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An alternative to DCS or PLC/SCADA?

06 September 2011

Traditionally, users have had a choice between a DCS or a PLC/SCADA approach when selecting a control system for use in process control applications. Richard Sturt, Rockwell Automation business development manager, process Industries, believes that Rockwell can offer a solution for users needing elements of both of these approaches.

A key benefit of traditional DCSs was that the suppliers took a ‘systems approach’ and it was designed for large scale applications. Much of the work of integrating different parts of the system was taken care of by the DCS vendor by using a single database for the complete system.

The DCS approach is to configure the system using standard control objects and faceplates, reducing engineering and providing standardisation. Most DCSs also offer fieldbus communications, tools for process optimisation and asset management packages to improve maintenance. More recently DCS suppliers have introduced more scalable solutions for smaller applications

However, there are a number of potential shortcomings in the DCS approach. Many process plants have a large number of OEM packages that need to be integrated with the overall process control system and it is unlikely that the OEM will use a DCS for their control system. In some manufacturing processes there is a requirement for different types of control; process, discrete, high speed, drive control, motion control or machinery safety. The DCS is not designed to cope with these types of applications and, if a manufacturing process covers multiple disciplines, the DCS will only ever be suitable for part of the system.

DCS has, traditionally also been an expensive product to support throughout its life. Annual licensing and support costs can become a significant part of the overall cost of ownership. The majority of DCSs are engineered and supported by the original manufacturer and are not freely available though other channels, leaving the end user reliant on its DCS vendor.

Most DCSs had closed and proprietary system architectures. More recently suppliers have adopted more open standards but, in many cases, this approach has limitations which often makes it complicated to integrate with MES and other business systems.

The PLC/SCADA approach
Although PLCs were originally developed for discrete applications, most cover a wide range of applications. The term Programmable Automation Controller (PAC) is now used to describe many products to distinguish them from older systems that were focused on discrete logic. This flexible approach offers multiple programming languages, open communications, and scalability for different applications. There are many systems integrators able to integrate PLC/PAC technology, often with specific industry or application knowledge. This approach is also ideal for OEMs who can select a cost effective product to meet their requirements.

Although PLCs and PACs have become more powerful they still have a number of features that are missing when compared to the DCS approach.

A typical PLC/SCADA system has at least two databases. The user has to configure the PLC database and then separately configure the database in the SCADA system. At initial engineering stages there may be an option to import the PLC database into the SCADA system but this will require manual modification as changes are made during commissioning and when enhancements are made. If additional software packages are required they are likely to have their own separate databases too, that will need to be configured.

Another potential weakness is that not all PLCs have features required for demanding continuous process control applications. In some cases they will lack the ability to modify the system on-line or will not include high availability features like redundancy from I/O modules to supervisory systems.

Process optimisation and asset management is another area where traditional PLCs may be lacking compared to a distributed control system.

The best of both worlds
A true ‘Plant-wide Automation’ approach takes the best features from DCS and PLC/SCADA systems and combines them into a single system. It combines the ‘systems approach’ of a DCS and flexibility of a PLC/PAC to provide a solution for multiple applications.

This approach has benefits that can improve plant performance and reduce costs. A single architecture can be selected for any application whether it is continuous process control, high-speed OEM equipment or a small batch reactor. It simplifies integration and reduces engineering costs. The resulting system will have a seamless communications architecture allowing access to plant data for process optimisation, asset management and integration with MES. Support costs will also be minimised by reducing training requirements and the cost of holding spares.

Rockwell Automation has recognised the benefits of combining the features of Distributed Control Systems and PLC/SCADA solutions into a single Plantwide Automation system with its PlantPAx. As part of the companies Integrated Architecture, it allows a single architecture to be used for a wide range of applications, including process, batch, drives, motion and safety.

The importance of being able to address such a wide range of applications is key to a truly plant-wide automation system. Operating any combination of these applications through a purpose-designed single architecture and software environment makes it possible to achieve the benefits of both a DCS and a PLC/SCADA system without many of the traditional downfalls of either. It is this seamless inter-operability that makes plant-wide automation systems so effective.

At the core of the PlantPAx is the single database that is used for the Controller, HMI, Historian, Batch Management, Asset Management and Business Intelligence Tool/Reporting Package. Any data created in the system is instantly accessible to all the software packages that need it and a single security model simplifies the management of user’s profiles.

The key principles behind this solution can help to drive a lower cost of ownership. Using a single architecture reduces initial engineering and integration costs. The process optimisation and business intelligence tools help to maximise plant efficiency.

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