Pumps and Systems, May 2009
Read how WirelessHART can be implemented in large process plants despite the distances needed for the network.
When we think of wireless networks, we typically think of networks such as WLAN (Wireless Local Area Network) or Bluetooth. We may think of our office environment, perhaps the hotel where we stayed on our last business trip, or even the local coffee shop where we can sip a cup of coffee and surf the Web. For most users this is relatively seamless; you sit down, power-up and away you go.
However, both technologies have somewhat limited distance ranges: up to 30-ft for Bluetooth and 300-ft for WLAN. Additionally, behind the scenes, there are a considerable number of IT professionals making the system work by installing firewalls, IP addresses, subnet masks and security software. They make the difficult appear simple and transparent.
What about wireless communication in large process plants where there are long distances separating systems and equipment, electromagnetic interference and constant environmental changes? Is a robust, reliable, long-lasting and cost-effective wireless network possible in an industrial environment?
Wireless Networks in Large Process Plants-Is It Possible?
Enter WirelessHART. Built on the widely-used HART standard, an open standard, this wireless communication works for brown field or green field installation. Unlike other wireless technologies, WirelessHART is designed to meet the unique requirements of wireless networks operating in process plants.
Most process plant operators, field technicians and engineers are already familiar with the HART communication protocol. They understand it, and they have worked with HART for years. WirelessHART simply combines HART and radio technology, which adds wireless capabilities to the HART protocol. It is just as easy to use as the well-known HART standard. IT is much less involved with installations; they are typically involved only in providing encryption keys for the wireless communication or organizing the frequency bandwidth that must be held free.
Planning a Wireless Network
To better comprehend this technology, however, we need to understand what is involved in installing a wireless network. Like all widely-used standards, including Bluetooth and WLAN, WirelessHART operates in the 2.4-GHz band, which is the only worldwide free license.
Since all of these standards use the same frequency range, their coexistence must be ensured. Ensuring coexistence, however, means that it could be difficult to install a Bluetooth communication in an area where a WLAN system is already installed. The systems may disturb each other. Although there are error correction algorithms that cause the systems to repeat the single messages in case of collision until they are properly transmitted, they do slow down the operation. Therefore, it is highly advisable to plan on avoiding these collisions in advance.
When planning a wireless network, adequate bandwidth margins should also be considered. If the planned network is already at its limit in terms of bandwidth, a small disturbance might cause the communication to fail.
These examples illustrate that proper planning for standard wireless networks is necessary and should be done by a dedicated group. WirelessHART cannot just be randomly installed. Proper site survey and analysis of the process must be performed prior to any installation. WirelessHART does provide unique features such as redundant mesh routing, a network manager, TDMA (time division multiple access) and FHSS (frequency hopping spread spectrum) that make WirelessHART less intimidating, with less IT involvement needed.
Wireless HART Features
A wireless mesh communications network helps ensure reliability even in the presence of interference. Each device acts as a router for messages from other devices. A device does not have to communicate directly to a gateway; it forwards the message to the next closest device. This ability extends the range of the network and provides redundant communication routes to increase reliability.
The network manager functions as the central component and organizer of the network on start-up. It constructs the network, establishes the organization of the communication and determines the redundant paths. The network manager also detects newly arriving stations and integrates them into the existing network during actual operation so that problem-free expansion is possible. During operation, the network manager monitors each station, as well as the important functions of the network, and applies corrections in the event of faults.
TDMA coordinates communication within the flat mesh network. All device-to-device communication is done in a pre-scheduled time window, which accurately synchronizes the radio stations in a 10-ms timing cycle. This enables a reliable (collision-free) network. It reduces the lead and lag times during which a station must be active.
Finally, WirelessHART features FHSS. To avoid interference from other radio stations in the 2.4-GHz band, WirelessHART uses FHSS to "hop" across the 16 channels defined by the IEEE 802.15.4 radio standard. All 16 frequencies defined in IEEE 802 are used in parallel. Channels that are already occupied are entered onto a "blacklist" and are no longer used for communication.
The combination of the exact time synchronization in the 10-ms cycle with TDMA and the use of all 16 channels with FHSS enable the overall network to achieve 1,600 communications per second.
The coordination of these features allows WirelessHART to integrate easily in existing or new installations without extensive IT involvement. With decreased IT involvement and more plug-and-play operability, end users will find that WirelessHART enables better use of existing applications and new, cost-effective applications that were not previously possible.
Will It Work In My Plant?
To avid process engineers this may sound great, but how do they know that a wireless network, or WirelessHART, will work for them? To determine whether WirelessHART is appropriate for a particular plant, look at the size and the characteristics of the process plant.
The plant environment plays an important roll in planning a wireless network. Changes in construction or vehicles, tow motors or even railway cars moving through the plant can interfere with signal transmission. Choose sender and receiver positions carefully, considering future plant renovations and additions.
Basically, analog values, such as level, temperature and pressure, must be retrieved. These values can be represented by 2 to 4 bytes. Since these values typically change slowly, 100 ms to 1 s can be considered as real time for those applications. This can easily be met with wireless technologies; even multiple repetitions of the signal and hopping through a mesh network can be processed in the given time frame.
The distances between the devices in process automation facilities are much wider. Ranges from a few feet up to a kilometer are not unusual. Range is an important factor. It may be possible to bridge this distance with one point to point connection on a 900-MHz system, but it is not possible to bridge this distance with a single connection in the 2.4-GHz band. The range must be extended with multiple repeaters or the installation of additional access points. For a more reliable and accessible system, a mesh network should be used.
Reliability, safety, ease of installation, safe communication and cost effective solutions all play an important role in using WirelessHART. The focus for industrial wireless utilization today is concentrated on logging and parameterization of devices, alerting and open loop control applications. Tomorrow it will be supervisory and regulatory control, along with emergency action applications that wireless will handle. WirelessHART will be leading the field.