The fieldbus is the enabling technology of a new generation of process plants. It not only facilitates 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 a part of everyday life while downtimes are limited to an absolute minimum.
Andreas Hennecke
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 environmental extremes and aggressive process media. In addition, manual intervention in the system, such as maintenance work or system modifications, can result in changes that affect the dependable operation of the fieldbus physical layer.
The new generation of monitoring systems, like the FieldConnex Advanced Diagnostics Module from Pepperl+Fuchs, continuously monitors the physical fieldbus layer and reassures the operator that the fieldbus infrastructure is operating under the best possible conditions. These systems detect both sudden and creeping deteriorations in signal quality long before plant operation is threatened. This allows preventive measures without any time pres-sure and is the key to proactive and scheduled maintenance work.
Remote diagnostics without on-site troubleshooting
The Advanced Diagnostics Module (ADM) for monitoring the physical layer is basically 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 commissioning of a new or modified system, the operating condition is stored as a baseline along with the respective alarm limits. From then on, the module works fully automatically and is able to determine and document the compliance of the complete system with the design rules of the IEC 61158-2 fieldbus standard. Au-dible and visual alarms make sure the system operator or maintenance technician is aware of any deviations from the baseline that could lead to a faulty condition or even total failure of the fieldbus segment concerned.
Complex wiring diagrams can be replaced with a simple spreadsheet documenting all components belonging to the physical layer. In this way, the 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 is forced to go on site to open junction boxes, track down terminals and connect test or diagnosis equipment. It is also no longer necessary to disturb the control room cabinet wiring. As a result, the potential for errors is minimised and the time required for troubleshooting dramatically reduced.
Instead, the service technician simply has to run a software package directly on the maintenance station. The Pepperl+Fuchs Diagnostic Manager provides detailed information on all physical layer measurements. Clear text messages point to pos-sible problems and suggest remedies. 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 deal with the problem during the next scheduled process shut-down or integrate it into a standard maintenance plan. Since potential problems are known long before they impact plant operation, unplanned shut-downs are effectively avoided.
After maintenance work, alterations or extensions to 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 handles 90 % of all alarms due to problems with the fieldbus physical layer before they have a chance to do any damage. Since the complete fieldbus structure is under constant supervision, plant performance and system availability are significantly improved.
Tracking down the difficult cases
A single deficiency in the fieldbus infrastructure does not normally suffice to impair plant operation. Real problems tend to be intermittent and are therefore difficult to detect. Alternatively, they may be caused by multiple faults on a single segment. In such cases the Advanced Diagnostic Manager also allows remote online access. Maintenance technicians can therefore be supported by off-site fieldbus experts, who are able to access all information instantly and provide valuable assistance over the phone. Another option is to enter all status data in a file and mail it to the specialist. In either instance, help is available quickly and at reduced cost, since travelling time and expenses are eliminated.
A special history log shows the long-term degradation of the physical layer. Up to two years of historical data are stored by the module in a non-volatile memory. Information covering an even longer period can be archived on the maintenance station. Trend charts can be created there by maintenance staff and statistical analyses undertaken in spreadsheets.
Seeing the actual waveform of the data signal transmitted is sometimes useful for identifying the cause of a problem. The built-in oscilloscope features many fieldbus-related trigger events – such as telegrams sent to or from a particular address or the issue or loss of a specific telegram type. Fieldbus experts can use the oscilloscope to pinpoint problems in a few simple steps. This not only speeds up repair work considerably, it also allows deteriorating conditions to be detected before the plant is shut down and the availability of the process is affected.
Safety as an inherent feature
The fieldbus also allows safety equipment up to SIL 3 to be integrated. Standard and safety equipment can thus share the same physical layer for communication, but the fieldbus infrastructure has no influence on evaluations of safety 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. Redundant electronic components such as the power supplies and the use of Advanced Diagnostics for the fieldbus physical layer ensure maximum availability for the fieldbus infrastructure.
With all these advantages, retrofitting an existing fieldbus infrastructure with an Advanced Diagnostics system generates cost benefits almost from the start. The capital investment required to service 100 fieldbus segments supporting 1200 field devices is approximately equivalent to the cost of one skilled instrument & electrical engineer for just three months. Put another way, this moderate outlay leads to a huge potential saving in terms of operating costs that can be immediately realised in the complex environment of a modern process plant.
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