A self-healing mesh network with redundant pathways that can adapt to changing plant environments avoids such issues (Figure 6). ABB and other vendors have tested such mesh networks within plants and observed the mesh adapting to the changing environment and effectively coping with other interference sources.
As often is the case with emerging technology, other groups — here, ISA (via the SP100 Committee) and Foundation Fieldbus and Profibus — have similar efforts underway. This does seem to send a confusing message to end users who are looking for a single standard. (Incidentally, instrument vendors also would like a single standard and development path.) However, these groups have made huge strides in working together to allow us to obtain a successful wireless instrument network.
HART 7 and ISA. HCF has provided the WirelessHART specification to SP100 — a joint analysis team has already identified a pathway forward to use WirelessHART at the instrument level. Don’t forget that ISA SP100 is a family of protocols; WirelessHART would fit in very well at the instrument level.
HART 7 and Fieldbus. The Wireless Cooperation Team (WCT) is working towards a topology where WirelessHART can be used at the instrument level with Foundation Fieldbus or Profibus as the backbone to the host system. The WCT meets regularly and includes experts from all three protocols.
With such cooperative efforts progressing and end-user pilots complete, there’s no reason to wait much longer to consider a WirelessHART project.
Evolution not revolution
The HART 7 specification has built upon the existing HART specification by adding wireless connectivity and other features to ensure reliability, security and simplicity of operation (Figure 7). So, yes, we can employ existing software tools including HART hand-held configurators; yes, the network is self-building and self-healing; and, yes, existing instruments can be used via a WirelessHART adapter.
The WirelessHART topology. Every WirelessHART network has three main elements (Figure 8):
- A gateway: It connects the control system (via Ethernet, Profibus, etc.) to the wireless network
- A network manager: This normally is part of the gateway and automatically builds the wireless network and manages its operation
- Field instruments and devices: These usually consist of pressure, temperature, position or other instruments but also can include adapters.
WirelessHART is an instrument level network using existing and new HART commands to access information. The instrument network connects to a WirelessHART gateway that builds the mesh topology and also connects to the host system via standard high-speed backbones such as Ethernet.
How often is the information updated? The rate at which process information is transmitted over a wireless network directly impacts battery life for those instruments using battery power.
WirelessHART offers several mechanisms to maximize battery life and use of the network capacity:
- Transmit measurements at different rates for different instruments — for instance, level every 20 or 30 seconds, and flow every 1 or 2 seconds
- Send when data change — deliver alarm or alert messages only when they are triggered (time stamp within the instrument)
- Change measurement rates based upon alarm limits — i.e., increase the rate when a value approaches a set limit.
Building a WirelessHART network. Once the gateway is commissioned (connected to the host and asset management system), you’re ready to build the WirelessHART network. It’s really a very simple procedure that involves entering three variables into the wireless instrument. This is one example of a commissioning procedure:
- Obtain the wireless network name/identity (which would be a value set in the gateway)
- Connect the instrument to the process
- Power up the instrument (which could be externally or battery powered)
- Attach a HART hand-held configurator to the maintenance port (terminals within the instrument that look like 4–20-mA connectors). Follow the menu on the hand-held and enter the following:
- The network name/ID
- The instrument refresh rate (i.e., how often the PV is updated)
- The network join password.
Monitor the instrument as it automatically synchronizes to the network, is authenticated, and successfully joins the network.