Process control gets a makeover with new-generation DCS

Historically, there has been a clear divide between control strategies for process and discrete applications. However, with the recent development of a new generation of DCS systems, the distinctions are now not so clear-cut.

Where distributed control systems (DCS) have traditionally been the control system of choice for process control, and programmable logic controllers (PLCs) the preferred method for achieving high-speed discrete control, there is now a new generation of DCS systems that blur these accepted boundaries. These new architectures can deliver both process and high-speed discrete control functionality via a single platform, and look set to change the way industry addresses business-wide challenges of control and data management.

Process applications are quite distinct from discrete functions on a factory floor. ‘Processes’ typically involve the conversion of materials into different products; and to measure how much or how well a material has changed during the process usually requires measurement of parameters such as temperature, pressure, viscosity, flow or concentration. Discrete applications, by comparison, tend to involve more absolute ‘yes/no’ or ‘on/off’ operations. An example of the difference can be found in the brewing industry. Beer production itself is a process application, while the subsequent filling of the beer bottles is a discrete application.

DCS topologies evolved as a means of bringing together multiple different and distributed process operations into a single control environment. They focus on keeping track of multiple variable inputs and running highly complex control algorithms, while keeping the user interface simple for the process engineer.

However, emerging demands from the manufacturing industry are requiring that standalone discrete control applications are integrated into a plant-wide topology. These requirements have driven the development of single control architectures that include the networking and control of those aspects of a manufacturing plant normally outside process control. Unlike the ‘black box’ nature of the traditional DCS, the key requirements of such ‘integrated architectures’ include the provision of a single common solution to a broad range of users - from operators and plant engineers to the plant manager - and the ability to offer full modularity and scalability to meet the needs of large networked plant-wide systems.

Beyond the traditional DCS

The development of process automation controllers (PACs) has combined the functionality of discrete, motion, drive, safety and process control into a single platform. This means all the control functionality of a DCS can be achieved in conjunction with the associated benefits of multidiscipline control, scalability and modularity - more reminiscent of the open-based systems, used in discrete applications. This new breed of PACs has effectively paved the way towards the new-generation DCS solution.

While more traditional DCS systems have made some use of common off-the-shelf technology, such as PC hardware and operating system software, older systems commonly rely on custom manufactured parts which are not always readily available, and are increasingly becoming obsolete. Moreover, traditional DCS vendors tend to retain their engineering services in house rather than providing training for their customers, or utilising the skills of system integrators. The long-term effect of this is the ongoing requirement for vendor assistance to ensure adequate servicing and maintenance throughout the life of the system, which dramatically increases the total cost of ownership of the technology.

Due to the commercial and technical attributes of the traditional DCS and PLC systems used in the past, many businesses operate two or more separate systems - consisting of both DCS and PLC platforms. However, the installation, operation and maintenance of parallel control systems is generally not cost effective, and can often be restrictive when it comes to plant-wide integration.

Many businesses that operate a DCS system for their process plant and PLCs for discrete operations are beginning to see the opportunity to integrate all control functionality onto a single platform. What they want is the best of both worlds: the user-friendly front-end interface, with out-of-the box functionality for effective and efficient use by their process control engineers; and the flexibility of a modern integrated control architecture that their plant engineers can understand and maintain. Additionally, the merging of stand-alone systems means that plant managers can more easily consolidate and analyse their plant information, thereby facilitating the making of educated decisions to improve their operation. The new ‘DCS experience’ The heart of the new-generation DCS solution is thus the multidisciplinary PAC, which features all the process and discrete control functionality that might be desired. By developing a bundled, preconfigured and tailored solution - comprising an integrated PAC, advanced production and performance software, and sophisticated visualisation/HMI systems - the DCS experience can be achieved, while at the same time maintaining the flexibility, modularity and scalability of a PLC-style architecture.

Figure 1: The heart of the new-generation DCS solution is the multidisciplinary PAC, which features all the process and discrete control functionality that might be desired.

The ability to integrate all plant control and information systems is a key feature of these new-generation DCS solutions. This extends beyond process and discrete control to safety, control and configuration of drives, maintenance and asset management systems, warehousing and financial/business systems, and manufacturing execution systems (MES) - not to mention the flexibility to adapt to changing site and operating conditions.

The benefits that industry will derive from a single control platform with this range of capabilities are huge. It will facilitate a commonality of engineering practices across a site, and can substantially reduce running and maintenance costs. Moreover, a single platform requires a single training package, facilitates the holding of a single set of spares, and provides a single interface with which to further integrate to other systems.

Arguably, the best advantage that a single operating platform will deliver to industry is the greater efficiencies it will leverage - from initial implementation of the system, right through to operations. With all processes networked together, the resulting information-enabled system will facilitate the extraction of real-time information to support decision making and advanced reporting requirements. Increasingly, this is providing a significant improvement in the way business can be conducted. However, for this to be successful, the architecture needs to support a wide range of industry-standard network protocols to ensure information transference is possible across the entire system.

Figure 2: New-generation DCS systems look set to deliver significant benefits to all the traditional DCS process-centric applications, such as the oil/gas sector, mining, chemical, water/wastewater and food/beverage.

Single-platform future

Just as traditional DCS systems merely require configuring on the part of the user - rather than programming - so too are the new breed of DCS systems equipped with embedded libraries of information and commonly used templates to circumvent the need for complex programming and replication of information. These libraries typically include templates for migrating information from legacy DCS platforms, thereby ensuring that systems are easier and quicker to initially convert.

The use of templates also greatly simplifies system reconfiguration arising from changing and expanding system requirements over time. Examples of add-on functionality include batch control, advanced process control and information reporting. The modular design of these new systems allows functions to be added when needed.

Furthermore, single-platform control architectures permit better levels of integrated safety - including both machine and process safety requirements - to offer a broad coverage of safety integrity levels (SIL) and fault tolerance requirements. These systems also often offer the option to integrate third-party safety systems, for the delivery of the highest available safety solutions possible.

There has been a growing positive uptake of these new DCS systems in the last couple of years. In a great many instances, these new systems have been tendered for projects in direct competition with traditional DCS systems, and their expansive capabilities have been found to offer the most comprehensive and future-proof solution for the application. Over time, it is expected that the die-hard DCS aficionados will accept that this new generation of DCSs represents a viable and beneficial alternative to traditional DCS systems.

One of the strengths of legacy DCS has always been the perceived robustness, in terms of the redundancy of networks. With the new systems demonstrating a comparable or superior level of operational stability, new-generation DCS systems look set to deliver significant benefits to all the traditional DCS process-centric applications, such as the oil and gas sector, mining, chemical, water and wastewater, and food and beverage. The extended control capabilities, improved efficiencies and enhanced capacity for information sharing will convey substantial business advantages to these industries into the future.

By Andrew Sia, Rockwell Automation’s Business Development Manager for Process Solutions - South Pacific.