The new fieldbus-based control strategy and network architecture are set to transform existing instrumentation and control systems. By leveraging the core features of fieldbus technology, this paper analyzes its impact on traditional Distributed Control Systems (DCS). Combining the structural characteristics of DCS, three methods for integrating fieldbus into DCS are proposed.
Industrial control has evolved from local and centralized control to modern distributed control systems (DCS) over the past few decades. In the last 20 years, the process industry has heavily invested in DCS systems and related instrumentation, with widespread adoption and user approval. The 4-20 mA signal has been a key feature enabling communication between DCS systems and field devices, marking a significant advancement in control system development.
However, digitalization and networking have now become the main direction for modern control networks. It is widely recognized that traditional analog signals can only provide basic measurement and control data, while smart transmitters face limitations in transmitting additional information over the 4-20 mA signal due to its low communication speed. As a result, the digitalization and networking of process control systems have become an inevitable trend.
Fieldbus provides a fully digital, two-way, multi-node communication link between intelligent field devices and automation systems. Its introduction has accelerated the digitization and networking of field devices, making field control more powerful. This advancement brings openness to process control systems, transforming them into comprehensive control networks that include measurement, control, execution, and process diagnostics.
**1. Impact of Fieldbus on Traditional DCS**
The influence of fieldbus on traditional DCS stems from its inherent technical advantages over conventional DCS systems. According to IEC and Fieldbus Foundation standards, fieldbus technology offers five main features:
- Digital signals completely replace the 4-20 mA analog signals.
- Basic process control, alarms, and calculations are fully distributed at the field level.
- Additional non-control information such as self-diagnosis, configuration, and compensation is included.
- Site management and control are unified.
- System openness and interoperability are achieved.
Fieldbus is not just a communication technology; it integrates intelligent instrumentation, computer networks, and open system interconnection (OSI). These features make fieldbus control systems (FCS) superior to traditional DCS systems in several aspects:
- Simplified system structure and reduced costs.
- Enhanced on-site device autonomy and improved system performance.
- Increased reliability and accuracy of signal transmission.
- Fully distributed, digital control network.
- Users retain full integration rights.
These benefits are evident when comparing the network structures of DCS and fieldbus systems, as illustrated in Figure 1.
**2. Fieldbus Integration in DCS: A Growing Trend**
Although users appreciate the improvements in control systems, they prefer not to overhaul their existing infrastructure. During the early stages of fieldbus development, most users favored gradual upgrades rather than replacing their entire DCS systems. This approach is both cost-effective and practical. Therefore, the most feasible solution today is to integrate fieldbus with existing DCS systems, allowing for flexible configurations across various 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. The I/O layer includes controllers and I/O cards responsible for connecting field devices. To integrate fieldbus, a fieldbus interface card is connected to the DCS I/O bus, mapping fieldbus data to corresponding DCS data such as measurements, alarms, and settings. This allows fieldbus devices to appear as if they are standard DCS modules.
This approach is suitable for small-scale applications or when introducing fieldbus into an established DCS system. It also works with PLC systems. While the structure is simple, the integration scale is limited by the number of interface cards available.
For example, the DeltaV system from Fisher-Rosemount uses this method. It includes a fieldbus H1 communication module (31.25 kbit/s) in its I/O cards, enabling seamless integration of fieldbus technology. This reduces installation, operation, and maintenance costs, and the same controller can support 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**
Beyond the I/O bus, fieldbus can be integrated at the network layer of the DCS system. In this setup, the fieldbus interface card is connected to the DCS’s upper LAN, allowing operators to view and modify fieldbus control and measurement data from the DCS operator station.
This integration enables decentralized control functions previously handled by the DCS host computer, which can now be executed by field devices. It also minimizes changes to the original DCS system, reducing disruption.
For instance, Smar's 302 series fieldbus products use this integration method, as shown in Figure 6. This approach enhances flexibility and scalability, supporting complex industrial environments.
**5. Fieldbus Integration via Gateway**
When DCS and fieldbus systems operate side by side in a factory, a gateway can connect them, enabling data exchange between the DCS and high-speed fieldbus networks. This setup allows the DCS system to access and display fieldbus data on a new operator interface. Multiple low-speed H1 buses can be managed using a bridge, while the fieldbus interface unit handles control coordination, alarm management, and trend collection.
This parallel integration supports the full integration of the plant’s control and information systems, enabling connectivity through web servers. It enriches the network with valuable data and control information, and ensures that the DCS and fieldbus systems remain independent yet interoperable.
**6. Conclusion**
In summary, fieldbus technology is expected to play a major role in the process industry. With necessary modifications, it can be integrated into DCS systems, offering numerous benefits to users. Although fieldbus will handle many continuous control loops, DCS will still be essential in real-time and complex control scenarios. The coexistence of both systems offers users greater flexibility, leading to more efficient and effective control solutions.
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