WirelessHART signals a change at plants

Wireless technology is so much a part of our lives that we use words like Blue-tooth and WiFi systems with confidence and familiarity. So why has it taken so long for the industry to take advantage of the benefits a wireless connection can offer? Gareth Johnston and Alan Munns of ABB Ltd explain the benefits of WirelessHART, a new development in plant-wide communications:

London - Some things that we can tolerate as consumers, such as signal loss when driving through a tunnel or occasional interference from other wireless networks, are unacceptable at plants. Industrial users require reliable and secure transmission of information via a technique that’s easy to set up and flexible enough to adapt to changing conditions. WiFi, Bluetooth and others simply don’t meet these basic requirements. The result is that wireless instrumentation has found a few specialised applications at plants but hasn’t been widely deployed.

The wireless instruments available to date are either proprietary or early pilot developments that can only exist within a single vendor network. However, an open approach is emerging. In September 2007, the Hart Communications Foundation (HCF), Austin, Texas, released its latest specification, HART 7, which included wireless connectivity. HART 7 provides the process industry with a wireless network that can support instruments from multiple vendors in an industrial environment

The advantages

Traditional 4...20mA field instruments have been using extremely successful HART protocol to assist during commissioning and scheduled maintenance work; in fact, more than 24 million instruments have been installed to date. However, some 20 years since HART 5 became available we’re still not making the best use of the remote access HART offers, with instrument information remaining locked away for the vast majority of users. WirelessHART can provide the key to unlock this information and allow us to install instruments at a much lower cost, offering the following advantages:

The vast majority of 4...20mA instruments installed have no mechanism to allow remote access to the information they hold. Retrofitting a communications path back to an asset management system, would typically require breaking into the field wiring to insert a HART multiplexer. This poses some risk and cost. The use of a WirelessHART instrument adapter offers a more convenient, lower risk and lower cost alternative.

The need to run cabling around a plant makes installation and commissioning costly for traditional 4...20mA analogue and fieldbus (Foundation Fieldbus H1 and Profibus PA) networks. Taking into account the costs of routing the cable, mounting the cable trays, cutting holes and tunnelling, and generating the work permits, expenses can run to £2,000 per point. Wireless networks clearly can reduce these costs as there are no (or fewer) cables to run with instruments often being self-powered.

During the planning phase for traditional wired instrument loops, it’s common to add up to 20% spare capacity to account for future plant modifications because wired systems are expensive to modify and expand later. Often, much of this spare capacity remains unused five or ten years down the line. A wireless network offers a very scalable solution that can reduce the need for building in costly spare capacity at the onset.

Target applications

Many industry users historically have been cautious in adopting new technologies. Hopefully WirelessHART will jump-start the use of wireless instrument networks at plants. End-user surveys provide a good guide to likely wireless applications.

Adding WirelessHART to existing HART instruments in the field is very simple. Users now only have to connect the adapter to the instrument at either a spare gland or even at a junction box (the adapter could be self-powered or powered via the loop) and add the network ID and password to the adapter. It will then automatically join the existing WirelessHART mesh network. The original 4...20mA signal remains intact, but with the user now able to remotely access instrument information. This permits:

• Calibration checking and over-range reading. For a pressure transmitter, the user can monitor the instrument process value and compare it to the 4...20mA value. This can help confirm the instrument calibration status or provide a value when the 4...20mA signal is out of range (the WirelessHART value doesn’t depend upon the 4...20mA value – it comes straight from the digital value)
• Value condition monitoring. For a positioner, users have access not only to valve position feedback but also to other data to help analyse the valve’s condition
• Advanced diagnostics. The user can remotely access instrument information such as asset signatures, level echo trace and other advanced diagnostics
• Full use of complex multivariable instruments. For multivariable instruments (mass flow, for example), it is now possible to access process values previously hidden away.

Coping with aging infrastructure

The maintenance and support of an aging plant can be problematic, especially when the new measurements are required and the spare capacity within the cable runs and its conditions are unclear. A WirelessHART solution can overcome many of these issues because a wired infrastructure isn’t required and the mesh network overcomes many of the problems of point-to-point topologies. Some examples:

· Replacing local gauge indicators. A wireless instrument can provide a low cost way to report information back to the control room, reducing operator rounds to read local indicators and improving visibility of the process

· Supplanting obsolete field instruments. Installing wireless instruments can obviate difficulties in keeping old instruments working as spares become harder to find. Self-powered or loop-powered, the wireless network provides an information pathway without other major changes to wiring or control system hardware

The low cost nature of wireless technology allows users to consider temporary measurements to help diagnose process problems. This could be as easy as using a strap-on temperature transmitter or replacing a local pressure gauge with a new pressure transmitter.

One of the clear messages from customers is that they won’t consider closed loop control using wireless for at least five years, instead preferring to first get experience with the technology. While it’s possible to transmit process data from a measuring device over the mesh to a positioner/valve, this requires care in setting up the network topology to reduce time delays. Plus, the positioner would be constantly modulating the valve and would need significant power, potentially ruling out a self/battery-powered option. Given that the measuring instrument typically isn’t far away, a local controller using wire connection to the instrument and positioner would be a good solution when coupled with instrument WirelessHART connectivity back to the host.

Early adopters of wireless instrument networks have been using either proprietary solutions (e.g. for tank level or safety shower operations) or running early pilots of WirelessHART. The proprietary networks often relied on a star configuration with single line out-of-sight connection to a wireless hub/gateway.

Process plants are typified by obstacles such as steel vessels, exchangers, piping, structural steel work, reinforced concrete which make it tough to set up a star wireless network with clear lines of sight from the gateway to the instrument.

A self-healing mesh network with redundant pathways that can adapt to changing plant environments avoids such issues. ABB and other vendors have tested such mesh networks within plants and observed the mesh adapting to the changing environment and effectively coping with the other interface sources.

As is often the case with emerging technology, other groups, Foundation Fieldbus and Profibus, have similar efforts underway. This does seem to send a confusing message to the 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 the creation of a successful wireless instrument network via the Wireless Cooperation Team (WCT).

The WCT is working towards a topology where WirelessHART can be used at the instrument level with the Foundation Fieldbus or Profibus as the backbone to the host system. The WCT meets regularly and includes experts from all three protocols.

With such level of cooperative efforts progressing and ender-user pilots complete, there’s no reason to wait much longer to consider a WirelessHART project.

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. This provides many benefits including the ability to employ existing software tools including HART hand-held configuration; a self-building and self-healing network; and the ability to use existing instruments via a WirelessHART adapter.

The WirelessHART topology

Every WirelessHART network has three main elements:

1. A gateway, which connects the control system (via Ethernet, Profibus, etc.) to the wireless network
2. A network manager, which is normally part of the gateway and automatically builds the wireless network and manages its operation; and
3. Field instruments and devices, usually consisting of pressure, temperature, position or other instruments, although it can also 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.

The rate at which process information is transmitted over a wireless network directly impacts on battery life for battery-powered instruments. WirelessHART offers several mechanisms to maximise 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 changes – deliver alarm or alert messages only when they are triggered (time stamp within the instrument); and

· 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), the user can proceed with building 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 commission 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); and

· Attach a HART hand-held configurator to the maintenance port (terminals within the instrument that look like 4...20mA connectors). Follow the menu on the hand-held and enter the following:

1. The network name/ID;

2. The instrument refresh rate (i.e. how often the process value is updated);

3. The network password.

· Monitor the instrument as it automatically synchronises to the network, is authenticated and successfully joins the network.

The gateway automatically optimises the mesh network and redundant pathways. It continues to monitor the mesh and adapts it as new devices join or the radio environment changes. This self-building and self-healing feature provides the reliability of data communication that industry requires.

WirelessHART makes security of data transmitted a key part of its own core technology. For WirelessHART, security is always switched on. Built-in features address the issues of adapt security and outside interference. Some of the security methods used include:

· Keeping data transmission bursts short (10 millisecond time slots). This makes it hard to synchronise and read messages

· Authenticating devices as they try to join the network (join key, device key) and monitoring attempted joins. This will help to stop unauthorised devices joining the network

· Encrypting each message. WirelessHART also allows the user to rotate encryption keys to make it difficult to identify the keys and decode the messages

· Authenticating that the data hasn’t been altered as it passes through the mesh network

· Changing the transmitter channel after each message has been sent, making it harder to snoop at data

· Requiring a physical connection to the instrument (via a HART hand-held) for the instrument join procedure

Vendor testing and end user trials have shown that WirelessHART provides the reliability, security and simplicity that the chemical industry demands from a wireless network. Users can take advantage of the technology today to improve their vision of the process and to unlock valuable information hidden within the existing instruments.

The WirelessHART specification is available now. Many significant instrument vendors are at an advanced stage of product development. Indeed WirelessHART instruments and devices will start to reach the market in the middle of 2008 and the selection will widen during 2009.