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>


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