System redundancy with WirelessHART
In process automation, operators make decisions based on the data they have available. When one or more process variables or other types of information are not available, they must make those decisions without a complete picture of what’s happening. In fact, they may not even know a decision is needed.
In process automation, operators make decisions based on the data they have available. When one or more process variables or other types of information are not available, they must make those decisions without a complete picture of what’s happening. In fact, they may not even know a decision is needed. The same is true for control systems: without all the necessary input data, the output of a control algorithm may not reflect the best action to keep the process operating smoothly — or even worse, it could cause a process shutdown.
That’s why process operations sometimes use system redundancy to minimize the probability of data loss, especially where the process data is operationally critical or where a single component failure could result in the loss of a significant number of process variables.
Redundancy may duplicate any of several types of critical system components — from devices to controllers to communications. For example, a traditional wired system may have two cables carrying the same information.
Wireless systems can also provide redundancy to help prevent data loss. In the case of Wireless HART, redundancy is available at all levels of the network system:
In the wireless sensor network
At the network access point
At the gateway/network manager/security manager.
Redundancy in the wireless sensor network
Wireless HART provides redundancy in the wireless sensor network through several mechanisms. Each communication can have:
Multiple paths between the field device and the gateway (spatial diversity)
Multiple radio channels (frequency diversity)
Multiple timing possibilities (time diversity).
In Figure 1, if communication from devices TT101 to the gateway fails on path A-B-C, the device will retry at a slightly different time and channel on a different path — for example, D-E-F. If that fails also, it tries again, perhaps on path D-G-C. The system provides for three retries to get the communication to its destination. This redundancy is provided in both directions — from the gateway to the device and from the device to the gateway.
Redundancy at the network access point
An access point is simply a specialized Wireless HART device with a high bandwidth communication interface to the gateway. It provides an entry and exit point for system communication to and from Wireless HART devices.
Wireless HART allows a network to have multiple access points. Besides additional path diversity (as described above), multiple access points also provide additional network bandwidth and redundant communication paths for the gateway and network manager.
Another benefit of multiple access points is that they can provide low-latency access for connection to final control elements. For example, strategically placing an access point near a control valve enables that valve to communicate directly with the access point, providing a low-latency path from the gateway to the valve.
There is theoretically no limit to the number of access points a network can have. You simply make the tradeoff between cost, redundancy and overall network access bandwidth.
Redundancy at the gateway, network manager and security manager
The higher-level components of a Wireless HART system are the gateway, network manager and security manager software functions. Each of these components can also be made redundant.
One way to do this is by putting each function in its own physical execution device and then replicating each of those physical devices. This gives users flexibility to decide which component or components are most critical to their applications. For example, a user might decide the network manager is most critical and make that particular component redundant.
Figure 2 shows another approach: putting all three components (gateway, network manager and security manager) in one physical gateway device and then simply replicating that device.
In this arrangement, one gateway device is the primary and one is the backup. The two communicate with each other via a redundancy manager to keep the state of the network synchronized. If the primary physical gateway device fails, the secondary recognizes this and takes over the responsibilities from the primary device.
Adapters for redundancy
Wireless HART provides many ways to improve redundancy, including the addition of a Wireless HART adapter. The Wireless HART adapter provides a redundant communication channel to traditional wired HART devices. Devices connected to legacy I/O can have continuous low-latency communication to asset management with no change to the control strategy.
Wireless HART addresses redundancy at all levels of the network. The examples show that the overall performance of a typical Wireless HART network is comparable to that of traditional wired field buses. The Wireless HART protocol allows for secure, highly reliable, low-latency communication with almost no impact on the bandwidth and process performance. All of this is automatically built into the Wireless HART standard — to make it simple, reliable and secure.
The HART Communication Foundation ( www.hartcomm.org ) is an international, not-for-profit, membership organization supported by more than 200 companies worldwide. Founded in 1993, the Foundation is the technology owner and central authority on the HART Protocol and provides global support for application of HART technology. The Foundation manages and controls the HART standards including new technology developments and enhancements that benefit and support the needs of the industry.