Most networks are wired systems based on dozens of proprietary protocols and, more recently, on Ethernet. In an industrial environment, whether it’s manufacturing, process control, transportation, or building automation, these networks are used for monitoring and control in both open-loop and closed-loop control systems. Sensors monitor the physical states of the process. Control signals initiate or control the various parameters of the system. In most cases, the sensor and control points are far from the control system, usually meaning long cables and all of their attendant issues.
This has engineers taking a closer look at wireless networking options, which are more viable than ever before thanks to the wide variety of available technologies. They can offer benefits that were previously unthinkable in most monitor and control operations.
NOT YOUR USUAL OFFICE LAN
Industrial networks differ from the typical office Ethernet local-area networks (LANs), which are religiously administered by their organization’s IT dynasty.
First off, their environment is harsh compared to the comfy setting for most LANs. They’re located in factories, plants, remote buildings, and even outdoors along oil and gas pipelines. Thus, industrial networks are subject to weather, temperature extremes, vibration, chemical fallout, and all sorts of other nasty climates.
Second, industrial networks are usually mission-critical. They’re relied upon for the operation of systems that can’t tolerate any kind of failure or downtime. While an office LAN can go down, delaying an e-mail or Internet search, a failure in an industrial network may shut down a profitable production line, process run, or other 24/7 functions leading to crisis conditions. Reliability must be golden.
Furthermore, industrial networks are subject to noise, moreso than in an office LAN. The noise comes from high-voltage ac lines; the switching of motors, relays, and solenoids; switching power supplies; and various wireless sources. Industrial networks need more than standard noise immunity and protection.
Industrial networking also can be characterized by deterministic operation, where the timing of the various operations is critical. Security may be an issue as well, so operations aren’t compromised by outside nefarious sources or unintentionally by well-meaning employees. Finally, interoperability among multiple different networks is often an issue.
All of these special requirements usually add up to a wired network as the best choice. But with today’s vastly improved wireless technologies, designers can use a wireless solution that meets all of the typical requirements and brings some significant benefits.
WHAT’S IN IT FOR YOU?
Perhaps the greatest benefit of wireless networks is the cost savings, especially if you’re building a new network. Wiring is expensive— copper cables have dramatically increased in price over the years. Industrial installations require conduit and other special wiring considerations to ensure reliability in rough environments. And, as experience has shown, the weak links in most wired networks are the connectors.
Wiring must be installed by licensed electricians or certified technicians at a cost often exceeding $100/hour. Even short runs of twisted pair in a conduit several hundred feet long can cost tens if not hundreds of thousands of dollars and take weeks or months to install.
Despite the cost of the wireless equipment, labor expenses are minimal, and installation time is extremely short. Maintenance costs must also be considered. With no wiring, there’s nothing to maintain except the wireless transceivers. While they do need a battery change every now and then, wireless transceivers are very reliable. In older equipment, that replacement interval was often every few months. Modern wireless systems use very little power and may only need a battery replacement every several years.
Range was a problem with older wireless systems, too, but that’s lessened with recent technologies such as mesh networks. While line-of-sight (LOS) operation is required for most wireless today, signal blockage may still be a problem. Workarounds usually can be found, though. Repeaters, gain antennas, and other solutions are common.
Finally, security may be an issue. Most newer wireless technologies incorporate encryption and other security measures, making it far less of a problem.
SOMETHING FOR EVERYONE
So with all of these benefits, wireless should be the top consideration, especially when installing a new network or replacing or retrofitting an older network. The next step, then, is selecting the wireless technology standard—a decision likely predicated on data rate and distance (Fig. 1).
IEEE 802.15.4 and ZigBee: For short range (less than 30 m), radios based on IEEE 802.15.4 are a good choice. They usually operate in the industrial-scientificmedical (ISM) spectrum from 2.4 to 2.483 GHz, which is available worldwide. The maximum data rate is 250 kbits/s, which is more than adequate for most industrial applications. Other ISM spectrum options include 868 MHz in Europe and 915 MHz in the U.S. at lower data speeds of 20 and 40 kbits/s, respectively.
Built on the 802.15.4 physical-layer (PHY) and media-access-controller (MAC) standard, ZigBee is ideal for mesh networking. That’s because it can signifcantly extend the range and reliability through node relays. ZigBee also is a top choice for industrial sensor networks. The low-dutycycle operation translates into low power consumption and long (like years) battery life.
Wi-Fi, 802.11: Wi-Fi is the main choice of wireless networking in enterprise LANs. It comes in several configurations, with data rates exceeding 100 Mbits/s in some forms. The workhorse 802.11b version is the most widely used, capable of 11 Mbits/s up to a range of 100 m. The 802.11g version supports data rates to 54 Mbits/s at that same range. The most recent version, 802.11n, has yet to be ratified. However, a Draft 2.0 version is now selling and offers rates to 300 Mbits/s using multiple-input/multiple-output (MIMO) antenna technology.
All of these versions operate in the same 2.4-GHz spectrum. The 802.11a version operates in the 5.8-GHz ISM band at a rate to 54 Mbits/s. That band offers less interference and fewer co-existence problems, but its range is slightly shorter.
In the past, the 802.11 wireless standards were less desirable for industrial applications, mainly because of their higher power consumption. But low-power versions from a number of chip suppliers have made 802.11 viable even in sensor or actuator applications, where long battery life is essential for minimum maintenance. When your data-transport application needs high speed and long range, it becomes an excellent choice. It also matches up nicely with the corporate office LAN. The 802.11i security standard goes beyond the usually used WEP, WPA, WPA2, and other encryption standards to ensure a very high level of security in critical links.
Proprietary standards: An interesting alternative in industrial applications is to use a company-specific radio that doesn’t conform to any of the common wireless standards. These radios, which employ unique protocols or wireless versions of wired industrial protocols like Modbus, are often a better choice for some applications. Most still use the ISM spectrum in the 902- to 928-MHz or 2.4-GHz bands. If you don’t have to be compatible with a company LAN or the Internet, they’re a solid option. Usually, they’re a better match for specific applications using proprietary standards like Modbus, Profibus, or HART.
Cellular: For the long range, special cell-phone modules are available for connection to industrial networks. Both cdma2000 of Verizon and Sprint Nextel and GSM/EDGE/WDCMA of AT&T and T-Mobile are available. These standards use the data capability, which is usually fast enough for most industrial monitoring and control applications.
OPEN SOFTWARE OPENS YOUR CHOICES
From a systems point of view, data from wired and wireless sensors alike is transmitted to a measurement system. The measurement system is typically a programmable logic controller (PLC), an industrial PC, or some kind of programmable automation controller (PAC). Often, this sensor data is also needed at the enterprise layer.
According to Robert Jackson, product manager of National Instruments, you can use software that supports multiple standards. This allows users to select the best wireless protocols for the application. NI’s LabVIEW software can be used to connect ZigBee or other 802.15.4 battery-powered nodes through Lab- VIEW instrument drivers that are available for Banner, Crossbow, Accutech, and Accsense gateways. Or, they can select Modbus to connect to other wireless gateways.
By using LabVIEW on a PC or a real-time PAC, you can then integrate wireless data from different vendors, combine it with wired measurement data, and share it at the enterprise layer (Fig. 2). Wi-Fi 802.11 is often selected to share data between distributed measurement systems and PCs and laptops due to its broad availability.
New Wi-Fi devices like NI’s WAP3701 industrial-grade wireless access point provide what’s needed to deploy Wi-Fi in industrial environments. With this approach, you can integrate wireless technologies into existing systems and select multiple protocols to best fit the application.
THE LATEST PRODUCTS
B&B Electronics, a longtime supplier of wire, cable, and communications I/O for industrial applications, now carries a wide range of wireless options. For example, its proprietary Zlink radio modems and I/O module units use the popular Modbus protocol (Fig. 3).
Models are available to operate in the 900-MHz or 2.4-GHz bands with different power levels. Modulation is frequency shift keying (FSK), and data rates of 9600 bits/s to 115 kbits/s are available. Different power output options let you select a model for the range you need. The I/O serial communications can be RS-232, RS-422, or RS-485.
According to Dennis Fairfield, B&B’s wireless product manager, the Zlink products’ key feature is a wide range of matching I/O modules to accommodate both digital and analog I/O. Analog I/O may be 4 to 20 mA, 0 to 20 mA, or 0 to 10 V. Digital inputs in the 0- to 48-V range can be used, and digital outputs in the 10- to 48-V range are possible. Options for temperature sensors, including resistance temperature detectors (RTDs), are available.
Banner Engineering’s SureCross Wireless network is a rugged wireless I/O solution that can fit a wide range of applications. The SureCross Network uses a node transceiver to collect data that’s then transmitted to a gateway system controller. This bidirectional system can be used to build a variety of monitoring and control systems. The radios are available in either 900-MHz or 2.4-GHz models. They all use a proprietary protocol.
The technology is frequency-hopping spread spectrum with FSK modulation. A time-division multiple-access (TDMA) scheme provides two-way time slots for data and control signals. With standard antennas, the 900-MHz version can achieve a range to three miles, while the 2.4-GHz version can reach a maximum of two miles.
The SureCross system is compatible with an extensive range of sensors, including RTD, themistor, thermocouple, photoelectric, ultrasonic, capacitive, inductive, pressure, contact closure, and flow. Models are available with a mix of both analog and digital I/O lines. SureCross also supports other serial communications formats, such as Modbus RTU RS-485, Modbus RTURS- 232, Modbus TCP/IP, and Ethernet/ IP. Banner has a new wireless developer’s evaluation kit for the SureCross system, designated DX70, as well.
Digi International’s recent acquisition of Maxstream gives it a real ZigBee presence. Its XBee Znet and XBee-PRO modules are designed for OEM applications (Fig. 4). They drop right into other products or can be used to quickly and easily set up full-blown mesh networks. The company’s XBee Wall Router plugs into an ac outlet and provides repeater functions to extend a mesh network.
Meanwhile, the Digi Connect WAN 3G wireless router supplies connectivity to remote sites and devices (Fig. 5). This Ethernet- to-cellular router supports both HSDPA and EV-DO 3G cell-phone technologies. It also provides primary wireless wide-area network (WAN) connections to remote sites containing Ethernet and serial devices, such as construction sites, power utility substations, retail point-of-sale (POS) sites, temporary facilities, and other remote places where a wired network isn’t feasible.
The Digi Connect WAN 3G features a built-in virtual private network (VPN) for secure connections, one Ethernet port, one serial port, a sensor port for connecting Digi’s Watchport sensors, and a USB port for connecting to Digi’s Watchport USB camera or for an external GPS device. According to Lynn Linse, principal engineer with Digi, the company makes a full line of other wireless products, both standard wireless and proprietary, for just about any application, plus all of the support software.
Freewave Technologies’ licensed FGRIO-S30 Modbus Industrial radio uses the 900-MHz band. Also, its 1-W transmitter can extend range to 40 miles or more. It’s ideal for long-range remote control and monitoring of tanks, lifting stations, pumps, flow meters, fluid levels, water sources, temperature and pressure, and the water/wastewater, oil, and gas industries.
Like other wireless industrial products, its main function is to reduce costs for wiring while providing solid, reliable connections to the system. The FGRIO-S30 can deliver data from remote analog (4 to 20 mA or 1 to 5 V dc) and digital sensors or third-party equipment over a wireless link to a remote terminal unit (RTU) or PLC. It can operate in either the Modbus mode or the wire replacement mode.
In addition, it offers two analog outputs in the 0- to 22-mA range and two digital inputs up to 1000 Hz for counters. Furthermore, it can program digital outputs to self-shutoff after a programmed interval to safeguard intermittent-rated loads. All analog inputs are read as both 32-bit floating point and 16-bit integer formats to reduce the computation required at the RTU. The FGRIO-S30 is available now.
Micrel’s MICRF218 receiver, which is part of the company’s QwikRadio family, is the first programmable receiver with jam avoidance. This amplitude-shift-keying/on-off keying (ASK/ OOK) receiver operates in the 300- to 450-MHz range and targets garage door openers, tire-pressure monitoring systems, and a variety of critical industrial-control applications. It also has a selectable IF bandwidth.
Its maximum data rate is 10 kbits/s using Manchester coding. An analog received signal strength indicator (RSSI) output is provided. This unique chip can detect interference on one channel and switch to another. It also can accommodate two different crystals to set the operating frequency using an external switch. The IF bandwidth and data filtering are selectable. The chip is available now for $1.71 in 10,000-unit quantities.
RF Technology’s TinyOne Lite 433-MHz low-power radio module boasts an output power of 10 mW and receiver sensitivity as great as –102 dBm, achieving a maximum range of about 500 meters depending on environment and antennas. It supports data rates of 10, 38.4, 100, and 115.2 kbits/s. The modulation is Gaussian frequency-shift keying (GFSK) in the 433.5- to 434.7-MHz ISM range. The input is RS-232 and TTL.
Finally, One RF Technology offers the S-One and M-One stacks. The M-One stack is ideal for mesh networks in home applications, sensor networks, and irrigation systems. One RF Technology also provides other radio modems for the 868- and 915-MHz and 2.4-GHz bands, including ZigBee.
Need More Information?
B&B Electronics • www.bb-elec.com
Banner Engineering • www.bannerengineering.com
Digi International • www.digi.com
Freewave Technologies • www.freewave.com
Micrel Inc. • www.micrel.com
National Instruments • www.ni.com
One RF Technology • www.one-rf.com
Wireless Industrial Networking Alliance • www.wina.org