INTEROPERABILITY describes the ability to exchange a measuring or controlling device from one vendor, with a device from another complying vendor, without disruption to the process. In theory this should have been possible since the inception of Profibus PA, but this has not been successfully implemented so far. The new Profibus PA Device Profile 3.02 resolves this issue.
In spite of the ever increasing complexity of technology, the integra tion of field devices in control systems should still be as easy as for 4-20 mA technology. While 4-20 mA technology involves transferring only a single piece of information, the process variable (PV), between the device and the system, much more information is trans ferred on the fieldbus. Examples of data that may be transferred include the status of actuators for predictive mainte nance or more precise measured values for intelligent process control.
This information will be critically necessary in the future. The complexity of plants is rising, while the number of employees operating them is falling. At the same time, product quality require ments are becoming more stringent; vari ations in quality are no longer tolerated.
Analogue devices, although undeni ably simple, cannot meet these chal lenges. Rather, the diagnostic possibili ties of an intelligent field device are required. However, the problem of field device integration has proven in recent years to be anything but a selling feature for the introduction of fieldbuses in the process industry. Installation of new patches and service packs and switching to new device versions and control systems during the service life of a plant led to unmanageable situations in terms of device replacement. In particular, the ability of users to manage the life cycle of devices was limited.
“Device failures also happen at inconvenient times, such as at night or on the weekend or when the personnel on-site are not experts in automation systems," described John Immelman, President of the Profibus Association of Australia. “Nevertheless, it should be possible for a user to replace a field device without any special knowledge about digital communication tech nology. The new profile really makes life easier in these scenarios,” noted Immelman about the Profibus PA Profile 3.02 introduced this year. The application profile defines vendor- neutral device parameters and functions for a wide variety of device types. This creates a basis for ensuring the compati bility of description files (GSD, EDD, DTM) and field devices. For example, it is possible for integration tools to auto matically assign a description file to a device, thereby simplifying engineering procedures (such as during initial instal lation of a field device).
Up to now, the use of manufacturer- specific description files meant that users were encouraged to replace a field device with an identical one. The Device Profile 3.02 bridges this gap, as the new field device now automatically assumes the device version of the old device. To achieve this, the tasks of the predecessor model are determined and the device assumes its functionalities without any interruption in the process.
The replacement device thus presents itself to the control system as its prede cessor, even thought it may contain technology that is 10 years more advanced. Then, during a subsequent planned shutdown, the new function ality can be integrated by updating the description file. The same applies to the integration of an EDD or a DTM during a device replacement. It is also possible in this case for a new device to assume the functionality of several pred ecessor versions (Device Revision), which is designated by the device manu facturer in a standardised manner and checked during certification.
The profile was scrutinised in the test laboratory of BIS Prozesstechnik in Frankfurt. The laboratory houses one of the world’s largest multi-vendor fieldbus installations in which type tests of field devices are performed. The laboratory is also a test centre for EMC and actuator technology (SIL, CE, TA-Luft) and a PI Competence Centre for Profibus.
All test results were clear: the profile proved itself in real-world industrial scenarios and satisfied the user require ments. As Immelman concludes defini tively, “We regard the results of the test as very positive, and we view the PA Profile as an important step toward simplified device replacement.”
The main goal behind the develop ment of Profibus PA Device Profile 3.02 was to minimise the additional effort required by users when working with device descriptions on the fieldbus. At the same time, the maintenance effort over the entire life cycle of the device should be reduced.
The manufacturer describes the properties and functions of a field device on the fieldbus in a file (GSD, EDD, FDT/DTM), which the user integrates in an overall control system (for example, process control system and engineering system) using a software tool. The field device functionality is made available in the system and addi tional information can be generated.
To reduce the maintenance effort throughout the life cycle, a suitable version policy for devices, device files, and software platforms was necessary. It includes provisions for marking the soft ware version on the device, the auto matic adaptation to the functionality of predecessor versions in commerce, as well as vendor-neutral guidelines for modifications to device software and their effect on compatibility.
Additional profile specifications include the mandatory mapping of specific diagnostic information of field devices onto standardised categories and faster transfer of field device data, for example, during transmission of parame terised data during device replacement.
The aim of certification is to provide users with an assurance that devices from different manufacturers are capable of fault-free operation when used together. The large quantity of profile- compliant device functions facilitates uniform handling of standard device functions by users.