[Disclosure: This article has been supplied by Phoenix Contact.]
Field signals can be connected to the process control system (PCS) via fieldbus, wireless, or conventional 4-20 mA technology.
The choice of a technology must take into account the application and its particular requirements. Using wireless communication to connect field devices to the process control system is currently a topic of discussion in many media outlets and at conferences and trade shows.
However, wireless transmission is not a new technology. As far back as the 1990s, data was being transmitted to computer systems wirelessly.
Even back then, services such as private mobile radio and trunked radio systems made it possible to bridge distances of more than one kilometre.
In the last several years, GHz-frequency bands have been allocated for such applications, leading to the development of a number of proprietary wireless systems such as Trusted Wireless by Phoenix Contact.
WirelessHART and ISA100 provide facility operators with further vendor-neutral wireless standards from which to choose.
Process control technology determines the fieldbus system
Various fieldbus systems are also available for connecting field devices directly to the process control system. Foundation Fieldbus FF-H1 and Profibus PA have established themselves as process engineering standards.
The physics involved in both standards is the same: two-wire technology for the operating voltage with modulated data. However, they differ with respect to data communication, shielding design, and bus termination.
The required installation materials, cable, and connection technology are likewise identical, and may thus be used for both fieldbus systems.
The advent of modularised active fieldbus distributors allows them to be expanded while in operation or replaced if faults occur.
[Modular fieldbus infrastructure for Foundation Fieldbus FF-H1 or Profibus PA.]
The maximum current to the field device can be adjusted according to consumption, thus allowing the fieldbus to operate more efficiently and safely and also increasing facility availability.
Since process control engineering field devices and interface components are not bus-compatible, the user must opt for a fieldbus system.
The selection is often made on the basis of the process control system, as the various PCS vendors are currently capable of integrating both the Foundation Fieldbus FF-H1 and the Profibus PA.
However, they typically acquire more expertise with a system for which they provide better support in terms of tools, functionality, and integration. The process control system therefore determines the choice of the bus system.
4-20 mA technology more difficult to adjust and reconfigure
Many facility operators continue to install the conventional 4-20 mA field signals. However, only one field variable can be transmitted per 4-20 mA signal, which is a drawback compared to the fieldbus technology.
The Foundation Fieldbus FF-H1 and Profibus PA allow multiple variables to be collected per field device, for example, flow rate and temperature. In addition, changing the parameters in the 4-20 mA field devices via Hart functions is time-consuming and limited.
[The current to the Foundation Fieldbus or Profibus devices can be adjusted as required.]
This procedure is simpler and faster with fieldbus devices, and automation is an option. The price difference between 4-20 mA and fieldbus devices is nonetheless relatively small; in some projects, the costs are actually comparable.
The only advantage of 4-20 mA technology is that the installation – the field distributors, cable, and I/O system – maintains the same familiar form.
However, this poses a competitive challenge in the medium to long term, since the experience of end users of fieldbus technology shows that the production processes can be adjusted more efficiently and reconfigured more quickly.
Likewise, the argument that maintenance personnel are not familiar with bus systems no longer holds. Many young employees working in installation, maintenance, and production have grown up with cell phones and PCs.
They therefore have no reservations about working with fieldbus technology and the associated software tools.
Wireless as an alternative to wiring
Both fieldbus and wireless technology have their advantages and drawbacks.
Wireless communication is indeed more expensive to purchase and maintain, for example, when batteries have to be replaced.
In deciding between wired and wireless transmission, it is useful to draw upon the experiences of numerous users with wireless solutions:
- Radio waves in the GHz range exhibit shortwave characteristics, meaning that they propagate quasi-optically, much like light waves. If there is no line of sight between the transmitting and the receiving antenna, radio reception will be less than optimal.
- Data is transmitted via antennas by means of electromagnetic fields. In the GHz range, only the electrical field is effective. Buildings, pipes, or tanks may reflect this field, causing the signal strength to be amplified, attenuated, or even nullified.
- It is important to select the right antenna and install it correctly in order to minimize interference and achieve higher signal transmission and reception gain. The antenna should be mounted as high as possible and in the line of sight of the receiving antenna. Doing this will reduce reflections.
- Before making modifications or fixed installations, a trial operation is recommended in order to determine the radio signal field strength and the optimal antenna positioning.
- The quality of the wireless connection is also influenced by the environment and may therefore vary. Modifications or new installations of facility equipment may thus have a negative impact on existing wireless links.
Practical experience shows that wireless communication should be avoided for applications in which wiring can be installed with reasonable expense and effort. Otherwise, wireless data transmission is a viable alternative.
WirelessHART and ISA100.11a have limited range
Various wireless systems are suited for use in process engineering.
A comparison of the WirelessHART and ISA100.11A standards shows that both systems promise the same performance.
Differences become apparent when integrating them into existing applications. WirelessHART will gain a market foothold more quickly here, since HART communication already forms an integral component of many 4-20 mA field devices.
These devices can thus be integrated into the wireless network using wireless data adapters. The field variables can still be transmitted via the 4-20 mA signal.
New wireless field devices have an antenna and batteries for transmitting the variables wirelessly.
Since the field device antenna is screwed securely into the housing, it cannot be oriented optimally or installed in a better position. Before mounting, the user should therefore measure the field strength at the installation site.
WirelessHART requires the installation of an extensive wireless infrastructure.
So-called wireless gateways are installed in the field for this purpose that act as interfaces between the wireless field devices or wireless adapters and the process control system.
The wireless gateways are equipped with an antenna over which they exchange data with the field devices. The gateways have a copper-based Ethernet connector for connecting to the process control system LAN.
[WirelessHART gateway to LAN, WLAN, and WirelessHART links.]
Phoenix Contact’s WirelessHART gateway offers both a LAN and a WLAN interface, making LAN cable installation unnecessary.
However, WirelessHART and ISA100.11a have the drawback of being limited to 10 mW of transmitting power, which restricts their range.
For longer ranges of up to three kilometres, proprietary wireless solutions having 100 mW of transmitting power, such as Trusted Wireless, have to be used.
Wireless technology will not supplant 4-20 mA field devices or fieldbus technology.
Facilities that have evolved over a period of years most typically use a combination of all three solutions.
New facilities will presumably use the fieldbus instead of 4-20 mA technology. Wireless systems find use in areas where it is not possible to install cabling.