Wireless technology moves into industrial applications

26 April 2010

Wireless technology has become an everyday phenomenon in the home and office environments, but a recent trend is its foray into industrial applications. Although tried and tested standards such as WLAN and Bluetooth play a significant role here, other options are now being introduced. R. Stahl looks into the potential benefits and the role of wireless technology in hazardous areas.

For any industrial communications solution, the question arises as to where and how a new technology can operate cost-efficiently. In some cases, wireless radio is supposed to replace a cable-based network, and must directly compete with these established alternatives. There are other applications in which wireless transmission actually gives users all new tools to introduce completely new processes that were inconceivable in the past due to the limitations of electric line solutions.

One example for such applications is the use of portable operator terminals with wireless interfaces, which can considerably simplify maintenance and commissioning tasks. As a first step, it is therefore sensible to take a detailed look at typical routines in process plants, and identify potential application areas with a view to how and how much they may benefit from wireless technology. Once a case is made for wireless technology in general, there are various solutions to choose from for actual implementations, which will also be discussed below. And last but not least, there are additional safety considerations for applications in hazardous areas. The second half of this article will focus on this final aspect.

State-of-the-art logistics solutions depend on systems which acquire data on the flows of goods with the highest possible degree of precision, and preferably in the very instant when stock items are taken out or replenished. In process industries many raw materials and products are transported in containers such as drums, tanks, IBCs, etc. Most of these are marked with either barcodes or RFID (Radio Frequency Identifier) tags. Acquiring RFID tag information is obviously a model application for wireless technology. As of yet, however, most readers used for this purpose are hand-held terminals with a cable that needlessly impedes their operation. Portable devices capable of both acquiring data and passing it on via radio to a central management system save time and costs, This increases data reliability due to exact and all but instant acquisition. RFIDs, which were first applied in the retail sector, continue to increase their foothold in the process industry. One key RFID advantage over barcodes is that smudged and stained labels can still be read from, also, there are other convenient features which previous solutions could not provide, e.g. that data can be written to the tags more than once, or that it is possible to acquire several tags at the same time, etc.

Any activities having to do with operator or service staff access to a plant, can be of benefit when personnel can use portable devices with a connection to MES (Manufacturing Execution System) and/or ERP (Enterprise Resource Planning) servers, since that enables an optimisation of typical production processes. For example, maintenance instructions can be automatically dispatched, as all relevant information can be provided via radio to a portable device service engineers can carry with them in the field. Staff are then able to inspect equipment as needed and, upon completion or even while they are taking care of a maintenance task, enter the results of the inspection, or repairs made, directly into the portable device. That data is then instantly available in a central database which can be utilised for documentation purposes or even to speed up billing. Similar advantages apply to operating and monitoring tasks in industrial plants. Portable devices make it possible to read live measured values and therefore keep an eye on the actual state of the production plant on site. At the same time, operators in the field have access to ancillary information such as maintenance schedules, operating instructions, ATEX certificates, and much more. As a result, routine procedures can be modified to become considerably more efficient. Also, camera systems or sensors at measuring points somewhat further away, e.g. within pump stations, can be integrated into the HMI concept at a low cost and can be readily displayed where needed thanks to radio transmission.

Using wireless communication does away with the often complex and costly laying of cables. However, many process industry applications require wireless field devices with an independent, onboard source of power. A number of promising approaches are based on consumption-optimised electronic circuits and on alternative sources of power using accumulators or solar cells, or on recovering energy from vibration, temperature fluctuations, etc. Initial field trials by process control system manufacturers with self-configuring meshed wireless networks have produced an availability rate of up to 99 %, which is certainly a respectable result. In the chemical, petrochemical and pharmaceutical industries, however, typical processes almost without exception cannot tolerate any interruption or error, as any such occurrence might result in costly losses for the plant operator. Since uncompromised availability of a production plant is one of the most important criteria here, it will likely take some more time before the wireless transmission of critical signals in control loops becomes common practice.

What can and should be done today, however, is a selective radio transmission of specific signals that are particularly hard to acquire any other way, or of non critical sensor data used for purposes such as process optimisation. While core process signals are absolutely needed to ensure proper control of a plant, other measured values are acquired only to be used for process improvements or preventive maintenance. For the time being HART communication is most commonly used to transmit such signals, or no sensors may be used at all. Recently, higher level asset management is taking on increasing importance in the process industry. Live information about the current state of production equipment in a plant in as much detail as possible gives staff better means to anticipate imminent plant failures and to adjust maintenance intervals to actual needs. Wireless data transmission is an attractive option for this area of application.

Users can resort to a number of different wireless communications solutions. As usual, there is no one standard that meets all requirements. All radio technologies currently available on the market have specific advantages and disadvantages, some of which will be discussed below. It should be noted, though, that the most widely used solutions in particular have originated in the office IT sector, i.e. they were not originally designed for industrial applications.

WLAN is most suitable for use with portable equipment such as barcode scanners or handheld HMI devices. It provides the greatest bandwidth (802.11b — 11 Mbit/s, or 802.11g — 54 Mbit/s gross data throughput) and is designed for the transmission of Ethernet-based protocols.

In a WLAN network, an access client such as a PDA can also roam from one access point to another without any interruption in transmission. This means users carrying portable devices can move freely around the site without losing their connections to the network. State-of-the-art WLAN systems also do ensure secure data handling, unlike earlier revisions which only relied on Wired Equipment Privacy (WEP), an encryption method that was very easy to break.

Bluetooth does not provide a bandwidth that will match WLAN networks, but recent systems do achieve transmission rates of up to 2 Mbit/s. In addition, due to its synchronous communication modes, Bluetooth provides a very good basis for real-time applications. One key Bluetooth feature is the Frequency Hopping Spread Spectrum (FHSS) scheme, which makes this technology significantly less susceptible to interference than WLAN. FHSS also provides some additional protection against eavesdroppers. Bluetooth works well for networks with up to 8 users, while a greater number will require increased technical effort. Bluetooth radio consumes less power in operation than WLAN. Due to its characteristics it is particularly suitable for integrating fixed devices such as terminals or sensors. Both wireless standards discussed so far have one feature in common: their specifications have been internationally agreed upon, which ensures that devices from different manufacturers are fully or at least largely, compatible with each other.

ZigBee enables data to be transmitted at a rate of up to 250 kbit/s and requires significantly less power in operation than either Bluetooth or WLAN. The protocol profiles approved by the ZigBee Alliance at the time of writing are by and large tailored to applications in building automation. It is unclear which other areas of application ZigBee will eventually focus on in the course of its ongoing development. ZigBee constitutes the basis for the wireless HART protocols and some wireless solutions with self-configuring, meshed wireless networks, or so-called wireless sensor networks. Like Bluetooth, ZigBee features frequency hopping which makes it less susceptible to interference.

Besides the standards just discussed, there are numerous other proprietary protocols such as NanoNet, Trusted Wireless etc. However, users will more often than not be inconvenienced by them due to incompatibilities between devices from different vendors. Based on the existing standards for WLAN, Blue-tooth and ZigBee technology, various committees and organisations are currently trying to improve standardisation and provide users and manufacturers with implementation guidelines. In Germany, for example, institutions at the national level working in this field include the VDINDE GMA working committee 5.21, the ZVEI, and a NAMUR subcommittee.

The VDI/VDE 2185 standard e.g. provides information on how to evaluate existing wireless technologies. In 2007 and 2008, similar activities have also been and still are underway at the international level, e.g. in the ISA's (Instrumentation, Systems and Automation Society of America) SPI00 committee and in the HCF's (HART Communication Foundation) HART Wireless group. The number of organisations alone is testament to the growing interest in wireless technology from users in many industrial sectors. The eventual approval of standards can be expected to further facilitate the acceptance of and increase the number of wireless solutions in industrial applications.

Most common wireless solutions use the so-called ISM frequency bands, which are license-free and therefore help to reduce operating costs. However, the drawback of this situation is that different applications have to share the frequency band. The standardisation forums are aware of this fact and have already come forth with new approaches to resolve this problematic issue. Blue-tooth e.g. uses an adaptive frequency hopping scheme that skips frequencies where the transmission has previously been impaired by interference. As a result, it is possible to operate WLAN and Bluetooth networks at the same time in the same environment without experiencing interference. However, not only in this case, but also as a general rule, users need to have detailed knowledge of existing networks before they plan and deploy a new wireless infrastructure.
Radio devices emit electromagnetic radiation that is clearly a possible source of ignition in an explosive atmosphere. The main risk lies in the induction of currents in metallic objects or inadequately EMI-protected electronic circuits. These currents can result in excessively high temperatures and the formation of sparks. Other dangers, such as direct ignition of an explosive atmosphere, are much less relevant. Laboratory experiments have shown that RF sources with several hundred Watts of energy are necessary for an electromagnetic field to directly trigger an explosion in a hazardous atmosphere. On the other hand, IEEE studies on electromagnetic radiation in hazardous areas have shown that even RF powers of 6W can become a potential hazard in terms of induction in metal objects. Still, for a long time, international standards have not provided any guidelines on this subject. The closest thing users will find are recommendations in some national standards, e.g. in BS 6656 (Guide on >Assessment on inadvertent ignition of flammable atmospheres by radio-frequency radiation<), CENELEC Technical Report CLC/TR50427, or German DIN VDE 0848-5. All of them merely discuss RF sources installed outside the hazardous area, which only transmit into this area. For RF sources inside the hazardous area itself, users are referred to EN 60079-14 (>Electrical installation in hazardous areas<). However, even that EN standard just contains a brief note stating that in designing electrical installations, the effects of electromagnetic radiation must be limited to a value that is not hazardous. No figures are spelled out, and the standard makes no mention of adequate means for actual implementations.

Only the recent draft IEC 60079-0, in Tables 4 and 56 in particular, has finally introduced useful, hands-on rules on wireless device operation in hazardous areas. The standard lists specific limits for continuous and pulsed RF signals. These limits apply to frequencies in the 10 kHz to 300 GHz range . Notably, though, there is no information on hazardous area zone/division classification and the related probabilities of the occurrence of an explosive atmosphere — which, of course, makes sense, since an RF signal will obviously not stop at the boundary between two zones. WLAN, Bluetooth and ZigBee all predominantly use the so-called ISM bands at 2.4 GHz, which are restricted to low power radio transmissions. Hence, WLAN access points using this band are limited by RF regulations to 100 mW. Bluetooth and ZigBee transmissions typically require only about 10 mW of energy. At face value, all of these technologies therefore need significantly less energy than the maximum allowed by the standard. However, the so-called antenna gain must also be factored into the calculation, as the ignition risk is also defined by the magnitude of the field strength. Antenna gain is a parameter that describes the concentration of radio energy emitted in a specific direction. Such directional gain increases as radio emissions in other directions decrease, i.e. the total energy emitted remains the same. Antenna gain is measured in relation to a specific reference. If the gain value is stated in dBi, then this value refers to an isotropic radiator, or omni directional radiator, i.e. the theoretical model of an antenna that evenly distributes energy in all directions from a point source. Typical values for rod antennas and directional antennas are between 5 and 9 dBi. Users have to take antenna gain into account when they refer to the values given in the tables, which can be accomplished by creating a signal level plan.

With few exceptions, automation components and devices currently available on the market must not be used in Zone 1 right out of the box. This restriction is largely a consequence of the rapid pace of development for new devices, which are released in very short intervals and are therefore often affected by incomplete standardisation. One possible solution to the problem is an installation of such RF equipment without Zone I approval in housings featuring a flameproof enclosure, i.e. Ex d type of protection, or another suitable type. The majority of these Ex d enclosures are made of metal, which shields electromagnetic radiation from the antenna as a side effect. Obviously, not just any antenna can be installed inside a housing of this type without additional measures. In some cases, a housing with a glass window can be used in combination with a directional antenna installed within. However, tests have shown that only antennas specially matched to a particular type of flameproof enclosure will actually work well, since the signal loss is otherwise excessive. The second option is the use of external antennas. However, hazardous area requirements demand that special explosion-protected antennas have to be installed in this case. In most cases, they have to be designed for increased safety (Ex e) protection, because, in the event of a short circuit between the power supply and the output or input stage in the RF device, no excessively high currents voltages are allowed to coincide with the explosive atmosphere without protection.

Wireless network installations cannot do without special attention to planning, and planning starts with the definition of the requirements for the wireless network. A range of aspects have to be considered, e.g. bandwidth, mobility, hard requirements in terms of real-time signal transmission, the encryption system, IT department demands etc. Using a floor plan, it is possible to assess the RF coverage in the area with the aid of planning programmes. Users deploying a new network also have to know exactly which wireless systems are already in use in the same place and in neighbouring areas. The location and selection of the antennas can then be established. In the next step, the deployment plan should be verified against an on-site survey. This is a live on-site inspection of the area to check the values previously determined on the computer.

In the real environment, using a portable access point. In this confirmation process, some additional information can be gathered that cannot be anticipated in a floor plan, e.g. on the effects of vehicles passing through, or of mobile containers which may have appeared in unexpected places. The survey will also allow users to realistically determine the effective bandwidth in the central and the outer areas of RF coverage. Finally, the RF system can be installed, commissioned, and put through a final test under real operating conditions to avoid unpleasant surprises. While the many steps of this procedure might appear to incur significant extra expenses, they have proven to be by far the most reliable way to ensure that a new wireless system really works as expected and brings about the desired process improvements.