Communications equipped for the future
26 April 2011
Process plant operators demand a high level of reliability and profitability in production. Plants must function in a harsh industrial environment over many years without interruption. The automatic interaction of different systems and components of various manufacturers must be ensured on a continuous basis. Communication systems provide the basis for this type of interaction…
Plant operators in the process industry demand industrial communication that is uniform and consistent over the entire production chain, including for continuous and batch processes and in hazardous areas, has a high level of availability and reliability, is flexible when it comes to modifications and expansions, and is economical to use throughout all phases, from planning to operation.
The technologies of PI (PROFIBUS & PROFINET International) meet these requirements. The basis for this is PROFIBUS, a time-tested industrial communication system. Specifics of the process industry sector are covered by the well-proven PA Device application profile. For plants that use Ethernet, PI provides the PROFINET solution, which, on the one hand, utilises existing protocols from the IT world such as TCP/IP or DHCP and, on the other hand, defines supplemental protocols in order to satisfy the more stringent requirements of industrial environments for availability, real-time capability, etc.
Against this backdrop it is evident that PROFIBUS and PROFINET are not competing solutions, but are actually complementary solutions. PROFIBUS is used in continuous processes and is suited for use in hazardous areas. PROFINET can also play out its strengths in batch processes and is of primary interest when requirements exist for integration all the way to the corporate management level and for remote maintenance and diagnostic capabilities, and the like, via the Internet.
Positioning of PROFIBUS and PROFINET
PROFIBUS and PROFINET are the only communications systems to fully cover the automation engineering requirements for both production and process automation. 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. Process automation applications, in particular, can benefit from both communication systems.
PROFIBUS is optimally equipped for use in process automation. With its integrated PROFIBUS DP communication profile, intrinsically safe energy supply of devices via the bus, network components customized for the specific application, standardised and thus interchangeable devices based on the PA Profile, comprehensive range of diagnostics for devices and bus, and many other features, PROFIBUS is the most powerful solution for process-related applications in the process industry sector. This solution includes complete integration of centralized process-related operations of a plant with upstream and downstream applications involving mostly discrete processes, such as filling and packaging (hybrid automation).
With advances in development, increased use of Ethernet systems in process automation is right around the corner. The range of uses extends from the corporate management level and the process control level all the way down to the field level. The exception to this are continuous processes, which often require intrinsic safety. Since there is currently no serviceable intrinsically-safe Ethernet solution available, this will continue to fall under the domain of PROFIBUS PA.
PROFINET is already being used successfully as a communication standard in production automation. PROFINET satisfies a wide range of requirements with its integrated Ethernet-based communication, ranging from data-intensive parameter assignment up to extremely fast I/O signal transmission. Communication takes place over the same cable in all applications from simple control tasks to highly demanding motion control applications. In addition, a direct interface to the IT system is always available. With its flexible line, ring, and star structures and its wireless and fiber-optic cable solutions, PROFINET conforms to the existing plant situation, thereby eliminating the need for expensive custom solutions. PROFINET integrates automatically reacting redundancy solutions and intelligent diagnostic concepts. Acyclically transferred diagnostic data supply important information about the status of network and devices, which is essential for correct operation and preventive maintenance. The proven PROFIsafe safety technology of PROFIBUS is also available for PROFINET. Hence, the basis exists for using PROFINET in process automation applications.
Innovation of PROFIBUS PA Profile V3.02
PROFIBUS communication has remained unchanged for years. Only the recent PROFIBUS PA Device V3.02 application profile has seen an innovative change, which was motivated by a series of NAMUR (international user association of automation technology in process industries) requirements. It puts forward innovations to make life easier for users over the entire life cycle of their plants. These involve two areas that are of particular concern to users in the process industries at the moment: standardized diagnostic information and easy device integration. For device manufacturers, PA Profile V3.02 means that every PROFIBUS PA device must handle standardized diagnostics. All suppliers are obligated to uniformly map the specific diagnostic information of their intelligent field devices onto the standardized categories according to NAMUR recommendation NE107 "Self-monitoring and diagnosis of field devices" (consists of four descriptive, vendor-neutral categories: failure, function check, out of specification, maintenance required). Accordingly, the mapping of vendor-specific diagnostic information is carried out in the field device itself, which means that only the standardized information has to be integrated into the higher-level process control or maintenance system. This significantly reduces the effort required for any future device replacement.
NAMUR Recommendations NE105 and NE121 contained specific requirements and rules for device integration in fieldbus systems. The first rule is that device integration must not cause any maintenance effort. The second rule relates to life cycle and investment protection. Specifically, according to this rule, if old devices are replaced after a period of 10 years or so by technically advanced versions, suitable standards must assure the existing device integration. Technical advancement of devices is thereby accompanied by stable device integration over the long service life of plants in the process industry sector. The challenge then is to ensure that advancements in device technology are designed in such a way that they do not interfere with stable device integration.
This is accomplished in PA Profile V3.02 by automatically transferring the functions of predecessor models. The standard mechanism in the replacement device identifies the predecessor and then automatically sets itself to the desired variant. The important thing here is that the intelligence resides entirely in the field device. As a consequence, the mechanism is independent of the control system, which in turn means that existing systems do not have to be upgraded in order to assure the device integration. The replacement device thus presents itself to the control system as its predecessor, even thought it contains technology that is more advanced. During a subsequent planned shutdown, the new functionality can be integrated, if required, by updating the description file. As a result of this, users can benefit from enhanced devices over the plant life cycle without having to disrupt ongoing plant operation.
Another positive aspect is the optimised transmission times for uploading/downloading parameters. These are beneficial primarily during commissioning or in the event of a future device replacement. Transmission using the new mechanism can achieve data rates up to five times faster.
Once the specification work was complete, tests were performed on prototypes in the test labs of BIS Prozesstechnik in Frankfurt, Germany, with the aim of verifying the practicability of the new profile functions. The testers focused on two scenarios: first, the replacement of an existing device while the process is running and, second, the display of status signals according to NE 107 in the event of an error. In order to test the quality of the specified functionality, two field devices – an actuator and a temperature transmitter – were tested in combination with five different control systems. All test results were clear: the profile proved itself in real-world industrial scenarios and satisfied the user requirements. The testing experts at BIS gave a positive score to all test results. The goal of easy device replacement has been achieved without question.
PROFINET for process automation
For optimal use of PROFINET in all sectors of process automation, PI (PROFIBUS & PROFINET International) has created a requirements catalogue in collaboration with users. Main focus of attention was given to investment protection for the end user because, as mentioned previously, instrumentation in a process control system typically has a life cycle of several decades. In this manner, it is ensured that plant operators can already rely on a future-proof system based on PROFIBUS and can change to PROFINET at any time.
The requirements mainly include the functions for cyclic and acyclic data exchange, integration of fieldbuses, integration and parameterization of devices including Configuration in Run, diagnostics and maintenance, redundancy, and time stamping.
The energy-limited bus feed of devices in hazardous areas on Ethernet has not been formulated as a requirement, since PROFIBUS PA already provides a proven solution for this. In addition, proven, field-tested Ethernet solutions currently do not exist for this.
In light of everything presented, PROFINET is equipped for use in process automation applications. The emphasis here is not only on the higher performance demanded by the paper and metal processing industries, but above all on the desire for optimum integration into the Ethernet-dominated corporate world and the Internet. A direct path to the MES and ERP systems and use of Internet services for things like remote maintenance, integration of wireless communication, and intelligent network management are driving forces here.
Integration of fieldbus systems
Alongside the technical advantages that PROFINET offers to users in the process automation sector exists the intrinsic demand to protect current and future investments in fieldbus technology and instrumentation.
To meet this demand, the PROFINET proxy concept has been developed to serve as a fieldbus integration solution. This concept enables integration of the three communication systems encountered in process industries – PROFIBUS PA, HART, and Foundation Fieldbus. These existing systems contain standardized mechanisms for mapping the respective fieldbus-specific properties onto PROFINET. The integration of the bus systems is accomplished by using proxies. As gateways, the purpose of the proxies is to link the higher-level PROFINET network with the particular 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 well as alarms with the field devices.
In process automation applications, the availability of automation systems is of central importance. This applies in particular to the use of continuously running processes in which, for obvious reasons, plant operation must never be interrupted under any circumstances. To avoid automation failures caused by wire breaks, short circuits, and the like in these types of plants, redundancy concepts were developed for PROFINET, which can be structured differently depending on the application (hence the origin of the term "scalable redundancy"). The basis for all of the redundancy 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.
Some applications require 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 synchronization of the components involved: By means of a central system master clock (e.g., based on GPS or DCF77), a master selected specifically for this purpose transmits a cyclic equidistant clock signal to all bus nodes, thereby synchronizing 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 standardized time of day. By acquiring events at a comparable time, an exact description and analysis of a possible fault is possible.
Configuration in Run
Like redundancy, uninterrupted plant operation – including when reconfiguring devices and networks and when inserting, removing, or replacing devices or individual modules – plays an important role. All of these “Configuration in Run” measures (CiR) are carried out in PROFINET without any interruption and without 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.
Thanks to the auto-sense and topology detection features, devices are identified 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.
Diagnostics and maintenance
Fieldbus systems are characterized by comprehensive diagnostic capabilities, which explains why corresponding requirements have been placed on PROFINET as well. These include, for example, requirements for providing and transmitting identification and maintenance (I&M) data, as is familiar from PROFIBUS applications, or for communication of events and transmission of device status, as standardized in NAMUR Recommendation 107. Changes that are made manually on the device itself or by means of external parameterization tools are signalled via PROFINET to the control system as parameter change events. Thus, the control system is able to recognize parameters that differ from those in the centralized data management, inform the user, and perform updates, as required. The important point here is that each change is registered by the control system so that all actions are reliably detected and documented in a central location.
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