Discussion on PROFINET in the context of Process Automation
Dr. Peter Wenzel, PI (PROFIBUS & PROFINET International)
The move to Ethernet-based communication systems is in full swing. This is true especially for PROFINET as proven by the latest figures on installed PROFINET devices. With 1.3 million new PROFINET devices sold on the market in 2011, the total installed base has now risen to 4.3 million devices. Factory automation projects account for almost all of these figures. It is the goal of PROFIBUS & PROFINET International (PI) to make PROFINET up the task for the full range of industrial automation applications.
In order to optimize PROFINET for the wide range of requirements of factory and process automation applications, the new PROFINET V2.3 version has been supplemented in two respects. First, advanced functions for integration and parameter assignment of devices (for Configuration in Run), scalable redundancy, and time stamping (for determining Sequences of Events) have been added that open up the market for PROFINET to process automation applications. Second, a performance upgrade has been implemented with the addition of the Fast Forwarding, Dynamic Frame Packing, and Fragmentation functions that extends the market for PROFINET all the way to high-end motion control applications, while still ensuring its coexistence of with IT applications.
Innovation of PROFINET for process automation
In its quest to make PROFINET fit for use in process automation, PI collaborated with users to develop a set of requirements. In addition to user-friendly operation, protection of investment for the end user is an essential requirement because instrumentation in a process control system typically has a life cycle of several decades. This ensures that plant owners using PROFIBUS today can rely on a future-proof system and can change to PROFINET at any time.
The requirements that apply to PROFINET for process automation mainly include the functions for cyclic and acyclic data exchange, integration of fieldbuses, integration and parameter assignment of devices (Configuration in Run), diagnostics and maintenance, redundancy, and time stamping (Sequence of Events).
For two-wire conductor systems used both in standard applications and in applications involving energy-limited bus feed of devices in hazardous areas, PI is continuing to rely on its thoroughly proven PROFIBUS PA solution. The question then arises as to the optimal gateway from PROFIBUS PA to PROFINET. A proxy concept for the integration of fieldbuses in PROFINET, which was specified several years ago, is available for this. This concept can be used to integrate the three communication systems used in the process industry, namely PROFIBUS PA, HART, and Foundation Fieldbus. It is based on standardized mechanisms for mapping the fieldbus-specific properties onto PROFINET. The bus systems are integrated using gateways (proxies) that link the higher-level PROFINET network to the fieldbus system to be integrated. The proxy becomes responsible for implementing the physics and protocol and ensures the exchange of all I/O and diagnostic data as well as alarms with the field devices.
The availability of automation systems is of critical importance in continuous processes in particular because, for reasons that are known, plant operation often must not be interrupted under any circumstances. To avoid automation failures caused by wire breaks, short circuits, and the like in these types of plants, a scalable redundancy concept was developed for PROFINET, in which the redundancy solution can be structured optimally to meet the specific requirements of the application.
Some applications require a time stamp for digital and analog measured values and alarms that is accurate to the millisecond. A precondition for this is an exact time synchronisation of the components involved. The purpose of this is to ensure 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 standardized time of day. The time recording of events is the basis for determining the Sequence of Events, thereby enabling an exact description and analysis of a possible error case, for example.
Like redundancy, uninterrupted plant operation – including when reconfiguring devices and networks and when inserting, removing, or replacing devices or individual modules while the plant is operating – plays an important role (Configuration in Run). The actions are performed in PROFINET without causing any interruption or adversely affecting network communication. This ensures that plant repairs, modifications, or expansions can be performed without a plant shutdown in continuous production processes, as well.
Fieldbus and Ethernet systems provide extensive possibilities for diagnostics, e.g., for maintenance. These include, for example, the provision and transmission of identification and maintenance (I&M) data, as is familiar from PROFIBUS and PROFINET applications, or the communication of events and transmission of device status according to NAMUR Recommendation 107. Manual changes made directly on the device or via external parameter assignment tools are signaled to the control system via PROFINET as parameter change events. This allows the control system to detect deviations from the central data management, notify the user, and perform updates, if necessary.
Performance upgrade for machine building
With the latest PROFINET V2.3 specification, a performance upgrade is available to users. This was made possible by the incorporation of intelligent functions in the new version of the specification, namely Dynamic Frame Packing, Fast Forwarding, and Fragmentation.
PROFINET real-time communication (RT) uses the prioritization methods of Ethernet and can therefore be implemented on standard Ethernet controllers. The accuracy of the firmware implementation determines the jitter of the transmitter clock, just like on other Ethernet systems. With a data rate of 100 Mbps and full-duplex transmission, bus update times that are faster by several factors compared to today’s fieldbuses are possible. As a result, RT is usually fully sufficient for typical factory automation applications. For applications whose requirements include the need to synchronize nodes to within 100 µs or less or to form a highly dynamic control loop via the bus, additional measures become necessary. The highly accurate isochronous real-time (IRT) synchronization process of PROFINET eliminates Ethernet transmission delay times of differing and fluctuating lengths.
PROFINET enables parallel TCP/IP communication for standard data, diagnostics, or parameter assignment purposes alongside both RT and IRT communication, without the need for additional modules or firmware measures. This is made possible by a free time slot in the update cycle. Data access, diagnostics, and parameter assignment are the same with RT and IRT communication. The user only has to specify when configuring whether RT or IRT communication will be used.
PROFINET V2.2 meets the real-time requirements of more than 95% of applications. Only applications involving specific configurations in which many nodes with few bytes are connected in a line topology may have more stringent performance requirements. For example, the possible bandwidth utilization is not optimal when padding is used, i.e., filling of frames to the minimum 64 byte length in compliance with standards. Additional measures have been taken in the PROFINET V2.3 specification for this case. These measures at different starting points produce high-performance communication with exact deterministic behavior at update rates as fast as 31.25 μs, without affecting the openness for TCP/IP communication.
The decision whether to forward a frame in the integrated switch of a device requires address information in the frame header. With Fast Forwarding, the FrameID (FID) address information is integrated once at the start of the frame header so that instead of having to wait for a large number of bytes it is possible for forwarding to take place early on. As a result, the current standard delay times of 3-6 µs per node can be reduced to 1.2 µs.
To optimise the ratio of frame to user data, the Dynamic Frame Packing function was defined. For this, improvements were made to the summation frame method already used in several fieldbuses in which the I/O data for several nodes on the network are integrated in one frame, thus requiring only one frame header and trailer (FCS). In contrast to ring bus systems, PROFINET uses the full-duplex principle of data transmission common in Ethernet systems. Here, input and output data are sent simultaneously on the 2-pair cable. When a single summation frame is used, this complete frame is sent, received, and checked all the way to the last node, including the checksums. With Dynamic Frame Packing, the data of the first nodes in the line, which are not relevant for the nodes placed further at the end, are removed during the passage. As a result the frame becomes shorter when passing through each node.
Time scheduling ensures the unlimited openness of PROFINET for TCP/IP frame transmission alongside IRT communication. Specifically, it ensures that the network is reserved for TCP/IP frames rather than user data during a defined time phase. With Fast Ethernet, the transmission of one TCP/IP frame can take up to 125 μs, which defines the minimum cycle time. The Fragmentation function defined in PROFINET V2.3 takes large TCP/IP frames in the individual nodes and, if necessary, divides them into smaller individual parts prior to sending. These fragments are then sent in consecutive cycles. The counterpart then reassembles the fragments into a complete TCP/IP frame. In this way, it is possible to configure bus cycles of 31.25 μs with shared user data and TCP/IP communication.
With Version V2.3, PROFINET now meets all requirements for automation applications, ranging from process automation and factory automation to high-performance motion control applications. This technology development paves the way for developing cost-optimized automation solutions and is especially important for meeting the demand for investment protection, both for existing plants and expansions to existing plants. PROFIBUS and PROFINET are not competing solutions but rather are complementary solutions. While PROFIBUS is used in continuous processes and hazardous areas, PROFINET is primarily of interest in applications requiring integration all the way to the corporate management level or whose real-time communication requirements cannot be met by conventional fieldbuses.