hundred bits) at low data rates, and power down again. They operate in the ISM bands, frequently around 900 MHz to minimize attenuation, although Zigbee, for example, also has a 2.4 GHz offering.
These networks are primarily aimed at process industries such as oil and gas, utilities, mining, and paper processing. Facilities may be spread out over acres and contain thousands of assets that need monitoring. Your average facility for discrete manufacturing is unlikely to incorporate assets on that scale but there may be instances that call for use of these technologies.
802.15.4 networks are architected as mesh networks built out of P2P topologies. The nodes deliver data to a hub, typically part of a higher-speed network. These hubs or edge devices process and relay the data for use.
An additional entry that is not part of the IEEE standard is the Long Range Wide-area Network (LoRaWAN), as defined by the LoRa Alliance. LoRaWAN is limited up to 250-byte payloads but can achieve ranges on the order of 10 km. It uses a star topology with nodes feeding into one or more gateways connecting to centralized controller. It is also focused around the process industry and other use cases such as remote meter reading, etc.
The sudden easy access to data from assets where it was previously unavailable can lead to overaggressive use?and frustration. If those sensors send small burst of data hourly, they will probably achieve their advertised battery lifetime. If they are configured to power up and send data too aggressively, the battery will need to be swapped much sooner, and suddenly the “set it and forget it” sensors are constantly top of mind. For success, these devices need to be applied as intended. This also holds for battery-powered Bluetooth devices.
Industrial wireless represents a substantial investment. To reap the benefits of these investments, organizations need to follow a strategic approach.
- Start with the application. What are your priorities in terms of reliability, latency, and data rate? Do you need a solution for one geographical region or do you need to have a globally harmonized standard? What are your cost constraints? “Focus on the use case,” says Grossetete. “When we have a good understanding of the use case, then we can look at which technologies fit it best.”
- Make friends with IT. Given the density and complexity of wireless networking in the modern enterprise, communications is essential. “The biggest restriction any wireless team has is spectrum availability,” says Shade. “If you have a big facility and you’re running multiple different applications, there is only so much RF spectrum available. Who owns it? Who can make those decisions? And how can you divvy that up in a way that everybody’s happy?”
- Perform an audit. Before installing a new network, bring in a spectrum analyzer and find out what’s on the floor. It’s almost a guarantee that you will discover multiple devices or networks no one knew existed. These need to be folded into the overall plan and potentially updated to optimize spectrum use.
- Diagram the plant floor. Detail the layout, sharing network cabling, network jacks, control cabinets, concrete pillars, etc. This is crucial information to help your vendor begin the design process.
- Run a small pilot project. Make sure the solution works for your application, but make sure that your vendor and solution can scale. “It’s good to start small, but you should also have a plan for a full-cycle deployment ready when you do your pilot project,” says Elmgren. “If not, it’s easy to get stuck in the proof-of-concept area.”
- Don’t forget cybersecurity. The 2013 Target data breach caused by the theft of credentials from an HVAC vendor who was logging in to systems remotely. Make sure to follow best practices and avoid introducing vulnerabilities.
Adopting industrial wireless is about more than simply convenience. It equips operations to respond to customer demand more rapidly and effectively. “Businesses are looking at how they can reorganize assets more quickly with wireless connections than with wired, so it is clearly how they can be more agile,” says Grossetete. “At the same time, they also want to enhance their ability to innovate.” The right wireless network can help industrial word achieve both goals.
Part 2 of this article focuses on cellular solutions over licensed spectrum.