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REVERECONTROL SYSTEMS

DCS Versus PLC: A User’s Guide To Selecting The Most EffectiveControl Platform For Your Application

Introduction

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 deci-sion, the selection process becomes more effi cient, and a more eff ective outcome results. Th is 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.

DCS and PLC – A Historical Perspective

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 confi guration is through function blocks, which after the advent of microprocessors controlled even more concurrent tasks across a distributed network of controllers. Th e 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. Th e adoption of PCs in the 1980s and 1990s enabled programming from the PC via ladder logic programming applications. Th e 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.

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DCS vs PLC

DCS HMIDCS

PLC HMIPLC

HybridDCS

Superior speed makes PLCs a better choice for applications involving fast production startup Using discrete I/O. They also offer range in I/O granularity and maintainability.

DCSs have the built-in infrastructure to perform advanced regulatory control on a plant-wide scale. Slower processes typically require coordination across various production units.

This approach optimizes cost and efficiency without compromising safety. It applies DCS technology where process material risk and cost of downtime are high, and PLC technology where changes in output or product variation. require flexibility.

HMI

PLC

Distributed control systems

Programmable logic controllers

Machines

Processes

Controllers

Key

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:

1. 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.

2. 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.

3. 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.

4. 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.

DCS, PLC or Hybrid? How Applications Affect Decision Making

As process automation advances, so too does the need for guidance in how to apply each technology effectively. Given the complexity of today’s control applications, the choice of controller technology isn’t limited to DCS or PLC. On the contrary, advances in process automation and controller technology make it possible for PLC and DCS to coexist in a networked environment—in hybrid control systems.

Hybrid controller integration offers plants more options to cost effectively optimize existing systems, whether the goal is to add capabilities, streamline processes, increase capacity, or improve operational efficiency. Consider the following application criteria:

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Speed – Controller speed is vital to safety and quality, particularly in processes involving interlocking and motion control, where controllers operate at millisecond speeds. PLCs historically perform more cost- effectively than their DCS counterparts when it comes to processing high-speed discrete data such as in assembly lines and equipment sequencing. However, DCSs have been more cost effective at crunching analog signals in more complex operations involving more than single-loop control strategies. One typically selects a PLC to allow flexibility and different methods of performing similar operations. DCSs are configured to trade customization for repeatable and dependable performance.

criticality – Risk relative to safety, cost, and regulatory compliance drives redundancy decisions. Consider the probability of a controller failure and the potential outcomes that may result. Processes where failures could result in loss of life, equipment, or expensive materials often include redundancy and triple modular redundancy. DCS technology typically has been the preferred choice for high-risk unit process applications where redundancy is necessary for product integrity. However, for safety interlocking and SIL rating per IEC 61511 and 61508, the PLC is the most often used and cost-effective solution. Also, given the cost of redundant DCSs, a hybrid approach is becoming a more popular and economical alternative to a single controller technology. For example, PLCs are used in high-risk machine-level process phases and for safety interlocking, while the DCS is used in high-risk plant-level process phases.

interoperability – This is a key consideration relative to legacy systems in terms of understanding whether the network environment is open or closed to third party and non-native devices and protocols. Despite the standardization of programming languages and network protocols, both DCS and PLC maintain mostly proprietary architecture. Many applications today involve retrofitting to an existing network; therefore, it is important to know the devices’ interfacing capabilities and protocols.

training – Assessing the skill levels of technicians, operators, and plant engineers is needed to determine the learning curve. If personnel already know PLC ladder logic control, moving from one PLC platform to another will be easily accomplished. DCSs, however, are more complex, requiring a longer learning curve and more extensive skill sets to properly configure a system. The learning curve affects day-to-day operation, which factors into the cost to deploy and maintain the system.

Following are some common controller applications and general guidelines for selecting the most effective controller technology for each:

oem controller unitS – These represent a smaller part of a larger system, such as skid mounted equipment. In the water industry, common applications for skid-mounted PLC usage include grit removal equipment, aeration systems, pumps, filters, centrifuges and filter presses. OEM controller units tend to be cost-sensitive applications that do not require extensive reprogramming, which make them more conducive to PLC technology for its lower installation costs, easier maintenance, and attractive total cost of ownership (TCO).

applicationS witH large complex proceSSeS (e.g. cHemical, power, and pulp and paper) – These tend to include highly complex control loop interactions and batch control, and require high redundancy for process uptime, making DCS technology a preferred choice given its robust capabilities. Today, PLCs often manage many of the areas within these large industrial sites and communicate to the DCS for operational control.

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applicationS witH motion and interlocking proceSSeS (e.g. conveying SyStemS and macHine and motor control) – This application category includes numerous discrete control devices such as proximity switches and motion detectors. PLC technology tends to be more compatible with such operations, particularly since many may undergo ongoing lineup changes.

DCS and PLC Use-Case Scenarios

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?

Trends to Watch in Controller Technology

The paradigm of specifying PLCs only for discrete control applications and DCS for proportional-integral-derivative (PID) loop control no longer applies. Driven by advancements in microprocessors and COTS, technologies have converged, and in some cases, the differences between the two are barely discernible. As a result, it is imperative to fully understand the process application to ascertain the best control strategy for the end user. Each technology and each vendor has a sweet spot. This is what needs to be fleshed out and matched with the operational requirements.

Today we are beginning to see secure cloud computing technologies, virtualization, and mesh networks migrating from the IT world to the realm of process control. The convergence of PLC and DCS will continue, and many of the functions that have been the domain of control systems will be pushed to intelligent field devices, including the use of more bus and wireless I/O.

These trends will continue slowly. Production facilities are wary of leading edge technologies; they will maintain a conservative viewpoint and continue to demand their control systems are proven technologies.

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Convergence

Convergence of the two technologies precipitates the need to consider application and customer requirements. Often the customer’s needs and wants go beyond the application itself and involve a business requirement such as lower TCO or flexibility to meet changing market demands. A hybrid system can often address manufacturing requirements and business needs. For example, apply DCS technology where process material risk and cost of downtime are high, and PLC technology where changes in output or product variation require flexibility.

Both technologies continue to trend away from proprietary systems toward open standards. PLC technology, for example, uses IEC 61131-3, which standardizes programming languages. Standardization contributes to cost and time savings by making PLCs more interoperable. DCS technology, too, has adopted PC architecture and open networking standards such as Ethernet and Fieldbus. Taken a step further, it’s possible to interface with today’s DCSs or PLC systems from a laptop or iPad.

Collaborative Efforts

As industrial control systems become more distributed, interconnected, and reliant on the Internet, they become targets for cyberattacks. Many may recall the Stuxnet worm that infected SCADA systems. The process control system can no longer be considered an isolated island of automation. It is an integral part of the business unit and must meet the same rigors of an enterprise’s risk management and IT security/ business continuity and disaster recovery plan. The lines become blurred between the IT department and operations, each with unique risk requirements relative to safety, availability, and fault tolerance.

It is not always the faceless hacker attempting to create chaos as a political statement or for bragging rights. Harm can originate from disgruntled employees as well as the unintentional introduction of malware. These scenarios must become part of an organization’s business continuity and disaster recovery plan.

Key Considerations when Selecting a Control PlatformThe following matrix provides general guidance for narrowing down the selection of controller platforms and is not intended to replace a consultation by a trained knowledgeable professional.

Criteria PLC DCS Notes

Product Value – We know a pound of gold is worth more than a pound of coal, so if you lose a batch of product, will it be bad or cata-strophic?

Good Better Is the added cost of redundancy, hardware, and engineering worth the benefit? The DCS has redundancy capabil-ities from the processor down to the individual I/O point.

Production Startup – Does your process start with the push of a button and return to speed quickly, or do you have to stage and/or charge production units over a lengthy period of time?

Better choice on fast startups

Better choice on slower startups

PLCs are fast; they run an input-process- output cycle in millisec-onds. Slower processes typically require coordination across various production units, which matches the capa-bilities of the DCS.

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Criteria PLC DCS Notes

Data Analysis – Enterprise-wide data his-torization and manufac-turing execution system (MES)

Good Better PLCs have begun to close the gap with SPC (statistical process control), historian, and data storage capabilities; however, these functions are typically integrated tightly within the DCS platform.

Discrete I/O – Motor controls, fans, interlocks, and other on/off equipment

Better Good This is where PLCs first gained experience, and they remain very effec-tive in this environment with fast scans and low cost. DCS providers can offer micro-DCS to com-pete in this arena.

Analog I/O – Pressure controls, flow rates, and PID control loops

Good Better PLCs are fine for simple PID loops; however, DCSs have the built-in infrastructure to perform advanced regulatory con-trol on a plant-wide scale.

Exception Notification – Alarming based upon defined limits; limited operator interaction

Better Good Many PLC applications can run for days or weeks with little, if any, opera-tor interaction.

Centralized Control –Operations continu-ously monitor process conditions and change setpoints based upon condition changes.

Good Better DCSs provide a system-wide database and can handle large loop counts in a multitasking mode.

Maintainability – Engineering and technician capabilities

Better Good PLCs utilize symbology common to electri-cal technicians, which typically requires less training. Many technicians are already familiar with common PLC ladder logic programming.Along with the added capabilities of the DCS comes the need for highly trained personnel. DCSs typically require more spe-cialized training involving IT-related networking and database functions.

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Conclusion and Summary

When designing, specifying, and sourcing controller technology, keep in mind that it’s no longer necessary to choose one or the other. On the contrary, for many applications, a hybrid approach can optimize cost, efficiency, and performance without compromising safety, security, or regulatory compliance.

Trends including advances in microprocessor technology, open standards for interoperability, and web-based applications are driving the need for new and different approaches to applying existing controller technologies. The result can be a more cost-effective control system from an initial cost and overall operating cost perspective, as well as from improved efficiencies.

Biographical Information:

Jim Hazelwood is a project engineer at Revere Control. Jim has spent the past 30 years helping manufacturing companies improve plant throughput and reduce operational costs through the design of robust automation systems. His certifications and memberships include ISA Certified Automation Professional (CAP) and PMI Project Management Professional (PMP). He holds a bachelor’s degree in computer information systems and a master’s degree in electrical engineering from the University of Alabama at Birmingham.

bill butler is a business development manager at Revere Control Systems (Birmingham, Ala.). Bill is a 45-year veteran of the manufacturing automation controls industry and has helped manufacturers optimize their assets by designing integrated control systems. Prior to joining Revere Control, he served in various engineering functions for Rust Engineering Co., The Foxboro Company, Fisher Controls, and Maxson Engineering Company. Bill is a senior life member of the International Society of Automation (ISA) and a licensed Professional Engineer (PE) in electrical engineering in Alabama, Georgia, Florida, and Mississippi. He holds a bachelor’s degree in electrical engineering from the University of Alabama at Birmingham.

Sources:

http://www.prnewswire.com/news-releases/analysis-of-the-global-distributed-control-system-market-201039331.html

http://www.amazon.com/Cybersecurity-Industrial-Control-Systems-ebook/dp/B0071ART60

http://en.wikipedia.org/wiki/Stuxnet

http://www.ncsafewater.org/Pics/Training/AnnualConference/AC09TechnicalPapers/AC09_SpecialTopics/ST_T.AM.10.30_Dodson.pdf

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