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."
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.
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.