Dec. 06, 2023
Automobiles & Motorcycles
Many factors come into play when selecting a new or upgraded control system, with careful consideration of each resulting in the best outcome.
Please note that this article was migrated from a previous system, and not all images could be transferred.
Two main types of platforms form the foundation of modern process automation: the distributed control system (DCS) and the programmable logic controller (PLC). Today’s decision process for a new or upgraded control system, however, requires more than simply choosing between the two. Contemporary control systems could also include other technologies, such as the safety instrumented system (SIS) and the supervisory control and data acquisition system (SCADA)—as well as Industrial Internet of Things (IIoT) functionality and other connectivity to enterprise business systems.
Taking these other technologies and required functions into account, along with additional factors as detailed below, will result in an improved decision-making process, leading to a successful implementation over the multi-decade lifecycle expected from modern control systems.
The business case for modernizing a control system usually boils down to a timing issue. Modernizing too soon reduces the maximum value of the system over time, while modernizing too late can lead to excessive operational expenses and downtime.
Arriving at the decision point usually requires extensive research and analysis. The cost of migration to a new system must be weighed against the operational costs of shutdowns, equipment failures and plantwide performance degradation. Modeling these costs can determine appropriate financial metrics, such as net present value, return on investment or payback period.
Some control system suppliers can provide tools to assist in determining future CAPEX and OPEX costs, but the cost of maintaining versus replacing should be simple enough to model, even over a long-term planning horizon such as the useful life of the new control system, or the life expectancy of the entire plant (Figure 1).
Figure 1: Example financial analysis comparing operating expenses, initial CAPEX and savings between suppliers.
The decision process should be aimed at obtaining the best control system solution for the price, not necessarily the most advanced system on the market, while reducing expected equipment maintenance and associated expenses. Studies have shown four key challenges in the selection process:
The decision process should determine the lowest lifecycle costs to support a safe, reliable and profitable manufacturing enterprise. If this is used as the decision objective, the selection criteria can be clarified around this financial goal.
The company’s strategic outlook should be the overarching consideration in selecting a control system, with the flexibility and forward compatibility of the new control system key considerations for selection.
With these high-level factors decided upon, the next step is to look at competing platforms.
PLCs and DCSes were initially developed for entirely different applications. The PLC was created to control a large number of discrete I/O, with programming simulating hard-wired relay ladder logic. The DCS was developed to control large continuous processes requiring more sophisticated control schemes such as cascaded PID control loops, process optimization and advanced or multivariate process control.
The differences between PLCs and DCSes are less clear today, although significant differences remain, with mixed systems often appropriate. Some questions to be answered during the selection process include:
The BPCS (basic process control system) is the “brain” of the plant, and any selection criteria must include how well the BPCS will support commissioning, startup and operations
DCSes are typically capable of more advanced functions than PLCs, often using function blocks to combine multiple PID controllers, as in a cascade configuration. The key architectural element of the DCS is the ability to distribute control to individual nodes, isolating segments of the process to increase system performance and reduce response times.
DCSes and PLCs also have differing approaches to redundancy. DCS manufacturers typically offer fully redundant systems with “hot switchover” in the event of a fault. This level of reliability is more expensive, but redundancy and reliability are often primary considerations for selecting a DCS.
PLCs can also be used in a distributed architecture, and reliability can be improved by configuring backup PLCs running in “shadow mode”—if a failure is detected the backup PLC assumes control.
The DCS architecture is designed to implement management of change (MOC) in the controllers, which becomes increasingly important as the scale of the system increases. DCSes use databases to enable visibility and modifications of DCS components and control schemes across the plant. By contrast, PLC systems do not have a centralized database, making it more difficult to manage change.
Depending on the operation, a mix of PLCs and DCS' may result in the best control infrastructure. Plants with PLCs for specialized equipment control, and whose operational elements are spread over a wide area, are examples of this type of infrastructure.
It is important to consider the capabilities of the prospective control system provider for delivering a broad set of integrated technologies, while at the same time ensuring future forward compatibility for both software and hardware.
PLCs have traditionally been preferred for fast response times, but this distinction between PLCs and DCSes has blurred because a DCS can now be used to control fast-responding processes.
The use of standardized Windows and Linux operating systems has enabled greater compatibility among system components, but this same standardization has opened both types of control systems to cyber vulnerabilities like Stuxnet and Flame. Cybersecurity has thus become one of the most important selection criteria.
Security practices and policies for the BPCS and SIS differ in implementation for DCS and PLC-based control systems, and cybersecurity for control systems differs greatly from general-purpose IT systems. IT system priorities are, in order, confidentiality, integrity and availability, while in a control system, availability, integrity and confidentiality are critical—and in that order.
A variety of schemas can be used to defend the BPCS. Some manufacturers isolate the HMI and control network using a secure gateway, and adopt a proprietary protocol for the control network. There are good guides from NIST, ISA and other organizations for recommended security practices.
Control systems typically separate the BPCS from the SIS. The SIS is a separate system used to safely shut down the BPCS and the process. Some suppliers provide a consistent operator interface between the BPCS and the SIS, although each system should have independent event historians, asset management and network communications (Figure 2).
The concept of a “digital twin” has led to the increased adoption of operator training simulators (OTS). The OTS can be applied throughout the lifecycle of the control system, from conceptual design through commissioning and ongoing operations. As a high-fidelity model of the system, the OTS synchronizes with the plant control system to predict plant internal states and responses. The development of the OTS starts in the early planning stages, so its value can be realized in both CAPEX and OPEX reductions.
Figure 2: This system architecture diagram shows an implementation of a safety instrumented system.
The IIoT offers tremendous promise in many different guises. For example, the application of artificial intelligence, machine learning and data analytics applied in near real-time using multi-node cluster computing including graphical processing units (GPUs) has disruptive potential. Using analytics in a cloud computing environment, artificial intelligence and machine learning can be applied across an entire plant for asset management, energy management, predictive maintenance, process optimization and safety management.
The concept of “edge devices” has changed design architectural thinking. Edge devices operate at the perimeter of the control network, close to the process. Analytics and computing control at the edge is called “fog computing” to distinguish from cloud computing architectures. Fog computing can also be used to isolate the edge from the Internet, and can be used to protect intellectual property.
Several new architecture initiatives have emerged from the Open Group and the NAMUR Open Architecture initiative. The Open Group is developing a standards-based, open, secure, interoperable process control architecture to address both technical and business issues for process automation.
The NAMUR Open Architecture describes a pyramid (Figure 3), similar to the four levels of automation identified in the ISA95 standard. NAMUR standards will address content exchanged among levels, including a safe and secure protocol for handling communications.
Figure 3: The NAMUR open architecture automation pyramid can be used to determine safe and secure protocols for handling communication among the four layers.
There are several other important considerations when choosing a BPCS or SIS supplier.
When purchasing a system, the company is entering into a long-term partnership with the supplier. Are the company’s and the supplier’s strategies and business models aligned? Which supplier can best support the system over the extended lifecycle of a modern control system? Which supplier will best support new technologies and architectures?
Answers to these and other questions can be found in a White Paper titled Control System Selection Key Criteria.
Kevin Finnan is a System Consultant at Yokogawa Electric Corporation. He was previously an independent consultant serving the automation and measurement industries, Vice President of Marketing for CSE-Semaphore, and Director of Marketing at Bristol Babcock. He has over 30 years of experience with oil & gas measurement, process automation and SCADA systems, and has been active as an author and presenter for numerous industry publications and events.
Learn More
Check out our free e-newsletters to read more great articles..
SubscribeThe process control system (PCS) ties all the automation elements of a process manufacturing unit together and affects every aspect of operations. As the brain and central nervous system of a facility, its performance level and quality is crucial to keep operations running smoothly.
As new technology is introduced in the automation and control industry, now is the perfect time for manufacturers to modernize a process control system (PCS) platform to improve efficiency and operational performance. They can innovate—not duplicate—the functionality of the existing legacy infrastructure. However, manufacturers must carefully select the PCS and a third-party vendor who understands their specific platform requirements that can optimize and upgrade their facility.
First, it is necessary to weigh all system infrastructure options. Making an educated decision will have a positive impact on the facility’s ability to produce effectively and profitably. The operators will find the system easy to use and there will be room to improve and optimize the process. A bad decision, however, can lead to results in the opposite direction.
The assigned project team, which may include outside consultants, should know the challenges inherent in the decision-making process. They must plan early and set a positive course, steering clear of subjective or emotional views based on past experiences and focus on delivering the best strategy. Tools and methods exist to help shed more objective light on these types of projects so groups can make more informed decisions. Teams can then decide based on the facility’s specific needs and process, while considering the three following critical factors:
A strong, experienced vendor selection team can generally compile a good list of CTQs, but most may still miss a variety of critical items.
Few people working in a process manufacturing facility for any length of time will have participated in more than one major PCS installation or migration project. It’s not an easy task because these happen so infrequently. In addition, a PCS vendor may lack experience with some of the project’s site-specific elements.
In these instances, a platform-independent automation partner with a track record of executing successful PCS installation and migration projects is invaluable. They typically have specialists who have worked with the main vendors being considered and also can bring objectivity to the team. The partners work with key stakeholders to help outline the CTQ elements and then objectively rate the respective vendors. They also can help with the request for proposal (RFP) process, which is often complex.
A PCS installation or migration project is a major investment. It requires an enormous amount of work and the risks are high. A poor decision will remain with a facility for several years or could require great expense to fix. Choose wisely when selecting the next PCS platform and consider the experienced people who will help ensure the project is successful in terms of budget, schedule, and performance.
Lynn Njaa is a senior consultant for Maverick Technologies’ DCSNext. Lynn leads team efforts to provide consulting and other front-end engineering services. Maverick Technologies is a CFE Media content partner. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media, eguenther@cfemedia.com.
KEYWORDS: process control system (PCS)
Factors to consider for PCS implementation projects
Installing a PCS is an infrequent and complex process.
Bringing in an experienced and independent platform automation partner can be invaluable.
Consider this
What elements would your facility require for a PCS installation?
Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our CFE Media editorial team and getting the recognition you and your company deserve. Click here to start this process.
Previous: 15 Control Systems Engineer Skills For Your Resume
Next: None
If you are interested in sending in a Guest Blogger Submission,welcome to write for us!
All Comments ( 0 )