Competing for the future - does wireless play a part?

There can be no doubt that the heart of any operation is the process control system. More recently, industry is embracing a paradigm shift away from localised operations via distributed control, to embracing the need for remote operations centres controlling and monitoring multiple facilities.

Broader control of multiple facilities would not be possible if it weren’t for advances in sensor-based technology, fieldbus communications and in some cases wireless sensors and adapters. Sensors now not only provide information on the health of the device but also the health of the process, making it possible to determine how the process can be improved. This information is communicated up to the control room and to the remote operations centre personnel tracking things like production, pending issues in the process and in measurement devices and the like.

Recent examples include the coal seam gas industry where, despite marginal well heads and the scarcity of experienced personnel, the use of remote diagnostics has made possible timely, cost-effective decision-making. Further examples include applications developed to retrofit and proactively monitor essential assets which have long been overlooked but are at the centre of production slowdowns or even shutdowns. Such applications help to ensure losses in the hundreds of thousands, if not millions of dollars, are avoided.

There are many issues that need to be considered in the implementation of a wireless sensor network. This article looks at these issues from the perspective of WirelessHART technology, but whatever wireless technology is used, the end user needs to take the following factors into consideration.

Network availability, reliability and management

A plant is often a challenging environment for radio frequency (RF) communications. RF systems must contend with having to communicate in the presence of 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.

Modern wireless sensor systems, such as WirelessHART, have therefore been developed to overcome these issues by providing self-organising, self-healing, adaptive networks featuring multihop, deep mesh architectures. At the heart of such a 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 blocks the path, an alternate path is used for the information to reach the gateway and control system. All transmitters - regardless of manufacturer - participate in the mesh topology to ensure a multitude of communication paths are available and that reliability needs are met. If required, redundant gateways are available to provide redundant communication masters that self-monitor and can perform ‘hot swaps’ - change over in the advent of a failure - reporting the event 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, for example, 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. The user therefore has control over the devices’ reporting rate (and power module life) with time-stamped communications. This functionality is all achieved behind the scenes, without user intervention, by the gateway and devices reporting the network availability and alternate path options.

Coexistence

Being based on global standards, a wireless field network potentially has to operate in close proximity to other wireless network technologies in the same 2.4 GHz band (such as Wi-Fi, Bluetooth and ZigBee etc) that may cause in-band interference. To help overcome interference issues, WirelessHART and other wireless sensor technologies use the IEEE 802.15.4 standard utilising direct sequence spread spectrum (DSSS) but also with 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 will migrate to another available channel and re-establish communications. If broad use of a selected number of channels is evident (eg, a Wi-Fi network), the WirelessHART blacklists those channels and communicates within the known set of available channels. Intelligence such as this embedded into the communications protocol ensures coexistence in the event of of in-band interference.

Security

Understandably, security has been at the forefront of end-user concerns in the adoption of wireless technology. In the case of WirelessHART, security measures may be classified into data protection and network protection.

Data protection

Data protection (or confidentiality) is concerned with the privacy and integrity of the data communicated. When transmitting, 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*1 technology to check for tampering during transmission (superimposing or altering data), attacks trying to change the network routing information and to ensure devices and the information shared are authenticated (proven to be from a known source on the network). 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 (eg, 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 to disrupt 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 of a predetermined known size, so that checks via CRC and CCM*¹ mitigate this threat. With respect to denial of service, using a random hopping algorithm with channel blacklisting helps to make DOS attacks more difficult.

In both cases (data protection and network protection) these security mechanisms are on permanently and transparent to the user. All that site personnel have to do is to ensure they follow routine procedures such as not giving out password access to the gateway and then configuring the transmitters via a wired maintenance terminal in the normal fashion. It should also be pointed out that when wireless sensor/instrumentation networks use a Wi-Fi backhaul network, the end user should also consider the security of the backhaul network.

Interoperability and interchangeability

Using a wireless sensor network based on an IEC standard ensures that multivendor interoperability is possible. Process applications require many types of measurements such as flow, level, valve position, pH, conductivity, vibration, temperature, pressure and acoustic, as well as contact input and level switches. These measurements may come from different transmitter manufacturers and all vendors using WirelessHART, for example, undergo certification from the HART Foundation. Therefore, certified WirelessHART transmitters of many different types, from many manufacturers, integrate into the system in the same way using the same application protocol.

Transparent system integration

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, using one of the common device description languages available on the market. However, using wireless technology that supports a common device description 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 that utilises EDDL. 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.

Figure 1:  The use of EDDLs and user-friendly management software allows field personnel and those in remote operations centres to quickly diagnose issues and save on overheads.

Forward and backwards integration

A control system has an expected lifespan of 15 years or more. Over its life cycle, 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 in order 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. With technologies that are text-based, such as EDDL, this means that new versions of WirelessHART transmitters can be deployed without having to upgrade the Windows version on the control system.

Power module considerations

Preserving power is important to extend battery life in remote wireless instrumentation. 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 transmitters in a mesh topology may provide a battery life of up to 10 years (depending on sensor type and the configured update period).

Wireless network management diagnostics

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 wireless technology that is chosen should provide communication status for all of the above and provide for user-friendly graphics to aid interpretation of information.

Diagnosing wireless devices

A question often asked by end users concerns any potential differences in diagnosing device issues in a wireless system. This is another area where it is important that the wireless devices comply with a known standard. The WirelessHART standard forms part of the HART 7.1 standard and, as such, no new equipment, training on devices should be required. Universal commands and specific commands are used to access diagnostics in the transmitter, making the 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 wireless interface to a legacy non-wireless device to provide insights into the device and possibly the process’s health (depending on the revision date of the legacy device).

Wireless today and in the future

The increasing uptake of wireless instrumentation and sensor technology due to standards such as IEC 62591 WirelessHART means that it is now applied to a plethora of applications by a diverse range of end users. It is used in diverse industries from the traditional petrochemical, metals, mining and manufacturing to food and beverage and, more recently, retail operations. While a generalisation, wireless sensor use could be classified as conforming to applications of:

1. Process and asset reliability monitoring and control, including motor, pump and valve automation monitoring.
2. Process throughput and efficiency improvements, including automated steam trap monitoring, tank level, rotating kiln and rotating device measurement, and better boiler profiling.
3. Personnel productivity improvement by replacing manual gauge monitoring labour, reducing the need to access hazardous areas.
4. Environmental, health and safety applications including emissions and discharge monitoring, pressure relief valve monitoring, and eye wash and safety shower activation.

In summary, the historical aim has been to optimise production relative to demand needs. Forward-thinking organisations operating in a globally competitive environment are now looking to reduce energy and utility costs to optimise production costs. Wireless sensor technology such as WirelessHART forms part of that planning as firms look to create a competitive framework for the future.

Notes: 1. CCM mode (Counter with CBC-MAC) is a mode of operation for cryptographic block ciphers. It is an authenticated encryption algorithm designed to provide both authentication and confidentiality. CCM mode is only defined for block ciphers with a block length of 128 bits. Cipher block chaining message authentication code (CBC-MAC) is a technique for constructing a message authentication code from a block cipher. The message is encrypted with some block cipher algorithm in CBC mode to create a chain of blocks such that each block depends on the proper encryption of the previous block. This interdependence ensures that a change to any of the plaintext bits will cause the final encrypted block to change in a way that cannot be predicted or counteracted without knowing the key to the block cipher.