The Highway Addressable Remote Transducer (HART) protocol is one of the most widely used industrial standards. Millions of devices around the world utilize the HART protocol for process-measurement applications. However, according to the HART Communications Foundation, as many as 90 percent of these devices are stranded, using only the 4 to 20 mA signal and leaving the HART data inaccessible. To help rescue stranded data from these inaccessible HART devices, the HART Communications Foundation introduced the WirelessHART protocol. This relatively new addition makes the popular HART protocol more usable, flexible and cost effective.
WirelessHART is part of HART7, introduced by the HART Communication Foundation in 2007. It is the first wireless protocol designed specifically for process specifications. The technology provides an easy and secure means of accessing stranded data without the hassle of running wire and cable. With the widespread usage of HART protocol, WirelessHART serves as a natural progression. It uses the same maintenance and diagnostic tools as traditional HART devices and requires little additional training. WirelessHART also does not require extensive radio frequency (RF) planning.
WirelessHART offers opportunities for a range of applications — from adding measurements where they were previously out of physical or economic reach, to enabling platform-wide functions such as asset and personnel tracking, security and enhancement of overall productivity.
How does WirelessHART work?
The core technology behind WirelessHART has been established over many years of development, and it continues to improve. The true strength of WirelessHART originates in the Time Synchronized Mesh Protocol (TSMP). By utilizing time-synchronized communications, each device in a network maintains a precise sense of time and thus remains synchronized with neighboring devices in the network. TSMP also schedules each transmission to occur on separate frequencies, providing a vast increase in effective bandwidth. This also serves to reduce power consumption so that the devices can run for several years on a small battery.
WirelessHART combines two different types of wireless transmission technologies: frequency-hopping spread spectrum (FHSS) and direct-sequencing spread spectrum (DSSS). FHSS sends data as it “hops” from one frequency to another, and it has the greatest tolerance for interference. The disadvantage of FHSS is low data rates. DSSS uses a “spreading and de-spreading” technique that offers higher data rates while suppressing interference — but not to the same degree as FHSS. By combining the interference rejection of FHSS and the coding gain of DSSS, WirelessHART creates a robust way to handle interference.
There are three types of WirelessHART devices: instruments, adapters and gateways.
• Instruments transmit information over the WirelessHART network. They rely on battery power, which makes them completely wireless.
• Adapters connect the WirelessHART network to the wired HART network. They bolt on to a wired HART instrument.
• Gateways are the link between the adapters and the instruments. The gateway manages the network, defines routes, ensures security and converts industrial protocols such as MODBUS to HART. This enables communication between remote devices and the control room.
Network planning
WirelessHART is a mesh-style network — every device can talk to every other device. This simplifies its design and deployment. By following recommended best practices, the network designer can increase reliability.
The HART Communication Foundation recommends that every WirelessHART device be within 650 feet (200 meters) of at least two other devices. A WirelessHART device generally can communicate about 100 meters (about 300 feet). Every device should have a backup (or secondary) path in case something should happen to its primary path. The gateway will define the primary and secondary paths, so the designer does not need to define these actions when designing the WirelessHART network.
Generally, more devices in a network will increase the possible distance and redundancy. However, with a larger network, the wireless devices will introduce more chatter. This can decrease network speed and battery life.
Clustering is one way to optimize a large network with wired and wireless HART devices. By adding gateways to a network, the designer can break a large network into smaller clusters. Clustering will reduce the number of “hops” a signal needs to get to its destination. By using the cluster method, more time slots will be available for remote devices. This allows faster update times when needed. With less node routing required, battery life will increase, ultimately improving the efficiency of the entire network.
Benefits in the process industry
Process instrumentation has undergone striking advancements over the past 70 years. HART protocol has played a significant role in the process industry for nearly three decades. As discussed earlier, many early wired HART devices were stranded.
The introduction of WirelessHART in 2007 led to various technical advantages and cost benefits, providing opportunities for process monitoring in various applications. The system meets the critical wireless requirements of industrial plant environments — including reliability, noise immunity and latency — while still using the same maintenance and diagnostic tools as the traditional HART devices. In addition, its flexibility and adaptive capabilities make it well suited for harsh industrial environments.
The time taken to expand or construct a process-monitoring loop can be reduced by installing a wireless system to replace infrastructure and signal cabling. In addition, the up front cost of installing a wireless network is often lower than cabling and conduit costs, and the savings in labor and permits are significant. A signal that took days to bring online using traditional wiring can now be commissioned in hours. The time savings and flexibility allow maintenance crews to deploy wireless nodes for temporary troubleshooting or add previously stranded measurement points for safety and improved efficiency.
WirelessHART enables the collection of HART data that otherwise would be lost. This data provides a more detailed look into an application, providing greater resolution of the primary variable while providing access to extra process and diagnostic data. This information can be used for process improvement or to better utilize instrument technicians by having a clear understanding of what instruments need to be serviced. Overall, WirelessHART is an ideal technology for instrument-rich networks.
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