Total insight for total control

Pepperl+Fuchs

New technologies for fully transparent fieldbus installations

The fieldbus is the enabling technology of a new generation of process plants. It not only allows the integration of all plant instrumentation, control loops and I/O devices. Due to new solutions like Advanced Diagnostics, it also offers total control of the physical fieldbus layer in combination with remote access to every detail of each instrument connected. As a result, the complete process control system becomes fully transparent to the operator, asset management and proactive maintenance are finally part of everyday life while down times are limited to an absolute minimum. It is therefore not surprising that even conservative plant operators consider switching to a digital fieldbus infrastructure in order to reduce costs while achieving a completely new range of advantages which were simply out of reach with conventional 4…20 mA technology.

With up to 31 field instruments attached to one and the same cable, modern fieldbus technology significantly reduces the amount of wiring needed for power supply and signal transmission. More importantly, it supports the demands of plant operators, who require utmost availability from such a crucial infrastructure that has a strong influence on uptime and safe operation of the plant.

Both fieldbus devices and the fieldbus wiring itself function at the heart of the process under demanding conditions. In many cases they are exposed to extreme environmental conditions and aggressive process media. In addition, manual interference with the system, such as maintenance work or system modifications can result in changes that effect the dependable operation of the fieldbus physical layer.

A new generation of monitoring systems, such as the FieldConnex® Advanced Diagnostics from Pepperl+Fuchs continuously monitor the fieldbus physical layer and reassure the operator that the fieldbus infrastructure operates under best possible conditions. These systems detect both sudden and creeping deteriorations of the signal quality long before the plant operation is at risk. This allows preventive measures without any time pressure and is the key to proactive and scheduled maintenance work.

Remote diagnosis without on-site troubleshooting

Basically, the Advanced Diagnostics Module (ADM) for monitoring the physical layer is a test instrument which is fully integrated into the fieldbus power supply and connected directly to the trunk of the network. After a first run immediately following the commission of a new or modified system, the operating condition is stored as a baseline along with respective alarm limits. From then on it works fully automatic and is able to determine and document the compliance of the complete system with the design rules of the fieldbus standard IEC 61158-2. Audible and visual alarms will make the system operator or maintenance staff aware of any deviations from the baseline that may lead to a faulty condition or even total failure of the respective fieldbus segment.

Complex wiring diagrams can be replaced with a simple spreadsheet documenting all components the physical layer is comprised of. In this way, numbers and locations of the wiring blocks and field devices connected to each trunk can be easily found. There is no need to refer to a wiring diagram for monitoring, troubleshooting, testing, or validation of the system. No service technician needs to go on site to open junction boxes, track down terminals and connect any test or diagnosis equipment. There is also no need to disturb control room cabinet wring. As a result, the potential to introduce errors are minimized and the time needed for troubleshooting is dramatically reduced.

Instead, the service technician only needs to run a software package right at the maintenance station. Pepperl+Fuchs calls it the Diagnostic Manager which provides detailed information on all physical layer measurements. Clear text messages point to possible problems und suggest remedies for the fault. It is then up to the maintenance staff to decide what kind of action needs to be taken and plan their activities accordingly. In most cases, it will be sufficient to take care of the problem during the next scheduled process shutdown or integrate it into a standard maintenance schedule. Since potential problems are known long before they effect plant operation, unplanned shutdowns are avoided effectively.

After maintenance work, alterations or extensions of the fieldbus infrastructure, a quick check with the Diagnostic Manager is all that is needed to verify the quality of the physical layer.
Advanced Diagnostics is able to sort out 90% of all alarms caused by problems with the fieldbus physical layer before causing harm. Since the complete fieldbus structure is under constant supervision, plant performance and system availability will be improved drastically.

Tracking down the hard cases

A single deficiency in the fieldbus infrastructure normally does not really affect plant operation. Real problem cases usually are often intermittent and therefore difficult to detect. Or they are caused by multiple faults on one segment. In such cases it is helpful that the Advanced Diagnostic Manager also allows remote access via the Internet. Maintenance technicians can therefore call on offsite fieldbus experts who are able to immediately access all information and give supportive instructions over the phone. Alternatively, it is also possible to write all status data into a file and mail it to the expert. In either case, help is available within a short time frame and at reduced cost, since time and expenses for traveling are eliminated.

A special history record is available to discover long-term degradation of the physical layer. Up to two years of history data are buffered by the module in a non-volatile memory. Even more historic data can be stored at the maintenance station. Maintenance staff is able to create charts showing trends or perform statistical analysis in the form of spreadsheets.

Sometimes seeing the actual waveform of the data signal transmitted is helpful to discover the cause of a problem.  The built-in oscilloscope features many fieldbus related trigger events – such as telegrams sent to or from a specific address or a specific telegram type being issued or lost. With the oscilloscope, the fieldbus expert can pinpoint problems and find them easily. This not only significantly speeds up repair work. It also allows detecting deteriorating conditions before a plant shut down can occur and the availability of the process is affected.

Safety as an inherent feature

The fieldbus also allows the integration of safety-related equipment up to SIL 3. In this way, standard and safety equipment can share the very same physical layer for communication, while the fieldbus infrastructure is left out of the evaluation of safety related equipment. This is possible because the safety protocols include additional checks and balances to ensure the reliability of the data exchanged between the PLC and the field instrument. Additional availability of the fieldbus infrastructure is ensured through redundant electronic components such as the power supplies and the use of Advanced Diagnostics for the fieldbus physical layer.

With all these advantages, installing an Advanced Diagnostics system for an existing fieldbus infrastructure is a cost benefit almost right from the start. The capital investment required to service one hundred fieldbus segments supporting 1,200 field devices, approximately equals the cost of one skilled instrument & electrical engineer for only three months. In other words: A small investment leads to a huge potential in operating cost savings that can be immediately realized during operation in the complex world of a modern process plant.

Dipl.-Ing./MBA Andreas Hennecke
Product Marketing Manager Fieldbus Technology
Division Process Automation