The new fieldbus-based control strategy and network architecture are set to transform existing instrumentation and control systems. By leveraging the core characteristics of fieldbus technology, this article explores its impact on traditional Distributed Control Systems (DCS). It also outlines three practical approaches for integrating fieldbus into DCS, taking into account the network structure of DCS.
Industrial control has evolved from local and centralized control to today’s distributed control systems (DCS). Over the past two decades, the process industry has heavily invested in DCS systems and related instrumentation, with their application being widely accepted by users. The 4-20 mA signal is a fundamental feature of DCS system integration with field devices, marking a significant advancement in control systems and instrumentation.
However, digitalization and networking have now become the main direction for modern control networks. While traditional analog signals provide basic measurement and control data, they lack the communication capacity to support smart transmitters’ ability to transmit additional information. As a result, the move toward digitalization and networking in process control systems is an inevitable trend.
Fieldbus offers a fully digital, bidirectional, multi-node communication link between intelligent field devices and automation systems. Its introduction has enabled the digitization and networking of field devices, enhancing field control capabilities. This innovation brings openness to process control systems, transforming them into comprehensive control networks that include measurement, control, execution, and process diagnostics.
**1. Fieldbus Impact on Traditional DCS**
The influence of fieldbus on traditional DCS stems from its inherently superior technical features. According to IEC and Fieldbus Foundation FF, fieldbus technology has five key characteristics:
- Digital signals completely replace the 4-20 mA analog signal.
- Basic process control, alarms, and calculations can be fully distributed at the site.
- Additional non-control information such as self-diagnosis, configuration, and compensation data can be transmitted.
- Site management and control can be unified.
- System openness and interoperability are achieved.
Fieldbus is not just a communication technology; it integrates elements of intelligent instrumentation, computer networks, and open system interconnection (OSI). These features make fieldbus control systems (FCS) significantly better than traditional DCS systems:
- Simplified system structure with reduced costs.
- Enhanced on-site device autonomy and improved overall performance.
- Higher reliability and accuracy in signal transmission.
- Fully distributed, digital control networks.
- Users retain full system integration rights.
These advantages are evident when comparing the network structures of DCS and fieldbus systems, as shown in Figure 1.
**2. Fieldbus Integration in DCS: A Growing Trend**
Although users appreciate structural improvements, they prefer not to overhaul existing instrumentation systems. During the early development of fieldbus, most users opted for gradual upgrades rather than replacing entire systems. Replacing DCS systems entirely would be costly and impractical. Therefore, the most feasible solution is to integrate fieldbus with existing DCS systems, enabling flexible configurations for broader applications.
**3. Fieldbus Integration on the DCS I/O Bus**
In a typical DCS system, the architecture is divided into three layers: management, monitoring, and I/O. On the I/O bus, there are DCS controllers and various I/O cards that connect to live 4-20 mA devices, discrete signals, or PICs. The controller handles on-site control.
Figure 2 illustrates how fieldbus can be integrated onto the DCS I/O bus. This involves using a fieldbus interface card to map data from the fieldbus system to the original DCS I/O bus. This allows fieldbus devices to appear as if they are traditional DCS modules, enabling seamless integration.
This approach is suitable for small-scale applications where DCS systems are already stable, and fieldbus is introduced gradually. It can also be applied to PLC systems. The advantage is a simple structure, while the limitation is the size of integration restricted by the interface card.
Fisher-Rosemount’s DeltaV system uses this method. It includes a fieldbus H1 communication module in its I/O cards, allowing integration with fieldbus instruments and reducing installation, operation, and maintenance costs. The same controller supports both H1 and traditional I/O modules, facilitating a smooth transition from traditional to fieldbus-based control.
**4. Fieldbus Integration in the DCS Network Layer**
Another way to integrate fieldbus is at the network layer of the DCS system. In this case, the fieldbus interface card is connected to the upper LAN of the DCS, rather than the I/O bus. This setup allows fieldbus control, measurements, and field instrument functions to be viewed and modified at the DCS operator station.
This approach enables some control and calculation functions previously handled by the DCS host to be decentralized to field instruments, improving efficiency. It also minimizes changes to the original DCS system.
Smar’s 302 series fieldbus products use this integration method, as shown in Figure 6.
**5. Fieldbus Integration via Gateway**
When DCS and fieldbus systems operate side-by-side in a plant, a gateway can be used to connect them, as shown in Figure 7. This allows information exchange between the DCS and the high-speed fieldbus network. The gateway facilitates data transfer, enabling the DCS system to display fieldbus information on a new operator interface.
This parallel integration enhances the overall control and information system of the plant, supporting connectivity with Intranet and Internet through a Web server. The benefit is richer network information and better data and control integration. Additionally, the fieldbus and DCS systems remain independent when integrated via a gateway.
**6. Conclusion**
In conclusion, we believe fieldbus technology will be widely adopted in process industry control. With necessary modifications, fieldbus can be integrated into DCS, offering numerous benefits to users. While fieldbus may handle many continuous control loops, DCS will still play a vital role in real-time applications. The coexistence of fieldbus and DCS at this stage provides users with more options, leading to a more efficient and balanced control system.
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