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Because every WirelessHART device is also a router, any device can direct and re-transmit data from any other device. Users do not have to maintain, stock, and learn how to operate two kinds of field devices.
Some of the most important process variables are those that today are not measured because the cost of wiring them into the control system is simply unmanageable. These include remote level measurements and asset-management information. In such applications, low-power sensors, many operating on batteries, are critical to making these measurements economically viable. Battery-operated WirelessHART devices make these measurements practical.
In keeping with the spirit and tradition of HART to “keep it simple,” the HART 7 standard requires that battery life be reported as a PV (process variable) and reported in “days remaining.” This key performance indicator enables operators and maintenance personnel to see simply and clearly when a battery in a given WirelessHART device must be changed.
WirelessHART, like all the rest of the HART 7 specification, is built on backward-compatibility and Electrical Interchangeability. HCF’s definition is rigorous: the freedom to use any manufacturer of HART devices, wired or wireless, and to mix manufacturers and products in the same wireless network.
With WirelessHART, users may mix any number of manufacturers’ devices with any vendor’s Registered HART wireless adapter and any other vendor’s Registered HART wireless access point or gateway. The same HART Universal Commands are required in all WirelessHART devices, just as they are in HART wired devices. The same Universal diagnostics exist in every HART device, wired or wireless, and may be used in exactly the same way. The same tools work with all HART devices regardless of manufacturer, and the EDD for that HART device (wired or wireless) works on all EDD-compliant hosts.
And any of those devices may be replaced in kind by another HART-enabled device from any manufacturer with instantaneous interoperability.
This incredible flexibility is provided on top of the largest installed user base of devices in the world—more than 26 million HART-enabled devices as of mid-2008.
As Sean Ireland, wireless product manager for Siemens Energy and Automation notes, “WirelessHART gives us the opportunity to provide flexibility to the customer for installation, a lower cost for installation, the ability to enable new applications previously not accessible due to technology or cost limitations, and improved information availability, which should result in process improvements for our customers.”
WirelessHART was built on proven industry standards. The radio is based on the IEEE 802.15.4-2006 standard. WirelessHART is also built on the IEC HART Standard, IEC 61158, and on the IEC 61804-3 standard for EDDL. The wireless portion of HART 7 also has been submitted for ballot to become a global IEC standard.
The 2.4 GHz frequency band used in WirelessHART radios was selected because of its international availability. It is available, usually without licensing, in almost every country, and is reserved for industrial, scientific and medical uses, and other commercial communications needs. Many cordless telephones also operate in this band, as do other IEEE radio standards, such as 802.11(a, b, g and n), the WiFi standard.
Many wireless applications and protocols are in use already in the modern industrial plant: cellular and VoIP telephony, WiFi, WiMAX, Bluetooth, and other wireless sensor networks.
WirelessHART communication is designed to result in low bandwidth usage. The communication protocol is a channel-hopping TDMA protocol. This protocol has 100 10-ms “communication slots” per second over 15 channels, allowing multiple devices to communicate in the same slot, but at different frequencies. This avoids interference and reduces multi-path fading. WirelessHART also provides priority-based message routing so the network stays up, and alarms and critical data packets get the appropriate priority. The Network Manager continuously assesses network health and retries and may “blacklist” any of the 15 channels to reduce latency and prevent interference from existing networks that might be in use.
“An upper bound on bandwidth consumption can be estimated, assuming a large number of devices being in a single geographic region, so that any device in the region can hear all other devices in the region,” says Wally Pratt, chief engineer of the HART Communication Foundation. “The HART standard provides estimates on communication range that would define the boundary size (diameter) of the spherical region containing that large number of devices or access points in the network. The number of devices in the region does not limit the number of devices or access points in the network. The network can have many devices with multiple hops and cover an area much larger than the ‘region’ we are considering here.”
For example, if one were to have 10,000 WirelessHART devices located in a spherical region, meeting the above criteria with an average update period of 60 seconds, communicating through 15 access points, the bandwidth consumed would be approximately 4.3% The diameter of the spherical ‘region” would depend on whether the installation is indoors or out and the configured transmit power setting for the devices. Consequently, the region can vary from 200 meters in diameter to less than 20 meters in diameter.
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