Vital link
Interface technology faces new challenges as the market continues to expand

The role played by interface technology is not immediately obvious. However interface technology actually helps drive innovation in the world of field communications. Interface solutions bridge the gap between controllers and the field environment, and they also perform signal conversion and monitor field devices.
Sabine Mühlenkamp
There have been major changes in the world of process automation during the past three years. Users and manufacturers have had to cope with Ethernet, wireless technology and fieldbus systems, and they have tended to give less attention to interface technology. It is easy to forget that interface systems ensure that the link between all of the automation systems remains intact. The market for interface products benefits from the growth in automation system sales. Long-term growth is expected to be in the 6–8 percent range. The market for traditional interface technology will also grow, albeit at a slower rate than the fieldbus segment.

The right information

The seamless flow of information on a variety of process variables makes a major contribution to efficiency in the chemical industry. Pressure, temperature, flow rates and levels are some of the key process parameters. Besides analogue (4–20 m) control signals, there is a whole range of digital message and command signals. Process control systems or PLCs process and forward the signals. On their way to the control layer, these signals pass through the interface layer where the signals are converted into a format which the controller can interpret. The interface can perform electrical isolation, power reduction or signal conversion. Interface circuitry ensures that the signals comply with specifications, and it prevents shorts circuits from effecting
other instrumentation. For information flowing in the opposite direction, the interfaces ensure that commands from the control system (PLC or controller) are understood. Thus interface technology is one of the core elements in an automation system along side of sensors, actuators and controllers. Interface components have to meet the same reliability and durability requirements as the other subsystems.

Compact design

Interface components also have to comply with some very specific requirements. Since control cabinet space is often at a premium in today‘s chemical plants, users prefer compact, low-cost modules. 20 mm to 45 mm was the standard just a few years ago, but products are now available that are only six millimeters wide. The height depends on what the components are designed to do. Ease of use is a crucial factor in field application. Engineering work is often outsourced, and fully trained technicians are in short supply at many companies. This means that interface system manufacturers will have to provide even more sophistication, and they will have to supply pre-packaged systems. The goal is to eliminate any potential installation errors. The importance of diagnostic functions continues to increase. If a fault occurs, it must be detected quickly and reliably, and above all the information provided must be unambiguous. It is up to the manufacturer to collect the necessary information, filter it and make it available to the service engineer, equipment operator or production manager. Diagnostic coverage must not be limited to simple sensors. It must include the entire field communications system, for example the bus system power supply and interface components. This is particularly important in markets such as the US where customers want automation solutions which are significantly less complex. The technologies have to be intuitive. Conventional 4–20 mA is still very common in Europe, but in China 300 engineers graduate each year who have an excellent knowledge of fieldbus systems but do not have the same expertise in conventional technology.

Unparalleled Atex-protection

Interface technology has a special roll to play in potentially explosive atmospheres. Conventional buffer stags are used to connect the 4–20 mA field devices to the I/O boards in the control system. Given today‘s remote I/O and fieldbus technology, the use of EXi isolation stages might seem like obsolete technology. However, this technology is still used on new installations when there is a need for electrical isolation or circuits which comply with functional safety requirements (DIN EN 61598)) Although the wiring requires more effort, it is a simple, proven solution. Fieldbus technology has come a long way. Just a few years ago, significantly fewer fieldbus devices could be operated in a fieldbus segment compared to safe zones due to power limitations. Fieldbus/multi barriers are now available which make it possible to supply more power and expand the maximum number of devices and the network size in Atex-zones while still complying with the standard. A bus segment for this area can have the same topology as a segment in an area which is not intrinsically safe. The choice of technology depends on the application, the general scenario and personal preferences. In addition to standard applications, interface technology plays a significant role when you want to mix different technologies. It is not unusual today for a company to use conventional point-to-point wiring, remote I/O and fieldbus technology.

Is the future wireless?

There is of course now an alternative to cabling. Wireless technology is a relatively recent addition to the process industry. Efforts are underway to adapt proven standards such as Bluetooth and WLAN to the requirements of the process industry or to develop new standards. The overriding goal is to ensure high system availability. Wireless systems will have to be very reliable and rugged, and they will have to offer sufficient range. Today, wireless solutions are used predominantly to bridge large distances and for critical areas such as remote tanks or pipelines or to span rivers. Currently the choice between wireless links and conventional technology is quite pragmatic. If a wireless application has substantial advantages compared to a conventional solution, then users will choose the wireless option. However wireless technology is unlikely to be used on a large scale.n