08/16/2011 | Autor / Editor: Steffen Ochsenreither / Sonja Beyer
In the past, several users of field devices chose to stick with the proven 4-20 mA technology, in spite of many advantages that Profibus offers. The criticism was that device replacement was perceived as overly complicated, and the large number of diagnostic messages was criticised as well. But an improvement in Profibus technology might silence these voices.
A nightmare scenario for any plant owner: a process-critical device fails and must be replaced as soon as possible. Of course, this usually happens on nights and weekends. The device can only be replaced by on-site personnel. Is a plant outage in this case inevitable?
Users of 4-20 mA technology are accustomed to replacing devices without having to shut down the control system or modify the software, but this turned out not to be possible with Profibus — at least until recently.
Profibus has undergone a steady process of development. With the introduction of the Version 3.0 Profile for process automation in 1999, many of the requirements identified by customers were integrated into devices. A decade later in 2009, the requirements profile for process automation field devices was revised. The sound and stable Profile Version 3.0 was well-accepted and established in the industrial arena, but a fundamental everyday requirement of industry remained only partially met: namely, the ability to replace drives. With Profibus, ist was not only necessary to replace the general station description (GSD) files but care had to be taken to use the correct GSD file for the corresponding hardware and software versions. Often, the GSD files were available in multiple versions, because they were constantly being revised and updated by the respective manufacturers.
As a result, the task of replacing devices in the control system, rather than the job of physically replacing the devices, emerged as the central problem. Customers operating high-availability plants, in particular, were forced either to keep a stock of identical devices on hand, which, in turn, resulted in high costs as well as creeping obsolescence of devices, or to risk a version conflict. The latter option requires accessing the control system because the related driver software has to be updated.
When the device has been replaced and the plant placed back in operation, the next requirement is to reload the customer-specific parameter sets in the devices. These parameter sets can easily contain as many as 600 parameters, owing to the increased level of functionality of devices, which means uploading or downloading the device parameter assignment takes its time.
But device replacement was not the only unforeseen challenge facing users. Although the diverse array of possible diagnostics is an intrinsic advantage of the technology, plant operators often reported being overburdened by it. The consequence was that diagnostics were incorrectly interpreted or ignored altogether. This also gave operators the impression that Profibus field devices were more prone to and/or sensitive to errors than the 4-20 mA technology.
But there is good news: the problems outlined above have been eliminated. With the completion of PA Profile Version 3.02 at the end of 2009, PROFIBUS & PROFINET International (PI) has addressed the problems and requirements of customers with a practicable solution.
For example, accessing the control system is no longer needed. Once the device has been physically replaced, the control system detects the device and the device version expected at the bus address based on the message frame of the master. The new device then answers with the appropriate Ident No. and is included in the cyclic communi- cation of the master. It makes no difference in this process whether the predecessor device was integrated in the control system with the manufacturer-specific or profile-specific GSD.
Tests have shown that the process value is available again immediately after device startup. Additional intervention in the control system is not required. User-specific parameters are loaded via the cyclic channel, and the device can then be operated as usual. In order to shorten the time required for uploading or downloading the device parameter assignment, the PA Profile Version 3.02 has introduced a solution. By grouping parameter sets, it is now possible to reduce the time required to transfer parameters by a factor of 5 to 10, thereby achieving the fastest transfer times for a fieldbus in process automation.
The device GSD can be updated during the next scheduled plant shutdown, after which all the new functionality of the device is available.
Even switched-off devices can be quickly and uniquely identified with PA Profile Version 3.02: manufacturers of these devices must print the name of the device’s hardware, software, and profile version on the nameplate.
This approach also opens up new possibilities over the entire life cycle of the plant. For example, devices can be replaced on a large scale without shutting down the entire plant. Plants can be renovated while in operation, and the new functionalities can be quickly introduced by adapting the software during the next maintenance cycle. This enables significantly shorter plant outages.
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