A user’s guide to selecting the most effective control platform for your application

Distributed control systems (DCSs) and programmable logic controllers (PLCs) are not mutually exclusive technologies. When the end-use application serves as the basis for making a sound decision, the selection process becomes more efficient, and a more effective outcome results. This white paper provides general guidelines and highlights key considerations when choosing a control system platform. While the details of each application are critical to the selection process, use the following as a guide when designing, specifying, and implementing controller technology.


Introduced in 1975, DCS is a widely used term to describe the monitoring and control of distributed systems in a manufacturing environment. A DCS is used to control continuous or batch-type manufacturing processes in a variety of industries such as food, pharmaceutical, and power generation. A DCS often includes redundant controllers for increased system reliability.
A DCS’s typical method of configuration is through function blocks, which after the advent of microprocessors controlled even more concurrent tasks across a distributed network of controllers. The 1980s ushered in limited layered Ethernet-based networking capabilities and the expansion of the UNIX platform, giving plants greater access to data. During the 1980s, PLC technology began to be interfaced within DCS applications. Today’s DCSs are capable of many advanced control functions including fuzzy logic, neural network, and multivariable control capabilities.
Introduced in 1968, the PLC is a digital computer that controls discrete production processes in industries including automotive, electronics, and packaging, among others. PLCs replaced relay logic systems and were programmed from proprietary panels using ladder logic, which documented the construction of relay racks. The adoption of PCs in the 1980s and 1990s enabled programming from the PC via ladder logic programming applications. The PLC historically has been the technology of choice in harsher conditions where humidity, temperature, and vibration are factors.
Early PLCs were only relay replacements and had no analog capabilities. While early DCSs had the capability to perform the functions of a PLC, their infrastructure costs were hefty, starting in excess of $100,000 back in the 1980s.
Generally, DCSs are found in systems with “invisible” processes, such as transforming raw materials, while the PLC is the dominant choice for “visible” processes, i.e. assembled items.
Today’s process automation systems in many ways represent a convergence or hybrid of DCS and PLC technologies. In the 1980s and 1990s, beginning with the advent of Microsoft Windows NT platform and using DCOM/OLE process control connectivity standard, Microsoft began its march to garner the largest slice of the human machine interface (HMI) pie. DCSs were once heavily dependent on proprietary hardware and network technologies supplied by DCS manufacturers. However, the introduction of commercial-off-the-shelf (COTS) components and standardized IT protocols placed downward price pressures on proprietary DCS communication interfaces and opened the door for PLC manufacturers to compete for controller business in some applications.
Unless the production facility is a greenfield application, it will have a PLC performing some tasks based on a number of factors:
Standalone Applications: These applications have a small input/output (I/O) count and require little or no operator interfacing. These applications do not produce data that would affect product quality, need to be historized, or benefit the business unit. Some examples are automatic doors, grinding machines, and sump pumps.
Harsh Environments: Many PLC controller brands and models boast of their ruggedness. It’s common to not only find these in washdown areas, but also in high vibration, electrical noise, and other environmentally challenging locations.
Skid Mounted: OEMs utilize the most cost-effective solution to perform the function required. These are small I/O count PLCs with or without networking capabilities. Applications such as pumping stations, ammonia skids, and compressor units use PLCs for their simplicity, cost point, and ability to standardize on a specific platform.
Safety Instrumented Systems (SIS): IEC 61508 and 61511 encompass many of the standards required for a certified safety system. Most existing facilities use PLCs that meet this criteria, which is used in applications such as burner management systems (BMS), high integrity pressure, and wellhead control.
Today, the majority of control system work is performed in brownfield facilities to expand production areas or replace legacy control equipment that is no longer capable of sustaining the necessary functionality required by the business unit. This requires control systems to assist the business in cutting product cost, minimizing quality variants, and increasing plant throughput.


Since the divisions between the technologies has decreased, what can go wrong? Let’s look at two examples.
Poorly Managed Complexity
Recently, we were asked to quote a replacement system for a small DCS at a small chemical plant. The system was not outdated and was operational, so why replace it? Discussions with the plant manager shed light on the subject. He stated that he had only one person with the skill set to work on the system, and that person was no longer with the company. This plant is also in a remote location with a limited skilled labor pool. Having maintenance and engineering provided by the vendor was cost- and time-prohibitive. Additional complexities were also an issue. Alarming schemes and control functions were implemented yet not understood by the operators. As a result, most control was done manually, thus negating any capabilities of the system.
Poorly Managed Technology
On another occasion, a plant site called us in a panic: “Our production line is down, and the guy we always call can’t get here for three days. Can you fix it?” The integrator they had always used was a one-man shop, which created a dependency. The integrator was also adamant about using a particular vendor’s equipment. The PLC platform was not the best fit for the production facility, so in order to make it work, multiple PLCs were hardwired together, while a VBA (Visual Basic for Applications) application and a Microsoft Access database performed recipe management on an antiquated PC. Multiple production days were lost while attempting to troubleshoot this chaotic and undocumented system.
What can we take away from these examples? Are you headed down a similar path? In the conclusion to this overview, we’ll examine coming trends in controller technology and also provide an easy-to-follow look at key considerations when selecting a control platform. ■
Jim Hazelwood is a project engineer at Revere Control. He has spent the past thirty years helping manufacturing companies improve plant throughput and reduce operational costs through the design of robust automation systems. Bill Butler is a business development manager at Revere Control Systems. He is a forty-five-year veteran of the manufacturing automation controls industry and has helped manufacturers optimize their assets by designing integrated control systems.
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