4-20 mA, Fieldbus, or Wireless Technology — There Are Pros and Cons Either Way for Transmitting Field Data

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DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

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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. 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 Expensive 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.

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