A fast synchronous network as the basis for achieving a high-performance overall system
23 November 2012
What are the factors for successful automation? Factors such as speed or the performance capability of a particular sensor are often mentioned. However, the outstanding features of an individual component are only an advantage if the design of the overall system is compatible. In practical terms, this means that high-precision sensors are of little use without a fast synchronous network, and vice versa.
Whenever performance is discussed, the overall system often takes a back seat. The result: the overall speed of the system is only as fast as the slowest link. In other words, you may have fast communication, but it is of little use if your controller or I/O system do not have compatible cycle times. It should be borne in mind that the terminal-terminal response time depends heavily on the bus update time. The critical factors are, therefore, the overall system accuracy as well as the synchronisation of controller, communication, and inputs/outputs. The basis for achieving such a high-performance overall system is the use of a fast synchronous network.
PROFINET technology, a communication system that reflects all facets of automation, is enjoying success across all industry sectors throughout the factory automation, motion control, and process automation markets thanks to its speed in addition to its real-world diagnostics, integration, safety, and wireless solutions. In 2011, 1.3 million new PROFINET devices were sold on the market, bringing the total installed base to 4.3 million devices.
In automation, the challenge lies in not knowing what the future holds in terms of requirements. For example, an end user may be completely satisfied at the moment with their automation and communication systems. But what happens five years later when that user's Quality Assurance Department requires certain production procedures to be transmitted over the communication system in real-time?
In order to be equipped for future tasks, PROFINET Specification V2.3 defined mechanisms that will further speed up communication with PROFINET. An important step of this definition is the performance upgrade of PROFINET to achieve cycle times of 31.25 µs. This upgrade is for applications that have more stringent demands on communication while also requiring isochronous operation. The key thing here is that the system remains scalable. Regardless of which level of performance will be required in the future, the user can rely on a single communication system without system gaps.
Faster to the goal
Three mechanisms make this possible: Fast Forwarding, Dynamic Frame Packing, and Fragmentation. As a result, cycle times of as little as 31,25 µs can be achieved together with high-precision isochronous operation. To maintain compatibility with the previous specification, three main tricks have been used. To optimise the IO bandwidth, the transmission time of messages was shortened from 6.3 µs to 1.2 µs by forcing an earlier forwarding decision (Fast Forwarding) during switching. Previously, a standard PROFINET frame could only be forwarded in the switch when the complete Ethernet header was received.
Like other communication systems, PROFINET uses the summation frame method for optimising the ratio of frame to user data, opening up further potential for optimisation. In contrast to ring bus systems, PROFINET relies on the performance advantages of a full duplex system, i.e., input and output data are sent simultaneously on the 2-pair cable. When a single summation frame is used, this would have to be sent, received, and checked completely down to the last node, including the checksums. This is where Dynamic Frame Packing comes in. Because the data of the first nodes in the line are not relevant for the nodes placed further at the end, these are removed during the passage. This shorts the frame in its passage through the network. The time-determining arrival of the frame at the last node is thus much sooner, thereby reducing the overall update time for all nodes.
A proven advantage of PROFINET is its unlimited TCP/IP communication, even when isochronous real-time communication is occurring simultaneously. For this, the architecture of PROFINET provides for time scheduling in addition to synchronization. The network is not loaded with I/O frames during a defined time phase but instead is free for any TCP/IP frames, which can take up a duration of up to 125 µs with Fast Ethernet and thus define the minimum cycle time.
Next, the fragmentation defined with PROFINET V2.3 takes large TCP/IP frames in the individual nodes and, prior to sending, divides them into smaller fragments, which are sent in consecutive cycles. The counterpart then re-assembles them so that the upper-level application layer receives an unaltered TCP/IP frame. This allows users to realise bus cycles of 31.25 µs with shared I/O and TCP/IP communication, without having to reduce the available bandwidth for the TCP/IP communication.
Applications exist today that can benefit from a cycle time of 31,25 µs, such as high-speed closed-loop motion control applications and applications in the measuring equipment sector.
A key aspect for the user is the compatible expansion options that allow it to update an individual controller or field device and still retain existing functions. Only when the user wants to make use of the new functions, for example. for performance optimisation, is it necessary to fully update controllers and field devices to the latest version. The user protects its investment, while remaining free to access the reserved performance at any time.
The resulting new generation of PROFINET modules will implement all these new functions in hardware. Accordingly, various technology suppliers will offer easy-to-integrate solutions in the form of ASICs, network controllers, or FPGAs and thus provide device manufacturers with the basis for producing high-performance solutions that meet customer requirements. As a result, users can rely on a coherent approach that uses both a fast, high-performance network as well as fast devices. A system designed with both in mind is essential for realizing the benefits of increased performance in practice – today and in future applications.
Comments on the fast cycle times of PROFINET Specification V2.3
For many users, a cycle time of 31,25 µs is almost unimaginable. Karsten Schneider, PI Chairman, explains the tools used to demonstrate this fast cycle time and the significance it has for real-world applications:
So, just how fast is a cycle time of 31.25 µs?
In fact, it is difficult to grasp just how fast this cycle time is, which is why we constructed a live model. Because LEDs react too slowly, we used an oscilloscope to visualize the cycle time of 31.25 µs as well as the slight jitter over the entire system. In addition, an analog signal was sampled, transmitted via PROFINET, and output at another station in our model.
Which applications will benefit of this cycle time?
It is of interest to highly dynamic measuring equipment applications, since sampling rates up to 32 kHz over the network are possible. It could be used, for example, to record torque characteristics in test stands.
Why will isochronous operation play an even more important role in the future?
The processes of the future will have to be tuned to each another with even greater precision. A typical example is the multi-axis closed-loop control process in printing machines. A more precise isochronous operation will not only increase the productivity of the overall printing machine but will also allow production of printed products with higher-resolution and thus sharper images. Another industry sector with stringent requirements for isochronous operation is the packaging industry. While the material filling process runs relatively slowly, the primary packaging process requires a very high speed. Both processes must be precisely tuned to each another to avoid disruptions in the overall process.
And how have you demonstrated this feature with the model?
Isochronous operation was demonstrated with a traditional stroboscope test. For this, we aimed a stroboscope at a variable-speed disk in such a way that a permanent image of a written text is produced.
Openness is often cited as being a highlight of PROFINET. Does this also apply to the short cycle time of 31.25 µs?
We have placed a high value on this during development. Even with the short cycle time, standard data can be transmitted without limitation via TCP/IP. We have a full HD video taken in our test setup that demonstrates undisturbed transmission of these data all the way through the PROFINET system. The ability to transmit standard data is necessary in order, for example, to transfer new parameters, quality assurance data, or images for production monitoring. An example of this is the transmission of data from high bay storage systems via a camera. In addition, there is a trend in assembly lines toward recording and storing torque characteristics of screws for quality control purposes. These data can also be transmitted without any problems.
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