On 17th of September 2012, AutomationWorld reported the unveiling of Emerson’s IEC 62591 compliant WirelessHART interface for use with its remote terminal units. Emerson has targeted this interface at upstream Oil and Gas applications and believes that the WirelessHART should make the sensing network extremely flexible without compromising on the communication reliability. While this news of a process controls giant taking a leap of faith as far as adoption of wireless networking for building critical sensor networks may seem a big step in the process industry setups, for some of us who have been following this evolution, especially that of WirelessHart, aren’t very surprised.
From its first release in 2007 to now, there has been a terrific momentum of its adoption and in one direction. While one of the principal driving forces behind the protocol has been the process giant Emerson, there are others like ABB, E+H and Nivis who have joined hands to build products that are WirelessHart based. The phenomenal growth is also fuelled by the proliferation of wireless sensor networks by the process industry and while there is a competing standard by ISA (100.11a) which is marketed as a future proof standard, WirelessHART because of millions of existing connected HART based devices is growing very fast. More than 8,000 WirelessHART networks are currently installed in major manufacturing sites around the globe, tripling the number of devices from 12 million to about 35 million in the last 2 years, signifying the acceptance of the WirelessHART standard by the process automation industry.
What is WirelessHart and how does the protocol enable reliable industrial grade wireless communication?
WirelessHart is a wireless sensor networking technology based on Highway Addressable Remote Transducer Protocol (HART) and uses IEEE 802.15.4 compatible radios operating in the 2.4GHz ISM band employing direct sequence spread spectrum technology and channel hopping for communication security and reliability, as well as TDMA synchronised, latency-controlled communications between devices on the network. Each device in the mesh network can serve as a router for messages from other devices extending the range of the network and provides redundant communication routes to increase reliability. The Network Manager determines the redundant routes based on latency, efficiency and reliability. To ensure the redundant routes remain open and unobstructed, messages continuously alternate between the redundant paths.
If a message is unable to reach its destination by one path, it is automatically re-routed to follow an established redundant path without data loss. WirelessHART supports multiple messaging modes including one-way publishing of process and control values, spontaneous notification by exception, ad-hoc request/response, and auto-segmented block transfers of large data sets. These capabilities allow communications to be tailored to application requirements thereby reducing power usage and overhead.
What makes the WirelessHART protocol a promising technology?
- First up, it is built on a solid HART standard foundation, ensuring that it addresses the basic challenges regarding handling process measurement and control problems. Also starting out with an established protocol reduces the risk of unforeseen problems with the technology or the development process.
- The HART protocol fundamentally supports on-demand communication as it is needed, making it a good choice for wireless applications where long battery life is important against most other bus protocols which require continuous communications that drain batteries quickly. It also permits selection of the power option that best meets application needs. Example options include long-life batteries, solar power, line power, and loop power. Other measures that are used to reduce communication overload are Smart Data Publishing and Notification by Exception.
- The onboard diagnostics in millions of installed HART devices mostly go unused because their host systems can’t access digital HART data. WirelessHART adapters unlock this ‘trapped’ data by providing a new communication path to asset-management systems, historians or other tools.
- WirelessHART includes several features to enhance reliable communications;
- Redundant mesh routing (space diversity): WirelessHART mesh topology with self organising and self-healing characteristics where if there is interference or other obstacles interrupt a communication path, the network immediately (and automatically) re-routes transmissions using path optimised, redundant mesh topology.
- Channel hopping (frequency diversity): WirelessHART ‘hops’ across the 16 channels defined by the IEEE 802.15.4 radio standard to overcome interference in the ISM band. Automatic clear-channel assessment before each transmission and channel blacklisting may also be used to avoid specific areas of interference and minimise interference to others.
- Time synchronised communication (time diversity): All device-to-device communication is done in a pre-scheduled time window, which enables collision-free, power-efficient, and scalable communication. Each message has a defined priority to ensure appropriate Quality of Service (QoS) delivery. Fixed time slots also enable the Network Manager to create the optimum network for any application without user intervention.
- Additional techniques such as DSSS technology (coding diversity) and adjustable transmission power (power diversity) also help WirelessHART provide reliable communication even in the midst of other wireless networks
- WirelessHART employs robust security measures to ensure the network and data are protected at all times. These measures include:
- 128-bit encryption prevents sensitive data from being intercepted
- Verification where Message Integrity Codes verify each packet
- Key Management where rotating keys can prevent unauthorised devices from joining or communicating on the network
- Authentication ensures that devices aren’t allowed onto the network without authorisation