Diagnosing Fieldbus Devices−PROFIBUS PA Paves the Way to Event-Controlled Maintenance

Process engineering production plants are heterogeneous in many respects: complex field devices of very different types alongside conventional motors and simple switches, long bus lengths with numerous system sections distributed across buildings and floors, potentially explosive zones with stringent safety regulations alongside 'normal' zones, e.g. for raw material delivery or product shipment, and a large number of suppliers involved − all that is characteristic of systems in process engineering. No wonder fieldbus technology is still regarded with a certain amount of caution in chemical engineering, as opposed to factory automation. For some users a standardized, completely consistent fieldbus solution is hardly imaginable for such industrial plants.
Roche AG of Sisseln (Switzerland), a production facility belonging to DSM Nutritional Products, has overcome these reservations in building a new Vitamin E production plant and implemented plant automation with PROFIBUS throughout.
The arguments in favor of fieldbus technology were not only the familiar aspects such as simple wiring, greater accuracy, and centralized parameterization but particularly the ability of fieldbus devices to use application-specific configuration of PROFIBUS PA (PROFIBUS for Process Automation) to pass diagnostic information from the fieldbus devices on to the process control system (PCS) independently and cyclically.
The strategy for automating this major project was based on positive experience gained previously with a small pilot plant (wastewater stripper) employing 25 PROFIBUS PA fieldbus devices.
The high expectations of PROFIBUS were fulfilled completely. The following article reports on practical experiences from planning through to operation and also addresses wishes concerning further innovations. The article should and can serve as encouragement to apply PROFIBUS for users with similarly large-scale objectives.

Project

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Production of Vitamin E in Sisseln
Vitamin E is an active ingredient that occur, for example, in vegetable oils, and prevent the oxidation of unsaturated fatty acids in the metabolic cycle. Nowadays Vitamin E is required in large quantities (one focus is on foodstuffs) and thus manufactured on a large scale. At the Sisseln plant of DSM work was commenced in 2001 to build a completely new production system that features a capacity of 25,000 t p.a. and therefore ranks among the relatively large systems of this type. In the light of the high level of productivity required special importance was attached to automation technology.

Plant topology − clearly arranged and universal
The plant to be automated with PROFIBUS occupies several floors of a building and also includes the tank storage depot. The nearly 5,000 I/O points at field level are distributed across 27 plant sections similar to one another, each with a controller (Siemens S7 400) for control purposes. Bus line supplies the simple actuators and frequency inverters while bus line supplies the sensors and closed-loop control valves, which provide extensive diagnosis.
All the 27 controllers are networked with one another via an Industrial Ethernet system bus and connected to the higher-level terminal bus via redundant servers. This terminal bus serves as a data bus for interfacing the operator control stations and the engineering station, and as a communication interface with the Office environment.

The wide range of field devices required becomes evident from the quantity listing of I/O points (Table 1). The aim was to interface and operate this wide range of field devices with the respective controllers using a single universal communication protocol in order to simplify planning, assembly, commissioning, and maintenance. In this respect the unique position of PROFIBUS in the field bus environment, which is still multi-layered despite IEC standardization, proved ideal particularly for process engineering production plants: due to its modular design PROFIBUS can be used in all areas of a plant for a very wide range of tasks, but with complete standardization of communication. This is achieved by the various alternative communication techniques or numerous dedicated application profiles.
These 'application-specific configurations' of PROFIBUS include the following:

• 'PROFIBUS PA' for process automation tasks, with the MBP-IS interface (Manchester coded, bus powered, intrinsically safe). The associated devices are termed PA devices.
• 'PROFIBUS DP' for factory automation tasks, with the RS485 interface. The associated devices are termed DP devices.
• 'HART on PROFIBUS' generally defines the operation of HART devices via Remote I/O systems as communication interfacing with PROFIBUS masters.

If one takes into account the possibility of interfacing motors as well as safety applications on PROFIBUS, which were not implemented in this plant, PROFIBUS currently constitutes the only field bus which, on account of its diversity, can consistently meet all the requirements for a process engi-neering plant.

Plant sections structured simply and unified
For all sections of the production plant a unified topology was implemented for interfacing the field devices. The different 'application-specific configurations' of PROFIBUS for interfacing field devices with the respective controller of the process control system (Siemens, PCS7) are as follows from right to left:

• Interfacing of PROFIBUS PA field devices via PROFIBUS DP/PA couplers, links and field barriers
• Interfacing HART field devices via a PROFIBUS DP Remote I/O system
• Interfacing Open/Close valves via a PROFIBUS DP valve island
• PROFIBUS DP for direct connection of motors to frequency inverters
• Direct interfacing of motors via input/output cards in the PCS

In this topology there are 170 devices or I/O points connected to each controller in each section of the plant, and most of them via the two PROFIBUS DP cards of the controllers for each one. The small amount of wiring from the controller to the field level compared with a conventional point-to-point connection can be seen clearly.
One should also mention the important function of barrier technology for the number of fieldbus devices that can be connected up in a PROFIBUS PA segment with intrinsic safety protection (Ex i). The maximum number of 32 devices per PA segment, which is for signaling reasons in compliance with IEC specifications, cannot be exploited to the full by any means on account of the power supply to the segment, which is limited in the case of Ex i. Help is provided by the technology of PROFIBUS fieldbus barriers, which are supplied at the input end with Ex e protection and thus with a higher level of power but which, on the other hand, allow intrinsically safe, reaction-free, short-circuit proof connection of up to four PROFIBUS PA field devices at the output end in the direction of the field. Consequently, it was possible to connect and operate with intrinsic safety up to 6 of these barriers or 24 fieldbus devices in a segment. At a data transfer rate of 31.25 kbits/s in the PROFIBUS PA seg-ment maximum access cycle times of less than 0.4 seconds were thus guaranteed for each fieldbus device.

The problem of assessing intrinsically safe systems for use in potentially explosive atmospheres has been fundamentally solved with PROFIBUS by means of the FISCO concept. If FISCO-certified fieldbus components are used, EEx i conformity is assured by the type test certificate. However, there remains considerable effort to meet the requirements of ATEX 137 (requirements made of system owners) because the field devices are only certified for operation in the atmospheric environment, which in chemical plants only applies in the aerated state but not to operation.

Diagnosis: powerful but not yet exploited to the full
The possibility of diagnosing fieldbus devices constitutes a considerable additional benefit of fieldbus technology. 'Diagnosis' is the interpretation of symptoms either measured or established in some other manner, derived from the devices or the process, the aim being to recognize and remedy malfunctions and their causes in good time.
The diagnosis of fieldbus devices was regarded as important especially for operation of the Vitamin E plant because, firstly, the production plant is to be essentially operated as a continuous system without any significant downtimes and is to be largely used without any changes over its lifetime.
This is where the diagnosis of fieldbus devices acquires major importance, from optimization of plant operation through to maintenance. Specific objectives on the part of users for exploiting the diagnostic information are, for example:

• Clearly organized state visualization for the operator and machine shop staff
• Logging for statistical fault detection and wear-related maintenance
• Standardized strategy in the PCS in the event of device failure and the need for maintenance

The prerequisites for utilizing the diagnosis of fieldbus systems are:

• Suitable diagnosis functionalities (sensors and computing power) in the field device
• Reliable communication of diagnostic information on the bus
• Standardized, cross-vendor definition (coding) of typical diagnostic contents

PROFIBUS, and the manufacturers supporting PROFIBUS, specified extensive diagnostic mechanisms at an early stage and developed appropriate devices so that powerful diagnosis is always possible. This applies not only to the diagnostic mechanism in the PROFIBUS DP communication protocol but also to the diagnostic contents defined in addition in the application profile of PROFIBUS PA field devices. However, in the latter case the definitions were not always formulated with sufficient conciseness so in some cases different bits are set for identical errors, depending on the manufacturer.
The need to establish strict standardization in this area has now been recognized by users and manufacturers and is currently being dealt with in a large body of representatives from GMA (VDI/VDE measuring and automation association), NAMUR (interest group for process control technology in the chemical and pharmaceutical industries) and PI (PROFIBUS International). The objective of this body is to specify the requirements in terms of self-monitoring and diagnosis in field instrumentation and harmonize further processing in the PCS.
Based on the classification of status messages already defined in 1966 by NAMUR in NE 64, the many different items of diagnostic information from the field devices are to be consolidated in this way to create general centralized alarms. The first results have already been presented at the general meeting of NAMUR in November 2003 and work is due to be completed in the second quarter of 2004.

For the Vitamin E plant the user developed its own I/O driver blocks for interfacing the heterogeneous fieldbus device environment and implementing the NAMUR 64-oriented diagnosis evaluation (Table 2), including the associated visualization at WinCC level. In this way the weak points listed were overcome within a short space of time and other requirements, such as message inundation suppression or the masking out of undesirable diagnostic bits, were integrated.

Device operation: two methods competing with one another
Fieldbus technology unfolds its complete user benefit only with systematic 'open architectures', which is synonymous with using devices from many different manufacturers. In order to control this diversity in installation, parameterization and operation, PROFIBUS has developed standards for centralized, standardized operation of field devices. This has produced two methods (EDD, Electronic Device Description, and FDT/DTM, Field Device Tool/Device Type Manager) which now compete with one another in the market and have their own advantages. It is the user's responsibility to make his own choice here, whereby in the light of the high pace of innovation of both technologies it is only possible to make temporary statements. We have acquired experience of both methods, formulated from our point of view. The operator control interfaces typical of both methods are shown in Table 3 (EDD + DTM).

Result

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Overview of experiences

• PROFIBUS is highly suitable for automating a complex process engineering plant because with a universal communication protocol a very wide range of field devices can be interfaced and operated. This covers practically all device types occurring in process engineering production plants, ranging from the PROFIBUS PA field device and the conventional HART device to motors and simple switches.
• Communication between the PA devices is reliable; despite the large number of devices used from different manufacturers there have been no initial difficulties for this technology. However, the amount of parameterization for all devices is quite substantial; one remedy in this area might be to suppress functions that are not required.
• The capital investment is about the same as with Remote I/O technology but up to 30% less than that for parallel wiring with a terminal board.
• Unexpected aspects were found with the technologies already introduced (HART, PROFIBUS DP). For example, the start-up phase produced technical problems, e.g. firmware incompatibilities and, in the case of HART devices, HART signal disturbance of the actuating signal of control valves. However, these difficulties were identified and remedied within a reasonable space of time with help from the manufacturers and with the aid of diagnostic options already integrated.
• Integration of PROFIBUS slaves from different manufacturers called for in-depth specialized knowledge of the customer in order to identify problems and address them accordingly.
• The complete breakthrough of the multi-vendor approach will only succeed when manufacturers have their products examined and certified by independent bodies, as proposed by the PROFIBUS User Organization and Namur working groups.
• Diagnostic capability can help boost productivity. In this area PROFIBUS already offers a good basis but there is a need for further development toward simplification and standardization in the coordinating control systems. Improvements are expected soon.
• In the competition between operator control methods and engineering methods we have positive experience of both methods (EDD and FDT/DTM), with certain limitations. In the long term the DTM/FDT concept has the potential to become an open industrial standard that is being requested by users. However, here too an accompanying test and certification procedure (DTM inspector) is without doubt absolutely essential in order to reliably rule out the possibility of malfunctions.

Positive experiences have paved the way toward equipping new future plants with PROFIBUS-PA throughout and omitting the intermediate step of Remote-I/O. The experience is largely consistent with the FuRIOS study presented in 2002.