OPT FOR SECURITY AND PERFORMANCE

Improve the security and performance of your plant with alarm management, performance supervision, asset supervision, web-based data visualisation, optimisation, advanced control, and tuning with the new TaiJi tool.

Rethink your working methods and obtain immediate results thanks to our services, trainings, and consulting expertise. Manage changes intelligently with our proven solutions.

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December 2008

In this issue

Trick


DCS or PLC: Which One is Appropriate?

For a new process control project, will you select a DCS or a PLC?
In fact, you have three choices:

  • DCS (Distributed Control System)
    • Examples: Bailey Infi 90, Foxboro IA, Honeywell TDC 3000
  • Hybrids
    • Examples: ABB AC800, Honeywell Experion, Emerson DeltaV
  • PLC (Programmable Logic Controller)
    • Examples: Allen-Bradley PLC5, GE 90-70, Modicon Quantum

New DCS's are closer to hybrids than to DCS's. Also, new PLC's (large- and mid-size) are closer to hybrids than to PLC's. These hybrid systems are designed for both process control and sequential control. They can be programmed in many languages:

  • SFC (Sequential Function Chart);
  • LD (Ladder Diagram);
  • FBD (Function Block Diagram);
  • ST (Structured Text);
  • IL (Instruction List).

Programming a PLC for process control requires more effort and programmers have to be aware of pitfalls and use best practices. We strongly suggest to define programming standards. Also, in many cases, the default values of functions are wrong.

An Example: Alarms!

With DCS's, alarms are defined with priority levels and format is standardized; on the other hand, most PLC's let the programmer decide how it will be done.

Control Strategies

If control strategies beyond simple PID control are used, it will be more complex to get the right behavior with a PLC. When using constraint control, each standby controller must follow the active controller. In a cascade scheme, if the secondary controller is switched from cascade to automatic mode, the master has to be switched (by the system) in manual mode.

Our experts can help your team design programming standards, design control strategies, and optimize your programs. Do not hesitate to communicate with us.

Performance

If your systems are properly programmed (based on best practices), performance will be similar for all systems.


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Courses

See our calendar: we offer workshops on control strategies, performance supervision, advanced process control, alarm management, etc.

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Articles And Webinar

New articles have been added to our web site:

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Webinar

TaiJi-PID: A New Era in Control Loop Tuning and Optimisation!

This free webinar presented by Michel Ruel has been recorded and is now available for download.

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TOP Control Is Certified To Go Offshore


Basic Survival Training (Including HUET)
40-hour course
The Marine Institute
School of Maritime Studies - a full member of the International Association for Sea Survival Training (IASST)
Fisheries and Marine Institute of Memorial University
St. John's, Newfoundland, Canada

Pacesetter


Operational Excellence in the Process Industries: Driving Performance through Real-Time Visibility
Industry Report

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