A guide to wireless sensor technology

Since its emergence, managers and engineers have done due diligence on IEC 62591 WirelessHART, considered to be a global standard for wireless sensor based technologies.

Many engineers now look to WirelessHART to cost effectively automate manual tasks, proactively maintain and monitor critical assets, comply with regulatory frameworks such as the EPA, drive productivity improvements and minimise production costs.

Thousands of WirelessHART networks with hundreds of millions of operating hours are in service around the world. How are engineers using wireless to create greater competitive advantage?

This article begins by understanding WirelessHART and then looks at innovative ways engineers are using the technology.

The heart of any operation is the process control system and there have been myriads of advances from pneumatic to electrical and electronic communications, from centralised control to distributed control in increasingly, remote operations centres controlling/monitoring multiple facilities.

At the core of all of these advances is information, and turning that information into timely, cost effective, decision criteria. At the centre of all of this have been advances in sensor based technology.

Sensors now not only provide information on the health of the device but also on the health of the process. This would not have been possible without HART communications.

Since its inception in 1989, it is estimated that here are approximately 30+ million HART devices installed/in service but this information was accessed in an ad hoc fashion and/or stranded in the device. With this installed base, in consultation with industry, vendors and regulatory bodies came together and IEC 62591 WirelessHART was born.

IEC 62591 WirelessHART was approved by the International Electrotechnical Commission as a global standard for wireless sensor based networks in April 2010.

Prior to this, NAMUR and its end-user membership, jointly developed NE124 in November 2009.

NE124 outlined end user requirements for wireless in process industries relative application class, network availability, coexistence, security, interoperability, power management, system integration, forward and backward compatibility, network management, diagnostics, configuration/commissioning, device replacement and certification.

WirelessHART addresses these requirements and to date is still the only IEC ratified wireless sensor global standard. The issues and the basis of thought leaders' decision making on NE124 and WirelessHART are addressed subsequently.

The NAMUR NE 124 recommendation identified three classes of applications. Wireless is predominantly used in class C applications (indication and monitoring) but with the recent advent of 1 second devices, also features in class B applications (process control).

While not used for class A (functional safety) WirelessHART is often times installed as a mechanism to monitor device/system health and reduce mitigation mechanisms and proactively provide warning on pending issues.

Network availability

A plant is oftentimes a challenging environment for wireless or Radio Frequency (RF) communications.

RF communications must contend with propagating/communicating over piping, vessels, structural steel, moving objects such as vehicles and other devices emitting an RF signature (which may or may not be noise at the communicated RF signal). It must also do this without user intervention in a self managed fashion.

A WirelessHART system is a self organising, self healing, adaptive network featuring multi-hop, deep mesh architecture. At the heart of the system is the wireless gateway, controlling communications and perennially challenging the network and devices for path optimisation and alternate path options.

Multiple paths are maintained such that when a new obstacle appears blocking the path, an alternate path is used for the information to reach the gateway and control system.

As it's a standard, all transmitters, regardless of manufacturer, participate in the mesh topology to ensure a multitude of communications paths are available and reliability needs are met.

All WirelessHART transmitters have standard COMM terminals for connecting a handheld field communicator and other tools.If required, redundant WirelessHART gateways are available providing redundant communication masters which self monitor and perform 'hot swaps' — changeover in the advent of a failure reporting this to the control host.


Naturally, some applications require faster updates and lower latency than others: some of course are the exact opposite in a relative sense.

WirelessHART is a user defined, time synchronised/scheduled communications protocol.

WirelessHART transmitters timestamp measurements from the original point of measurement allowing latency to be tracked and have a selectable update period adjustable from 1 second to 1 hour.

Therein the user has control over the devices reporting (by default power module life) with time stamped communications.

This is all achieved behind the scenes and without user intervention by the gateway and devices reporting network availability/alternate path options.

Coexistence

As a global standard, a WirelessHART field network potentially has to operate at the same time, in close proximity, to other wireless network technologies in the same 2.4 GHz band (such as Wi-Fi, Bluetooth and ZigBee and known as in-band interference).

WirelessHART uses the IEEE 802.15.4 standard using Direct Sequence Spread Spectrum (DSSS) but Frequency Hopping Spread Spectrum randomly channel hopping from one communications channel to another on a packet by packet basis.

If momentary use of the selected channel is detected, the network migrates to another available channel and re-establishes communications. If broad use of a selected number of channels is evident, the WirelessHART channel black lists those channels and communicates within the known set of free channels.

Intelligence such as this embedded into the communications protocol ensures coexistence in light of 'in band' interference.

Security

Understandably security has been at the forefront of end user concerns in the adoption wireless technology. WirelessHART security measures may be classified as (1) data protection and (2) network protection. 
Data Protection.

Data protection/confidentiality deals with privacy and integrity of communicated data. When transmitting, privacy in the WirelessHART standard uses end to end (data source to data recipient) 128-bit AES encryption on a message by message basis.

It also uses CCM* technology checking for tampering during transmission, attacks trying to change the network routing information and ensuring devices and information shared are authenticated.

In addition to this, a separate common network encryption key (autonomously routinely changed subject to site security policy) is shared by devices when broadcast information is shared across the network (for example challenging network path efficiency).

Devices attempting to join the network must pass a separate 128-bit 'join' encryption key test or their access will be denied. In effect information is checked on transmission for authenticity, packet size/alteration, source and destination and network verification.

Network Protection. Network protection is concerned with ensuring the network remains functioning. Attacks may emerge from devices impersonating authenticated devices to steal legitimate information, attempting to insert malicious code or disrupting network services in the form of a denial of service attack.

Regarding impersonation, as above, WirelessHART will look to authenticate and validate device communications and deny service to the unauthorised device(s). Moreover, the size of the data frame is small and a predetermined known size and checks via CRC and CCM* mitigates this possibility.

With respect to denial of service, again WirelessHART uses a random hopping algorithm with channel blacklisting using low power of 10mW in communicating. 

In both cases these security mechanisms are on permanently and autonomous to the user. Following from this, all site personnel have to do is ensure they follow routine procedures such as not giving out password access to the gateway and then as normal configuring WirelessHART transmitters via a wired HART maintenance terminal.

WirelessHART networks using a Wi-Fi backhaul network should also consider the security of the backhaul network. 

Interoperability

Process applications require many types of measurements such as flow, level, valve position, pH, conductivity, vibration, temperature, pressure and acoustic as well as on/off contact input and level switches.

These measurements may come from different transmitter manufacturers and all vendors using WirelessHART undergo certification from the HART Foundation. Certified WirelessHART transmitters of different types, from many manufacturers, integrate into the system similarly using the same application protocol.

There are many considerations in designing and commissioning a wireless network. Plants already have digital devices using hardwired and bus integration into intelligent device management software: predominantly using Device Description (DD) or the newer enhanced Electronic Device Description Language (EDDL).

Using another device definition technology and establishing new practices increases the cost of adoption. However using wireless technology that supports the EDDL standard can streamline integration yet provide for many of the benefits such as device and process diagnostics.

The WirelessHART standard supports EDDL technology enabling WirelessHART transmitter integration in existing intelligent device management software. When the EDDL file for the WirelessHART transmitter is loaded, the system automatically picks the correct EDDL file for the transmitter requiring no manual intervention. 

A control system has an expected lifespan of 15 years or more. Over its lifecycle, new types and versions of wired and wireless transmitters will come into the plant. The control system must be kept up to date with these to avoid technical obsolescence.

Therefore, using a device integration technology which has no dependency on version releases of Microsoft Windows ensures backwards and forwards compatibility between system and wireless transmitters.

Again WirelessHART uses EDDL technology which is a text-based standard (totally independent of Microsoft Windows) and new versions of WirelessHART transmitters can be deployed without having to upgrade the Windows version on the control system.

Power module

There are several considerations when selecting the power module in a wireless system. Firstly replacing power modules does require manpower and inserting old and new battery cells together in a transmitter, or using a battery which has been dropped, could result in a hazard.

A preferred wireless technology enables battery life of several years and provides keying to ensure correct insertion. Additionally battery cells which are permanently encapsulated into power module casing prevent the mixing of discharged cells with good cells.

Finally subject to application need, consideration needs to be given to power module certification. Failure to consider certification for a hazardous area will mean the device will have to be removed from service in order to change the power module.

WirelessHART uses the extremely low-power IEEE 802.15.4 radio communications with sensors turned off between measurements to preserve the life of the power module. Careful selection of vendor can mean that WirelessHART transmitters in a mesh topology may provide a battery life of up to ten years (depending on sensor type and update period configured).

Preventing network disruptions and providing for effective troubleshooting are key issues for network design, maintenance and selection.

Key metrics in network management diagnostics entail communication statistics such as missed updates, discarded updates, reliability, path stability, signal strength, latency, number of re-joins, timestamps for last join, maintenance of a "live list" of devices, service denials due to network load and power module status/health.

The WirelessHART standard provides communication status for all of the above and can provide for user friendly graphics to aid interpretation of information. 

Device diagnostics

An often asked question by engineering concerns any potential differences in diagnosing device issues in a wireless system. The WirelessHART standard forms part of the HART 7.1 standard. As such, no new equipment, training on devices is required.

Universal commands and specific commands are used to access diagnostics in the transmitter making WirelessHART transmitters easy to troubleshoot. If an asset management application is used, the richness and ease of use of the wireless system becomes apparent.

This can be exemplified by adding a WirelessHART THUM to a legacy non-wireless device to provide insights into the device and possibly the processes health. 

Certification

If wireless transmitters are unable to join the network, the plant commissioning and startup would be delayed. To avoid this, NAMUR in NE124 mandated the use a wireless technology where interoperability and standard conformance is tested by an independent third-party.

As such, all WirelessHART transmitters are interoperability tested by the HART Communication Foundation providing trouble free commissioning in the field.

End users have undertaken a process of due diligence in assessing IEC 62591 WirelessHART. Today WirelessHART is used across a variety of applications by a diverse range of end users across a range of industries.

NAMUR NE 124 application classes.

While a generalisation, WirelessHART use could be classified as conforming to applications of (1) process/asset reliability monitoring/control (2) process throughput/efficiency gains (3) personnel productivity improvements and (4) health/safety and environmental improvements and compliance with mandates.

Process/asset reliability monitoring/control applications include motor/pump, valve automation monitoring with benefits in limiting reactive instances of production slowd owns or worse — unplanned shutdowns.

Process throughput/efficiency gains include automated steam trap monitoring, tank level, rotating kiln and rotating device measurement and better boiler profiling; all realising in efficiency gains and huge savings in energy costs. 
Historically, manufacturers tended to optimise production relative to demand needs.

In the current globally competitive environment, strategic companies seek to manage energy and utility costs to optimise production at minimal cost. Even a 1 to 2% saving in energy costs can equate to savings of hundreds of thousands of dollars annually. 

[Brett Biondi is the Wireless Business Development Manager for Emerson Process Management across Australia and New Zealand. Jonas Berge is Director, Emerson Process Management.]