Choose Wireless Wisely
Jonas Berge,Director of Applied Technology, Emerson Process Management

Over the past few years we have seen several case studies of how IEC 62591 (WirelessHART™) transmitters have been used in process and asset monitoring tasks that previously were too risky or impractical to automate by laying cables in an operating plant. But what important characteristics of the WirelessHART technology makes it so uniquely adapt for process applications?
By: Jonas Berge, Director of Applied Technology, Emerson Process Management
There are many wireless technologies available, but choose wisely, for while the right technology will bring you piece of mind, the wrong technology will take it from you.

The Only Standard
Industrial wireless sensor networks have existed for more than ten years. However, the first generation of wireless sensors were impractical because every manufacturer had a different proprietary protocol. Products from different suppliers did not work together. Multiple gateways and configuration software would be required to use different kinds of sensors. This mirrors the early days of "smart" protocols for 4-20 mA transmitters: every DCS manufacturer used a different protocol and only supported their own devices, locking the plant in. Multiple handheld terminals were required. Eventually the HART protocol prevailed, and thanks to 4-20 mA/HART, most DCS can now communicate with any 4-20 mA device, and only a single field communicator is required. WirelessHART is doing the same for wireless.

IEC 62591 is the only international standard for wireless in process applications. It was originally developed by the HART Communication Foundation (HCF), released in 2007, and subsequently approved as an international standard IEC 62591 in April 2010. IEC 62591 is not a subset of any other standard.

Standard WirelessHART products are already here, thus there is no need to buy proprietary or "standards-ready" wireless products now, and later have to worry how to upgrade.

The same common handheld field communicators and laptop software that plants already have to configure all their 4-20 mA/HART devices, can also be used to securely commission all WirelessHART devices regardless of manufacturer or type before it joins the WirelessHART network. Similarly HART documenting calibrators also work for WirelessHART. This makes WirelessHART easy, requiring minimal training. Moreover, commissioning over the wires using a common tool ensures no data is exchanged "over the air" before wireless security has been established.

WirelessHART security measures include encryption, authentication, verification, key rotation, and sequence number etc., but if not enabled it would be useless. For this reason, WirelessHART security cannot be turned off, ensuring these security measures are constantly active.

Meeting Process Users' Needs
WirelessHART was developed to meet the requirements of process industry users. In the early phase of development the HART Communication Foundation (HCF) collected wireless requirements from end-users in the process industries. Users demanded coexistence, reliability, long battery life, security, and multi-vendor interoperability etc. – all that plants need to solve process problems - and end-users are experts on their process. However, end-users are not necessarily experts on RF design or communication protocol stacks, so HCF member companies sent their experts on RF and digital communication to design the WirelessHART technology to meet these process requirements.

Verified by End Users
The NAMUR organization of chemical companies has a requirement specification for wireless known as NE 124. A field trial was organized by NAMUR at the BASF plant in Ludwigshafen in Germany in 2009. The trial included gateways and devices from ABB, Emerson, Endress+Hauser, MACTek, Pepperl+Fuchs, and Siemens. The trial proved that WirelessHART meets process user requirements. By now, WirelessHART has been put into operation in more than 2,100 plants.

What about Wireless Plant Network?
WirelessHART is a wireless "Level 1" network for sensors and actuators that complements the existing level 2 industrial protocols that may use Wi-Fi (wireless Ethernet) such as Modbus/TCP, EtherNet/IP, FOUNDATION fieldbus HSE, and PROFINET etc. as well as level 3 and 4 standard protocols for web browsing, email, file transfer, voice, and video etc.

Large Networks
It is not a good idea to put all eggs in one basket, so a single wireless network covering the entire plant may not be the best way. A practical solution is instead having one network per plant area, just like plants have one DCS controller per plant area. It fits in with the DCS architecture in most plants and the division of job responsibilities within the plant. WirelessHART supports this architecture. It is even possible for the same area to have different gateways for process automation and asset optimization if preferred, but usually a single gateway is used. Data from gateways in different plant areas is backhauled over Ethernet or Wi-FI using the HART-IP protocol, Modbus, or OPC. WirelessHART fits well with existing plant philosophies.

Optimized for Process Applications
Because WirelessHART requirements were collected from end-users only in the process industries, not from factory automation, building automation, or power grid, the WirelessHART protocol is optimized for process application, not a compromise with the diverse needs of other industries. An advantage of this application optimization is that a multitude of communication options need not be set at commissioning. This prevents errors and reduces delays. WirelessHART is designed specifically for process applications.

A True Standard
Over the past few years WirelessHART devices have made their way into all kinds of plants in every industry around the world. A vast majority of WirelessHART devices in operation are so called "native" devices meaning the radio, antenna, and power module (battery pack) are built into the device as a single integrated unit such as a pressure or temperature transmitter. The other solution is wireless adapters that are mounted on conventional 4-20 mA/HART devices and tap into the digital HART communication, "tunneling" the device setup information and diagnostics to intelligent device management software. Such tunneling is only required for the HART protocol, and it is useful because more than 90% of the 4-20 mA/HART devices installed are not digitally integrated, they use 4-20 mA to the DCS. The HART communication is only used with a handheld field communicator at commissioning. A WirelessHART adapter is an easy way to add HART capability to an old DCS. However, FOUNDATION fieldbus, PROFIBUS, and Modbus etc. have no 4-20 mA, they are all purely digital and therefore already digitally integrated with the DCS. Therefore there is no need for wireless adapter or tunneling for those protocols, tunneling HART is sufficient.

By using a single standard protocol, WirelessHART avoids the problem of multiple protocols such as multiple gateways, numerous drivers, and different ways of mapping the process variable (PV) to the system database etc. Similarly, a single protocol avoids multiple ways to configure, multiple ways to calibrate, and multiple ways of diagnosing/troubleshooting device problems. The WirelessHART approach is to use a single common protocol for all wireless devices regardless of manufacturer and type. This makes WirelessHART easy to use. Moreover, native integration of the DCS with the gateway or wireless I/O card is possible without drivers and data mapping. Technicians will configure, calibrate, and diagnose all wireless devices the same way. A single common protocol is the only way to true interoperability. WirelessHART makes interoperability easy.

Manufacturers including Emerson, Pepperl+Fuchs, Stahl, Invensys, Endress+Hauser, Phoenix-Contact, ABB, MACTek, Siemens, RCS, Cooper, SMAR, and ProComSol etc. are rallying around the IEC 62591 (WirelessHART) standard to ensure all kinds of wireless devices required to meet application needs are available with the same protocol. These vendors are shipping interoperable WirelessHART devices today including transmitters for pressure, temperature, level, valve position, pH, conductivity, vibration, and acoustic, as well as on/off contact input, level switch, and adapters. There is no need for multiple protocols to solve plant problems. WirelessHART does the job for all process applications.

WirelessHART operates in the same license-free 2.4 GHz ISM-band as Wi-Fi and other wireless technologies, but since WirelessHART uses channel hopping and channel black listing, WirelessHART is able to coexist with these technologies.

HART Communication Foundation
The HART Communication Foundation (HCF) is a not-for-profit organization with more than 200 member companies including instrumentation manufacturers and users, which provide worldwide vendor neutral technical and application support for HART technology, new technology developments, education, marketing, and interoperability compliance testing. More and more of these members are developing WirelessHART products.

One misconception is that some wireless protocols use "standard D-cell" batteries. However, although the size is the same, no wireless devices use regular carbon-zinc or alkaline batteries. All wireless transmitters use Lithium Thionyl Chloride batteries which have very high power, long life, and wide temperature range but cannot be found in regular stores. They must be purchased from a specialty supplier or through wireless device suppliers. If individual cells are replaced by users, special care should be taken to replace with proper, new cells to prevent potential safety issues.

Technology Ready for Control when You Are
WirelessHART transmitters are predominantly used for process monitoring and asset monitoring. However, the technology is ready for control when you are.

WirelessHART is time synchronized and scheduled with a precisely periodic macrocycle (called superframe), and makes use of publisher/subscriber (called burst mode and catch) communication similar to FOUNDATION fieldbus. The result is deterministic communication. Most WirelessHART products available today support a 4 second update period with 1 second update periods available this year from many suppliers. Master-slave communication is used for remote configuration and diagnostics just like 4-20 mA/HART. WirelessHART is purely digital communication, there is no analog.

Redundant WirelessHART gateways ensure high availability. WirelessHART discrete output and on/off valve couplers are also becoming available this year, enabling wireless actuation.

Full Mesh Topology
Although opinions of wireless experts differ on many points, all agree on at least one point: that self-organizing mesh-topology is the most robust. Mesh-topology is more robust because devices establish multiple paths among themselves, routing messages at device level, in a "mesh". If one of the paths is disrupted, the network automatically switches to another path, thus maintaining a reliable connection. That is, redundant data pathways eliminate single points of failure. Actual installations consistently demonstrate greater than 99% data reliability. Mesh topology is therefore best-practice. Star topology is typically not used although supported by the technology.

WirelessHART uses a unique full mesh-topology as many as seven hops deep, providing an important advantage: WirelessHART doesn't need a costly infrastructure of multiple backbone routers to be installed throughout the plant within range of every wireless device. Running costly hazardous area power supply to backbone routers is therefore also not required. Mesh topology is truly wireless and low risk.

The self-organizing network optimizes itself for greatest reliability and minimal power consumption. The ability to self-organize is critical in a plant environment with intermittent sources of noise and temporary obstructions. With sources of noise ranging from motors and pumps starting and stopping to walkie-talkies, the RFI environment is constantly changing, but a self-organizing network can easily adapt and maintain high data reliability. It also makes adding and removing devices very easy as manual configuration is not required.

Going the Distance
Another misconception is that one protocol using IEEE 802.15.4 radio is longer range than another protocol also using IEEE 802.15.4 radio. This is not the case. The protocol has no impact on distance. All protocols using IEEE 802.15.4 have the same limitations. Range is determined by three factors:
1. Transmission power
2. Antenna gain
3. Receiver sensitivity

Transmission power and antenna gain are limited by national regulations, the only range difference is in radio chip sensitivity. For instance, a radio chip with -96 dBm sensitivity is longer range than a chip with -90 dBm. A future chip with -99 dBm will beat them all. The same IEEE 802.15.4 chip can be used for WirelessHART or any other protocol and they will achieve the same distance so there is no range difference between protocols. An unobstructed range of several hundred meters is possible with raised antennas. However, this is “line of sight” (i.e. no obstruction) as in an open field – which you may find in a flat desert oil field. However, inside a plant full of metal pipes and vessels as well as structural steel, there might be line of sight only above the pipe racks because there is less obstruction at that height. However, down among all the steel, the range is far shorter, as little as 30 m. Line-of-sight range of hundreds of meters becomes irrelevant. Once down among the steel, the extent of coverage is a matter of how well the wireless technology can get around all these steel obstacles. Mesh topology, with its ability to “hop” from device to device, circumventing obstacles, has proven very successful. Moreover, using mesh topology, overall distance is increased by automatically relaying messages from one device to the next. Many hops means longer total distance. For instance, seven hops cover longer distance than just two hops. When range is only 30 m in a dense environment, the ability to multiply this with 7 hops is important. A full-mesh topology allows for longer distance.

Deploying Wireless
After thousands of installations in industries around the world, users have found that choosing their wireless network can be simple with a few rules of thumb. Chose a wireless technology which is an international standard supported by your preferred transmitter suppliers and has all the device types required for your current and future applications. Narrow it down to a single protocol which ensures ease of deployment and long term device maintenance, and which makes use of your existing commissioning tools and requires less training. Make sure to use a self-organizing mesh topology for maximum robustness and ease of management. One common protocol customized for process automation users eliminates the need for superfluous configuration. Last but not least, security which cannot be turned off greatly reduces cyber security risks. Many sites have said that the question is not “if” they will use wireless, its merely a question of “when”. By choosing your wireless network wisely, the investment will be repaid not only on the first application, but for every additional application that is added to the same network in the future.