Choosing a control system: PLC or DCS

Today, the economic pressures facing us are at levels few have ever seen. These challenges make it all the more imperative to select the technology that is right for you. Focusing on the requirements of the application and of the key stakeholders should always be your guiding principle.

03/01/2009


Today, the economic pressures facing us (and our companies) are at levels few have ever seen. These challenges make it all the more imperative to select the technology that is right for you. Focusing on the requirements of the application and of the key stakeholders (engineering, maintenance and operations) should always be your guiding principle. No one can afford to select a system based solely on technology (what it can do). More importantly is what it can do for you. Productivity, efficiency, safety, energy savings and long-term supportability are among the criteria that should guide your decision.

Many of the classic stereotypes of when to use a programmable logic controller or when to use a distributed control system have become clouded. In fact, the key differences between a PLC and DCS are no longer in the hardware. Microprocessor advancements have made the controller in a PLC system virtually indistinguishable from that in a DCS. The real differences now are in the software and in the application libraries (domain expertise) that come with these products.

To help determine the best technology for your application it is important to objectively document your requirements. To enhance this process, ask some important questions of the major stakeholders in the plant. This will help you determine whether a PLC or DCS system is best for you.

Production

  • Downtime %%MDASSML%% Does it result in lost production or does it additionally create dangerous conditions for the equipment and personnel?

  • Batch %%MDASSML%% Is the process a “simple” batch application (single product, single procedure) or “complex,” requiring a high level of flexibility and recipe management?

  • Type of control %%MDASSML%% Does the process require high speed control such as fast interlocking or motor control, or does it require deterministic execution of analog control logic such as PID?

    • Operations

      • Role of the operator %%MDASSML%% Does the operator primarily handle exceptions or are they an integral part of the process (the person in the loop)?

      • HMI displays %%MDASSML%% Is the main purpose to display status information (On/Off, Run/Stop) or to provide a window into the process (using faceplates and analog trends)?

      • Alarming %%MDASSML%% Does the operator need exception-based information or management tools to help respond quickly and effectively to process upset conditions?

        • Maintenance

          • Configuration changes %%MDASSML%% Can the system be shutdown or must changes be made online without affecting operation of the process?

          • Diagnostics and troubleshooting %%MDASSML%% Is the maintenance team looking for diagnostics to tell them when something is broken or asset management capabilities to alert them before something might break?

            • Engineering

              • Configuration languages %%MDASSML%% Is the preferred tool for configuration and troubleshooting ladder logic or function block?

              • Philosophy %%MDASSML%% What is the mindset: programming of custom logic or configuration and reuse of pre-engineered templates?

              • Project engineering approach %%MDASSML%% Is it a “bottom up” or “top down” design and configuration?

                • Many applications require functionality that combines the capabilities of a traditional PLC %%MDASSML%% such as programming in ladder logic and the ability to perform high-speed control of motors and drives, and the classic DCS %%MDASSML%% such as PID control and redundancy. We call these hybrid applications. If you fall into this category, the good news is that there are hybrid systems on the market. No financial bailout is needed to produce hybrid technology for process applications.

                  The role of technology is to help people perform their jobs quicker, easier, safer and more productively. Keep this in mind as you evaluate which technology to add to your plant.


                  <table ID = 'id1499270-0-table' CELLSPACING = '0' CELLPADDING = '2' WIDTH = '100%' BORDER = '0'><tbody ID = 'id1499190-0-tbody'><tr ID = 'id1499192-0-tr'><td ID = 'id1499194-0-td' CLASS = 'table' STYLE = 'background-color: #EEEEEE'> Author Information </td></tr><tr ID = 'id1499320-3-tr'><td ID = 'id1499322-3-td' CLASS = 'table'> Todd Stauffer is DCS marketing manager, and Bob Nelson is PLC marketing manager at Siemens Energy & Automation Inc. </td></tr></tbody></table>


No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.