PROFINET gains process acceptance
03 June 2011
PROFINET is already a familiar feature of production automation and drive engineering applications. But what about the process automation sector? Dr Peter Wenzel of Profibus and Profinet International (PI) has reported a heightened interest from this sector.
Dr Peter Wenzel of Profibus and Profinet International (PI)
PROFIBUS PA is already widely used in process facilities. However, an integrated communication system such as PROFINET is essential to enable complete integration of centralised process-related operations of a plant with downstream applications involving mostly discrete processes, such as filling and packaging.
For this reason, PI established a working group of manufacturing companies whose initial task was to define the particular characteristics of process industries. Additional consideration was given to the requirements of NAMUR. Besides the extended cycle times, continuous plant operation, and complex actuators and sensors, another challenge is the quantity of devices (up to 100,000 I/O signals). Moreover, life cycles in the process industry are often very long too.
To establish PROFINET on a widespread basis the specific requirements of this sector had to be implemented. This effort focused on four main areas:
Investment protection – To protect investments, seamless integration into existing fieldbuses must be possible. Many plants already have a large installed base and continued use of this is the aim. This is ensured by means of a proxy concept, the three communication systems encountered in process industries – PROFIBUS PA, Hart, and Foundation Fieldbus – can be integrated into the higher-level PROFINET network. The proxy assumes responsibility for implementing the physics and protocol and ensures the exchange of all I/O, diagnostic, and parameter assignment data as well as alarms with the field devices.
Configuration in run – The chemical industry places top priority on plant availability. It must be possible to reconfigure devices and networks and to add, remove, or replace devices or modules during operation. The auto-sense and topology detection feature identifies devices automatically and their locations pinpointed. This enables convenient, reliable solutions to be developed for device/spare part replacement scenarios, in which the replacement device parameters are assigned automatically by the control system. All of these ‘Configuration in Run’ measures (CiR) are carried out in PROFINET without interruption and without affecting network communication. This ensures that plant repairs, modifications, or expansions can be performed without a plant shutdown in continuous production processes, as well.
Time synchronisation and time stamping – In power plant automation a high value is placed on time-correct tracking of individual process signals. This is especially critical when it comes to malfunctions in individual automation areas. The plant operator wants accurate information on the order that the signals were sent and at what time to allow him to perform a detailed ‘root cause’ analysis. An accuracy of 1ms is critical for this purpose.
This requires a time stamp for digital and analogue measured values and alarms that is accurate to the millisecond. A precondition for this is an exact time synchronisation of the components involved: By means of a central system master clock, a master selected specifically for this purpose transmits a cyclic equidistant clock signal to all bus nodes, synchronising them. This ensures that I/O devices can provide real-time information about alarms and other important events with a time stamp that is based on a network-wide standardised time of day. By acquiring events at a comparable time, an exact description and analysis of a possible fault can occur. Because not every field device has such a time stamp, a hybrid operation must also be possible. This is guaranteed.
Scalable redundancy – To avoid automation failures caused by conditions such as wire breaks or short circuits, redundancy concepts were developed for PROFINET, which can be structured differently depending on the. The basis for these concepts is the automatic switchover of communication paths to intact paths in the event of a fault, along with communication of status information regarding the cause of the communication interruption. The user can decide whether he wants to use controller redundancy, network redundancy, device redundancy, or device interface redundancy. Moreover, the recovery time of a communication system must be fast enough to prevent process disturbances. All redundant elements must have a diagnostic capability so that faults can be detected and faulty elements replaced.
Many users need an integrated communication system down to the field level. PROFINET enables a direct path to MES and ERP systems, while also facilitating the use of Internet services. New architectures can be realised with PROFINET. The flexible signal allocation allows signals to be assigned to controllers in the network without any rewiring. This aids in future plant expansions, even when continuous processes are involved. Entirely new paths are also possible: if, up to now, architectures have been based on the layer model of the conventional automation pyramid, flatter and thus more cost-effective architectures are now conceivable, especially for smaller applications.
With completion of the ‘PROFINET for Process Automation’ application profile, PI has taken a step towards a uniform communication concept for process and production industries. What remains now are a few housekeeping tasks, such as the review of security concepts, coordination with FDI, development of test specifications for devices, and certification of devices that meet the PROFINET requirements.
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