It’s been well over 100 years since Marconi, Tesla and others pioneered wireless communication technology. Commercial use of RFID (radio frequency identification) technology dates from the early 1970s. And Wi-Fi—wireless local area network products based on the Institute of Electrical and Electronics Engineers’ (IEEE) 802.11 standards—have been widely employed in commercial applications since the 1990s. Be it in the form of Wi-Fi, the workhorse of the industrial wireless sphere, or wireless field device standards like Zigbee and WirelessHART, wireless techology has invaded the industrial sector, rapidly progressing from oddity to useful technology. It enables applications that can’t be done—or can’t be done inexpensively—with wired communications. It’s also complicated. Electromagnetic radiation, antenna placement and other sources of radio signal interference—not to mention complex and changing security issues—can lead to broken signals and mysterious outages. These network design tips and sources of expertise can help you make it all work.
Proactive maintenance has long been a goal of manufacturing professionals, and wireless technology enables qualified professionals to tap into a wireless sensor network with a cellphone or tablet computer from anywhere in the plant, or perhaps from off-site, to monitor the health of components and initiate corrective action long before a potential problem becomes a real one. Of course, the thought of someone “tapping into” the system still worries a lot of people. It needn’t.
Mark Lochhaas, product manager for automation vendor Advantech, notes that “while personal wireless networks often remain insecure, security standards for business, industrial and government use have been developed over the last several years and adopted by most organizations.”
Lochhaas explains that the earlier WEP (Wired Equivalent Privacy) security algorithm, which was found to have serious flaws, has been superseded by much stronger and more secure transmission algorithms. Wi-Fi Protected Access (WPA), including the Temporal Key Integrity Protocol (TKIP), replaced the older WEP algorithm in 2003. The more recent WPA2, introduced in 2004, uses the even more secure Advanced Encryption Standard (AES) 802.11i algorithm.
WPA2’s AES algorithm is compliant with the National Institute of Standards and Technology (NIST) FIPS 140-2, required by some government agencies and corporations. “These standards,” Lochhaas notes, “can protect a robust communication system. For secure communications, WPA2-compliant products should be used for industrial wireless implementations today.” These should be coupled with good security practices, including using DMZs and firewalls, just as they do in the enterprise office environment.
The use of port-based authentication, based on the 802.1x standard, and the use of RADIUS authentication servers on top of WPA2 encryption make it extremely difficult to penetrate an industrial wireless network, says Lochhaas. But while security is no longer a deal breaker when it comes to wireless communication, electromagnetic radiation and other sources of radio signal interference often is.
“Non-industrial environments tend to lack the different sources of RF interference that industrial environments have,” says Tim Pitterling, product manager, industrial Ethernet infrastructure, for Siemens Industry. “Large metal machinery, walls and areas that can reflect RF waves; thick concrete walls that can block them; and a range of other sources of radio signaling—RFID, neighboring industrial WLANs and office WLANs, to name a few.”
Other sources of interference include the microwave in the breakroom and ubiquitous cellphones. All these sources of interference would be identified in the site survey that must precede system design and implementation, says Pitterling.
Mike Fahrion, director of product management at automation vendor B&B Electronics, cautions that during the initial design phase users need to quantify the “noise floor” in their facility. This is the background noise that issues from “high-frequency digital products or competing forms of radio communications. The background noise identifies a noise floor at which the desired signals are lost in the background ruckus,” he says.
The noise floor will vary by frequency. It will also “often be lower than the receive sensitivity of your radio, in which case it wouldn’t be a factor in your system design. But if you’re in an environment where high degrees of RF noise exist in your frequency band, use the noise floor figures, rather than the radio receive sensitivity, to make your calculations. Doing a simple site survey to determine the noise floor value can pay off down the road,” Fahrion adds.
Another Fahrion tip for the site survey stage: “Some obstacles are mobile. More than one wireless application has been stymied by temporary obstacles such as a stack of containers, a parked truck or material handling equipment. Plan for that.”
But large pieces of machinery, metal structures, tanks and the like that are extremely hard to propagate radio waves through, is a given. The scattering and bouncing of radio signals off these structures causes a type of interference called “multipath.” Advantech’s Lochhaas explains that this the reason that FM radio sometimes sounds fuzzy or has dropouts. “This is called picket-fencing signals, because the signal comes and goes with the regularity of a picket fence. Multipath is caused when the signal takes two or more paths to reach the receiver, and depending on the length of the paths, phase cancellation is produced and the signal goes to zero.
“One of the considerations in selecting wireless devices is output power. More power does two things. First, obviously, it lets the signal go farther. But higher power also allows the signal to cut through much more noise, and in the industrial environment with myriad sources of RF and electrical noise, that’s a key feature.” He notes that in the industrial environment, the use of high signal strength 802.11n based devices and the use of 802.11a on the 5 GHz band goes a long way to minimize interference.
Lochhaas points out that the most effective and simplest way to fix an interference problem is to either stop the source of the interference or move the radio to where the interference doesn’t swamp or destroy the signal. The availability of “bridges” can be used to place an additional router where it can be line-of-sight from the first router to the next, thereby removing the interference.
Antennas are key elements in RF performance, and Ira Sharp, product marketing manager for networking and I/O for Phoenix Contact, says that users must be sure to employ the right antennas for their application. “To decide which type of antenna to use, a few things must be considered: The distance the signal must travel to reach the destination, the number of surrounding radios and their distance from the destination point are all critical,” he says.
If the signal must travel longer distances, “directional antennas will provide better results, but aiming will be more difficult because the radiation angle will be narrow. If there are other radios in the area, omnidirectional antennas can cause interference with the surrounding radios, creating an overall weak communication system,” Sharp says.
After choosing the antenna type, the next step is to define what gain the antenna must have, says Sharp. Gain, which is measured in decibels-isotropic (dBi) units, describes how strong an antenna transmits and receives radio waves. Fortunately, help is available on this front, as propagation study software can be used as a guideline for selecting the antenna gain needed. This doesn’t, however, excuse the users from doing their homework. Sharp stresses that “understanding how gain affects the radiating properties of an antenna makes it easier to make the proper selections.”
Sharp adds these words of caution: “Antenna selection, alignment and placement can be very confusing, as many different aspects of the process must be taken into account. This process is absolutely the most critical part of an RF communications system because, if any of these elements are incorrect, the radio system simply will not communicate.”
Wireless systems always face the issue of data transmission delays, or latency. “Data transmission latency—mostly from data collisions within the WLAN’s shared medium that require data to be resent—can cause control problems due to delays between a sensor’s parameters sending a trigger signal for a PLC response and the PLC’s reception of that signal. This can compromise the effectiveness of industrial controls, in terms of response times,” Pitterling notes.
There is no way to extinguish latency, but choosing the media access method that is right for your application can help alleviate it. “Two standard IEEE 802.11 media access methods are the distributed coordination function (DCF) and the point coordination function (PCF),” says Pitterling. “Each defines a different method to coordinate data communications in the shared WLAN medium. While in DCF all devices have to request the right to talk, PCF offers the option to let the AP manage the nodes via the assignment of what are essentially ‘talking rights.’”
The latter, Pitterling says, is more suitable for applications requiring continuous data flows, because it makes sure that everyone gets to talk, but transition times in the case of roaming are not optimized and therefore are still in the range of several hundred milliseconds—which is unacceptable for industrial control.
B&B Electronics’ Fahrion observes, “Bit error rates for wireless communications are orders of magnitude higher than those for wired communications. Most radios quietly handle error detection and retries for you, but at the expense of software and variable latencies.”
He adds that protocols that are sensitive to inter-byte delays may require special attention or specific protocol support from the radio. “Do your homework up front. Confirm that your software won’t choke, that the intended radio can get along with your protocol, and that your application software can handle it as well,” Fahrion adds.
The beneficial mesh
Another factor promoting reliability of wireless networks is mesh technology, an innovation that all users and potential users of wireless technology should be aware of. “Industrial wireless products have been used within plant facilities for decades,” notes Soroush Amidi, with automation vendor Honeywell Process Solutions. “So what technological evolution has triggered the wider adoption and usage of wireless within the process industry? The short answer is wireless meshing. Wireless meshing permits neighboring devices communicating using the same protocol to be interconnected and exchange data amongst themselves, hence significantly increasing the availability and reliability of the data being communicated over a non-deterministic medium.”
Pitterling advises that “a mesh network can be designed using the routing technique. Routing propagates the message along a path, by hopping from node to node until the destination is reached.” To ensure the availability of a path, a routing network allows for a continuous connection and reconfiguration around paths that are broken or blocked, employing self-healing algorithms.
“Self-healing capabilities enable routing-based networks to operate when one node breaks down or a connection goes bad. As a result, the network is considered to be very reliable, since there is often more than one path between a source and a destination in the network,” says Pitterling.
Beware of COTS
Just because 802.11 is a commercial off-the-shelf (COTS) standard doesn’t mean that COTS products can be used in the industrial environment, warns Advantech’s Lochhaas. Industrially hardened access points “will survive in the industrial environment when exposed to extremes of temperature, dust, humidity, hazardous vapors and other common situations,” he says. “So when selecting your industrial wireless products, be certain to determine their suitability for the service you wish to use them in.”
Fahrion notes, “Industrial environments are harder on wireless than any office would ever be,” which can drastically shorten the useful lifespan of wireless equipment. You get what you pay for in this world, and using office-grade networking equipment to save small amounts of money upfront will turn out to be a huge mistake when you have to pay for it later with downtime and repairs.”
Fahrion’s comments could serve as a handy maxim to guide your approach not just to COTS components, but to wireless implementation in general.
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