First introduced in 2007, WirelessHART has become the standard of choice for wireless communication with industrial field devices. Also known as IEC 62591, WirelessHART is the first international standard for wireless industrial field devices. Daniela Roth, marketing communications for MESCO Engineering GmbH, explains why the standard has become so well-accepted in such a short time. "WirelessHART fits well to HART technology with wired communication."
"Most all industrial customers are already familiar with HART," says Moore Industries International national sales manager Gary Prentice. "I believe it is this embedded familiarity that has given WirelessHART its head start. Moreover, most all of the asset management systems and hand-held communicators used for programming field devices are already based on HART. Wireless standards or protocols other than HART have to be further explained when it comes to seamless integration with other devices or existing legacy control and management systems."
Gary Cusick, president of MACTek Corporation, says, "The industry recognizes the benefit of a wireless solution based on the industry-standard HART protocol. Users expect a wireless solution to be based upon their existing devices and skill level. Also, WirelessHART technology is supported and implemented by a large number of device suppliers around the world.
"Third," Cusick continues, "WirelessHART is now field-proven, with more than 8000 networks (and more than 10,000 devices) working in process applications worldwide. Lastly, "users have confirmed that WirelessHART, is a simple, reliable and secure solution that truly addresses the needs of the process industry. Users have come to know that a wireless solution that claims to be everything to everybody is not a practical solution and will add unnecessary complexity to the solution."
Figure 1: WirelessHART is a robust self-organizing and self-healing mesh network.
Many of the 280+ members of the HART Communication Foundation are offering or preparing to offer WirelessHART devices. Jonas Berge, director of applied technology at Emerson Process Management, says, "WirelessHART has more supporting instrument suppliers than any other wireless industrial protocol. Interoperability is assured because they all use the same common application protocol: HART commands. All features for all instruments regardless of vendor can be accessed thanks to the HART standard."
Robert Schosker, product manager for intrinsic safety, HART, signal conditioners, power supplies and surge at Pepperl+Fuchs, notes, "Nearly 70% of all products shipped today incorporate the HART protocol. Integrating a wireless technology on top of a hugely successful standard protocol that’s simple, easy and already understood makes HART a viable—and trusted—option as a wireless
provider. Secondly, getting to the market first helped immensely. And almost every manufacturer of field devices offers HART as part of its product portfolio, making the availability of products easier and offering less risk to the customer."
Having identified HART as a good basis for a wireless instrument network,, the creators of WirelessHART needed to take into account the needs and requirements of the major end users of instrumentation. The major instrument vendors and organizations, such as the HART Communication Foundation, carried out customer surveys and received the same top three requirements: "Make it simple, reliable and secure."
Customers also said they needed a network that can monitor itself and repair problematic pathways automatically and in good time. Reliability was a key focus of the WirelessHART development team.
Another key focus was security. Data must not only be encrypted, but as recent malware attacks on field controllers have shown, it must also be authenticated to make sure it has not been changed since transmission.
Finally, one of the most powerful features of HART, and what has made it so successful, is its simplicity of operation. As with the wired HART protocol, a master device issues a command and a slave instrument responds. WirelessHART has to be more aware of power requirements, so has included additional features, such as reporting on exception or changing refresh/update rates based upon conditions.
WirelessHART is simple enough to work with for commissioning engineers, operators and maintenance technicians. The network has many automatic functions built in to simplify engineering—it is as simple as 4-20 mA.
A Word About Power
While WirelessHART was designed for battery operation, there are many applications in which the transmitter will be wired for power, but not to signal. But it is in battery operation that users have to consider power.
The standard was designed specifically for low-power operation, less than 4 mA at 12 VDC. Update rate (how many times a minute the transmitter wakes up and takes a measurement and transmits data) affects battery life, as do cold, heat and other environmental considerations.
Several vendors provide WirelessHART devices with integral batteries that are approved for hazardous area classifications.
About the Mesh
Emerson’s Jonas Berge says, "In a plant environment full of steel, you need mesh topology with multiple hops and multiple paths to ensure reliability. Only WirelessHART provides both true mesh technology with multiple hops and star topology (direct connection to the gateway for faster updates) if needed. It is self-organizing, so no undue effort is required to make it work—it works out of the box. Routing devices also mean a single gateway covers a large plant area. Other wireless technologies are star topology with limited hops. This requires a large number of backbone routers, which is impractical and costly to wire up in an existing plant."
Every WirelessHART device, including the WirelessHART adapters now available from a number of vendors, is seen by the network as a traditional HART device. Wired and wireless devices can be mixed together, providing input to the control system. Every WirelessHART device can be used as a router. In other systems, devices can either be endpoints or routers. This can produce a serious issue for maintenance—having to keep multiple types of similar devices on hand and available and easy to differentiate at 2 a.m.
The gateway provides the connection to the host network. The input to the gateway is the WirelessHART mesh network. The output is the signal to the control system via the main host interfaces—Modbus, Profibus and Ethernet. The gateway also provides the network manager and security manager. The network manager builds and maintains the mesh network. The security manager manages and distributes security encryption keys and holds the list of devices that are authorized to join the network. If a network path becomes unreliable or broken due to an obstruction or interference, the network manager will automatically find an alternate route for the information.
HART, WirelessHART and Data Rates
Some may think that HART is slow. HART has been used to report slower moving process data (tank level, temperature) via multi-drop, or even the secondary process variable data from multivariable instruments. But WirelessHART has been targeted at process applications, including monitoring and control, and operates at 250 Kbps—significantly faster than the 31.25 Kbps rate at which Foundation fieldbus (FF) and Profibus PA operate.
Again, Jonas Berge comments. "Can you use WirelessHART for control? The technology supports it. It is time-synchronized and scheduled. One-second update period transmitters are now available. Discrete on/off signal and actuating devices are now available too. On/off valves are coming. Gateway routing algorithms are being optimized for ever lower-latency inbound and outbound traffic. Slower loops like large capacity analytical, temperature and level are possible candidates for wireless control."
Figure 2: WirelessHART network include gateways, process connected WirelessHART devices, repeaters and adapters.
Threshold Triggered Smart Reporting
Originally, HART was intended to be a polling system, with the host polling each slave instrument in turn. Since then, new features such as report-on-exception and smart reporting have been added. This permits the device to report when a pre-defined condition occurs, increasing battery life.
Network Security
The WirelessHART technology was designed to enable secure industrial wireless sensor network communications while making sure that ease-of-use is not compromised. Security is built in and cannot be disabled. Security is implemented with end-to-end sessions using AES 128-bit encryption. These sessions ensure that the messages are enciphered so that only the final destination can decipher and use the payload created by a source device.
To be a credible threat, an attacker must possess access, knowledge and motivation. WirelessHART security architecture addresses all three of these areas by minimizing, controlling and auditing access; requiring high levels of technical expertise to subvert the network; and reducing the consequences (span and duration) of any individual security breach.
WirelessHART does NOT use TCP/IP and is therefore safe from the typical Wi-Fi hacker.
Planning a WirelessHART Network
Because of the self-building and self-healing nature of the network, it is as simple as 4-20 mA. The mesh network reduces the amount of effort required at both the planning and commissioning phases. There is little difference between commissioning a wireless network and a wired one.
Site surveys are not as detailed as would be required with other topologies. A simple walk through of the plant to identify instrument locations will also identify possible obstructions. A pathway around an obstacle can be provided by adding a router. This is similar to the plant walk-through required for planning cable runs. junction box and marshalling cabinet locations.
Commissioning is also similar to commissioning 4-20 mA loops. There is very little information more than what would be entered using a hand-held communicator for a wireless instrument. The commissioning engineer completes the joining process and can monitor the join status on the hand-held. There is a loop test command just as there is in wired HART. And this is done using the same hand-held that is in your tool box today!
Making the Mesh More Robust
The way to increase the robustness of the network is to add more devices to the network. Each additional device geometrically increases the number of potential paths information can travel through the mesh. The WirelessHART mesh network is designed so that up to 10,000 devices can be added to a single network.
WirelessHART builds on the solid foundation of HART communication, enabling users to quickly and easily gain the benefits of wireless automation while maintaining total compatibility with existing devices, tools, skill levels and systems. That’s why WirelessHART has become the most commonly used process automation wireless field instrument network. It is simple, reliable and secure, and…it is HART.
