INTELLIGENT FIELDBUS POSITIONER EVOLVED TO MAINTAIN OPTIMUM PERFORMANCE

Authors

Ismo Niemelä, Oliver Jenkins, Ella Suojalehto and Jari Vehmas. Presented by Marko Ankerman

Company and address

Metso Automation, Field Systems Division, Tulppatie 7 B, PO Box 310, FIN-00811 Helsinki, Finland

email

ismo.niemela@metso.com

Keywords

fieldbus, smart devices, digital, diagnostics, control, maintenance

ABSTRACT

Fieldbus-based solutions constitute one of the most rapidly growing market segments in the area of process control and automation. As standards and specifications have been stabilized with general agreement across industry, suppliers have been able to develop a wider choice of products and solutions.

Customer benefits are self-evident when expressed in terms of savings achieved through reductions in cabling and I/Os, engineering, cabinet space and commissioning.  However, in the longer term, additional savings and benefits can result from the use of intelligent field devices, e.g. control valves which can create savings through the improved production efficiency that results from the flexible implementation of control strategy and from faster response. Fast, accurate control performance reduces process variability.

Smart field devices can also provide on-line diagnostics, which generate life cycle cost savings in terms of focused maintenance planning, rapid trouble-shooting of root causes and reductions in shutdown spares usage. Information exchange with field devices is faster and more reliable than ever before.

Fieldbus components and solutions developed by Metso Automation are based on open technologies, which offer flexibility for the end user. Numerous fieldbus deliveries have indicated that openness is the main contributor when quick and trouble-free start-up of automation projects is the target.

DEFINING A FIELDBUS

A fieldbus is a digital two-way multi-drop communication link or local area network (LAN) for field devices. Traditionally, control signals, such as process variables and set points, have been transmitted by varying the current from 4–20mA to correspond to the range of the device. In a fieldbus, this control signal is transmitted digitally. The use of digital transmission also allows for the transmission of additional status and diagnostics information.

The fieldbus (in one form or another) can be found throughout the whole plant structure. Whatever its form, which may range from Ethernet, through industrial LAN to control networks and discrete devices, the fieldbus brings a broad range of generic advantages.  It can also bring significant benefits to the process control of manufacturing systems.

MANY DIFFERENT FORMS

Our company supports both of the major process control fieldbuses, Profibus and Foundation Fieldbus, throughout its range of field devices and control systems. Alongside this, we also strongly support sensor device networks, AS-interface (ASi), DeviceNet, and Modbus.

Making a decision about the suitability of particular devices for specific applications is a complex one. Basically, Profibus evolved from PLC architectures and has advantages in manufacturing-system integration, installed base, component availability and relative cost. The strengths of Foundation Fieldbus lie in the areas of advanced control strategies, simplified interfaces and the potential for greater data flow when taking advantage of fieldbus-based control.

To date, the focus on the fieldbus has highlighted the savings in initial capital expenditure -- on wiring, I/Os, system hardware and system engineering. While these benefits are very important, it is also necessary to focus on the entire life-cycle of the plant, including the capability of fieldbus technology to improve production efficiency and reduce running costs.

GENERIC BENEFITS

Whether the protocol is Foundation Fieldbus or Profibus PA, the fieldbus offers significant benefits over traditional 4–20mA systems. The lower initial expenditure, achieved through reductions in cabling and I/Os, also impacts on engineering, cabinet space and commissioning. Furthermore, production efficiency is achieved through flexible control-strategy implementation and faster response, thanks to the elimination of A/D converters on the control side. Fast and accurate control performance reduces process variability.

LIFE CYCLE COST SAVINGS

On-line diagnostics enable focused maintenance planning, rapid root-cause trouble-shooting and a reduction in shutdown spares usage. Information exchange with the field device is faster and more reliable than before. In addition, the increased flow of information directly to advanced control systems has a significant impact on operational expenditure.

PERSONNEL SAFETY

Because the collection and storage of diagnostic data takes place within the device, there is a marked reduction in the number of trips into the field by maintenance personnel. This is a very important factor in keeping safety integrity levels (SILs) to a minimum.

EASY SET-UP AND COMMISSIONING

With a single segment running out to several devices, the need for cable checks is significantly reduced. Building or modifying the control strategy is considerably simplified by the introduction of concepts such as standard function blocks. Some device manufacturers have ensured that local keypad calibration, popular in HART devices, is also available for the fieldbus. Whilst supporting generic configuration software from the Foundation Fieldbus or Profibus, our company is committed to the only open-protocol multi-vendor device management software based on the Field Device Tool (FDT) specification.

FIELDBUS-BASED CONSISTENCY TRANSMITTERS

Easy access for calibrating or reading diagnostics data is essential to most analytical transmitters in the pulp and paper industry. Grade changes or unexpected process disturbances can often cause headaches for operators. For example, consistency transmitters on multi-grade machines may require several recipes to be configured in the transmitter memory in order to achieve successful consistency control.

Field-bus-based solutions can provide calibration windows on the operator's monitor which enable both recipe changes and diagnostics-reading simply by clicking the mouse. This is a real benefit when real-time data is needed for decision making.

Our company has now launched the first Profibus-based consistency transmitter, Smart-Pulp PA, on the market. A Foundation Fieldbus version of the transmitter will be launched shortly. Those who are already familiar with the analogue version of Smart-Pulp will not notice any difference in transmitter operation. The functionality remains the same, but users acquire data much more easily than before.

FIELDBUS-BASED VALVE POSITIONING

The ND800 Intelligent Fieldbus Positioner has evolved from our high-integrity positioner platform. To ensure the optimum solution for a wide range of specific applications, the new positioner has been developed for both Foundation Fieldbus H1 and Profibus PA. On-line valve diagnostics are continuously stored throughout the life of the device.

The ND800 Intelligent Fieldbus Positioner utilizes sophisticated software to continuously receive accurate data related to the performance and condition of the field device. Essential on-line valve diagnostics maximise run-time performance, and can reduce valve maintenance expenditure by up to 50%.

The performance trends of the valve and its impending need for servicing are closely monitored and maintenance can be planned for when it is truly necessary.

Unlike some other suppliers our Foundation Fieldbus device includes both AO and PID function blocks as standard. The AO control block application includes scaling and reversing options, while the PID control block application has advanced features including set-point ramp speed plus a derivation filter and selector. The closed-loop response speed is easily adjustable and multiple tuning options are provided. The device can be used either as a basic device or as a link master, allowing control in the field.

A NEW LEVEL OF PROCESS-PLANT EFFICIENCY

The ND800 Intelligent Fieldbus Positioner is just one element in an evolving dimension of process-care solutions will bring unprecedented process stability and efficiency.  The development of open-architecture device-management software will provide the ability to integrate all products, regardless of the protocol being used. By applying these latest technological developments to well-established, reliable products, the efficiency of process plants will be raised to a new level.

FIELDBUS IS A BUILDING BLOCK FOR FUTURE CARE

Fieldbus technology also plays an important role in Metso's Future Care concept of a long-term partnership with the customer to provide technology know-how and resources, which complement the plant's existing capabilities throughout the life cycle of the process and equipment. The integration of plant assets and production information, which the fieldbus can ensure, is a key element in strengthening plant competitiveness and profitability.

While the purchase of an asset or technology can be relatively simple, obtaining the best performance from it is more difficult. The ultimate objective is to provide tools and solutions which are designed to build long-term knowledge from the information provided by fieldbus devices. This knowledge can be used to improve process performance, drive maintenance costs down and minimise shutdown time.

Figure 1

Figure 1 Real-time diagnostics information from fieldbus-based devices substantially benefits process management

Figure 2

Figure 2 Fast and accurate control performance cuts down process variability

Figure 3

Figure 3 To ensure the optimum solution for specific applications, Metso Automation has developed the ND800 for both Foundation Fieldbus H1 and Profibus PA

SUMMARY OF THE BENEFITS

In the fieldbus projects much attention is paid to the capital savings; across a range of installations this has been in the region of 40% compared to traditional I/Os. This is not due to lower cost of individual components (which are generally 10-25% higher), but to a reduction of total engineering cost. A fieldbus segment can contain up to 32 devices, usually less in typical installations. The result is a reduction in I/O leading to simplified and reduced wiring which impacts on the complexity of drawings, easier modification to change requests, lower total costs for wiring, marshalling, and junction boxes. With correct training, the reduction in I/O's, terminations and support for reverse polarity installation cuts down on the commissioning phase.

Long term benefits in production efficiency can be achieved through:

1) Standardised control functions:

Flexible control strategy implementation and potentially more accurate response (elimination of A/D converters on the control side). Fast and accurate control performance, and strategy optimisation which reduces process variability.

2) Expanded view for the operator:

From the field through to Enterprise planning systems. This does not come only as a result of fieldbus, although the design lends itself to it. A loop retune can be carried out due to a sticking control valve or erroneous transmitter reading. The cost impact of loop de-tune can often be missed but in fact it often masks the root cause of the problem, which may deteriorate further and possibly fail catastrophically.

3) Information Flow:

Efficient information flow from field devices enables the operator to make effective decisions. For example a traditional transmitter with a transducer fault may continue to give a false signal, and the control output is modified in response, resulting in waste, recycle costs or additional chemical requirements. With fieldbus every control cycle (usually a couple of hundred milliseconds) has a status message enclosed, which indicates the validity of the signal.  If it is not good then a selector can switch to using the signal from a back up device.

Additional benefits can be achieved through life cycle cost savings:

1) Proactive planning during run time:

Although at 31.25 kbits/sec the control segment is still relatively slow, but if it is interfaced correctly into the higher speed non-powered bus, there will be a consistent flow of diagnostic data. This helps devices with on-line diagnostics to provide focused maintenance planning, rapid root cause troubleshooting and reduction in shutdown spares usage. Information exchange with the field device is faster and more reliable than possible before. An increased flow of information directly to advanced control systems has a strong impact on operational expenditure.

2) Selective instrument and valve replacement:

In a typical outage of the 100% of devices pulled out for overhaul usually approaching 50% would be unnecessary and thus incur unnecessary cost.  Although this type of planning was available with HART based instrument, I believe that the constant data flow from fieldbus devices makes this approach a more practical reality.

CONCLUSION

With an annual growth rate of more than 70% fieldbus solutions are one of the most rapidly growing market segments in the process control area. Fieldbus technology is now seen as the key to improving plant operations from the process control point of view. It can bring savings in capital investments and in maintenance activities. However the opportunity to increase income is what every user seeks and this is where fieldbus solutions can show clear benefits. Continuous information flow about device operation and diagnostics increases the runnability of many processes and device self diagnostics eliminates unexpected failures and shutdowns. The majority of users are looking for flexible control strategies where loop controllers are implemented at field devices. They allow faster and more accurate control resulting in lower variability and better quality.

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