In the past it was fairly easy to determine whether a distributed control system (DCS) or a programmable logic controller (PLC) based system was right for your application, because their strengths and weaknesses were well understood. In recent years, however, this has become more difficult, thanks primarily to the advancement of the microprocessor, which has allowed the technologies to converge.

The convergence of PLC and DCS has opened up a new set of options for those process plants that traditionally used PLCs to control their electrical infrastructure (such as motors, drives, and motor control centers), while utilizing DCSs for regulatory control. Also, because your applications may have grown or evolved, you may find that a traditional PLC or DCS no longer meets your requirements. Understanding the merging of PLC and DCS functionality is important for selecting the best system for your company.

Please realize that we will be using broad generalizations in the following analysis, and that every individual application will have exceptions to these “rules.” However, the logic is still sound. Since the authors work on different sides of the PLC/DCS fence for a supplier that has delivered both DCS and PLC solutions to the market for over 25 years, we feel that we are in a unique position to deliver both sides of the story.

At first glance, system architectures look very similar — and they are. Both systems share the following components: field devices, input/output (I/O) modules, controllers, human machine interfaces (HMIs), engineering, supervisory control, and business system integration. Differences become more apparent when you consider the nature and requirements of an application, however.

For example, in a DCS architecture, redundancy is often employed for I/O, controllers, networks, and HMI servers to ensure 100% availability. One of a PLC’s most common applications is the control of discrete field devices such as motors and drives. Effectively doing that requires that the controller be able to execute at high speeds (typically a 10 to 20 msec scan rate), and that the electrical technician responsible for maintaining it be able to read and troubleshoot the configuration in a language that he is familiar with (commonly relay ladder logic).

Because PLCs and DCS are not that different from a technology point of view, we must look beyond technology to the application expertise and domain knowledge built in to these systems by suppliers. This can help you better understand the sweetspots where each is best applied. The seven questions that follow are designed to make you think about your company's operating philosophy and application requirements, taking into account the point of view of all the major stakeholders in your plant (engineering, operations, maintenance, etc.). The table on page 41 presents the questions and possible answers in a quiz-like format; the discussion of the questions on subsequent pages provides additional food for thought.

This may seem like a very basic question, but it is fundamental to determining the requirements of the application and, therefore, the best-fit automation system. Typical factory automation applications, for which the PLC was originally designed, involve the manufacturing and/or assembly of specific items. These applications may employ one or more machines and a fair amount of material movement from machine to machine. A typical characteristic of this type of process is that the operator can usually monitor the items visually as they progress through the manufacturing line. The process is, by nature, very logic-control-intensive, often with high-speed requirements (throughput = profits). This type of process is often controlled by a PLC and HMI combination.

Process automation applications typically involve the transformation of raw materials through the reaction of component chemicals or the introduction of physical changes to produce a new, different product. These applications may be composed of one or more process unit operations piped together. One key characteristic is that the operator can’t see the product. It is usually held within a vessel, and may be hazardous in nature. There is usually a large amount of simple as well as complex analog control (e.g., PID or loop control), although the response time is not exceptionally fast (100 ms or greater). This type of process is often controlled by a DCS, although the analog control capability of a PLC may be more than adequate. A determining factor in the selection process is often the scope of the control application (i.e., plantwide versus single unit, and number of I/O points).

Process applications may also have sequential (or batch) control needs. A PLC can be used effectively for “simple” batch applications, while a DCS is typically better suited for “complex” batch manufacturing facilities that require a high level of flexibility and recipe management.

If the value of each independent product being manufactured is relatively low, and/or downtime results in lost production, but with little additional cost or damage to the process, the PLC is the likely choice. If the value of a batch is high, either in raw material cost or market value, and downtime not only results in lost production but potentially dangerous and damaging conditions, the selection should be DCS. In some chemical applications, maintaining the process at steady state is critical, because if the system goes down, the product could solidify in the pipes. If the cat cracker in a refinery goes down it could be days before it can be brought back on-line and that means lots of lost revenue.

In contrast, the bottling operation of a brewery that only needs to run 10 hours a day to meet production schedules, and which can be shutdown for system maintenance, troubleshooting, or upgrades, is a classic PLC application.

Dangerous downtime is clearly another deciding factor. The more volatile the application, the more it may require a solution with lots of redundancy to ensure that the system is available when needed.

Typically, the heart of a factory automation control system is the controller (PLC). It contains all the logic to move the product through the assembly line. The HMI is often an on-machine panel or a PC-based station that provides the operator with supplemental or exception data. Operational information from data analysis is increasingly needed, however, driving demand for more robust HMIs.

In process automation, where the environment can be volatile and dangerous, and where operators can't see the actual product, the HMI is considered by most to be the heart of the system. In this scenario, the HMI is a central control room console that provides the only complete "window" into the process.

In a PLC environment, the operator's primary role is to handle exceptions. Status information and exception alarming help keep the operator aware of what is happening in the process, which in many cases can run "lights out." The DCS plant requires an operator to make decisions and continuously interact with the process to keep it running. In fact, leveraging the operator's process knowledge is often critical to operational excellence and keeping the process running optimally.

The speed of logic execution is a key differentiator. The PLC has been designed to meet the demands of high-speed applications that require scan rates of 10ms or less, including operations involving motion control, high-speed interlocking, or control of motors and drives. A DCS doesn't have to be that quick most of the time. The regulatory control loops normally scan in the 100 to 500 millisecond range. In some cases, it could be detrimental to have control logic execute any faster — possibly causing excessive wear on final control elements such as valves, resulting in premature maintenance and process issues.

The extra cost for redundancy, an insurance policy of sorts, may be well worth it in the case of the typical DCS system, but is often not cost-justified for PLC-based systems.

Taking the PLC system offline to make configuration and engineering changes may have less impact, since the platform is not running continuously or because the process can be restarted easily. In contrast, configuration changes and tweaks to the DCS system are done on-line, while the process is running virtually non-stop. A blast furnace, for example, is planned to stay on-line continuously for five to seven years.

The expectation and desire to be able to create a customized application varies greatly between DCS and PLC users. Because the PLC was originally designed to be a jack of all trades, it's understood that the development of customized routines and functions is required to meet the unique needs of an application.

Pre-engineered solutions, consisting of standards, templates, and extensive libraries, are what DCS application engineers expect out-of-the-box when working with a new system. The highest priority of a DCS is to deliver reliability and availability, which often results in a design which trades unlimited functionality for repeatability and dependability. The significant tradeoff with the DCS is its inability to accept many custom modifications without creating compatibility issues.

Factory automation engineers want customizable control platforms, which offer the individual components that can be quickly programmed together to accomplish the task at hand. Often integrators and engineers open the PLC toolkit, roll up their sleeves, and start programming. The tools provided by a PLC are typically optimized to support a bottom-up approach to engineering, which works well for smaller applications.

DCS engineers, on the other hand, are typically most effective using a top-down approach for engineering, which forces them to put significant effort into the upfront design. The ability to propagate libraries and templates throughout the application is very important to minimize rework and promote the use of standards.