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  1. 1. Decision : PLC or DCS Vivek Koshti
  2. 2. Selecting the technology and the right supplier can help your company : • To respond quickly to changing market conditions in a way that creates a sustainable competitive advantages. • Minimize Total cost of ownership over the life of plant • Create a system which is easily maintainable / upgradeable for the long term. • Achieve its future goals & vision. • Selecting an automation system based on a review of available products is the typical course of action for someone in the market for a new automation system.
  3. 3. The hybrid application typically requires a process Control system that can deliver both PLC & DCS Capabilities. A successful evaluation should start by developing a Clear picture of the requirements of your application & the needs of your engineering, maintenance and Operations personnel. This presentation outlines the seven key questions that will lead you to make the right choice.
  4. 4. Typical DCS System Architecture
  5. 5. • In DCS Architecture diagram, redundancy is often Employed for I/O Controller, Networks, HMI servers. • Since redundancy adds cost & sometimes complexity DCS users must carefully evaluate their need for redundancy in order to achieve their required system availability and to prevent unplanned downtime. • DCS is typically better suited for complex batch manufacturing facilities that require a high level of flexibility and recipe management. • Plant operator can’t see the product which is within the vessel and may be hazardous in nature. There is usually a large amount of simple as well as complex analog control i.e. PID or loop control, although the response time is not exceptionally fast (100 ms or < )
  6. 6. Typical PLC System Architecture
  7. 7. • PLC Architecture diagram illustrate one of its most common applications, the control of discrete devices such as motor & drives. • To effectively control motors and drives requires that the controller be able to execute at high speeds (10-20 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 (relay ladder logic). • From a technology point of view, one can see that PLC & DCS are not that different. We must look beyond technology that is built into these system by the supplier, so that we can better understand the “sweet- spots” where each is best applied.
  8. 8. At first glance, system architectures look very similar. Both system share following Components : • Field Devices • I / O Modules • Controllers • Human Machine Interfaces (HMIs) • Engineering • Supervisory Control • System Integration The differences become more apparent when you consider the nature & requirement of an application.
  9. 9. The seven questions to ask yourself before Choosing a system : PLC or DCS
  10. 10. Q. 1 – What are you manufacturing & how? • No. of Products manufactured : Single / Multiple • Recipe parameter : Constant or Variable • Procedure : Single or Different • Equipment Utilization : Fixed or Flexible • Frequency of changes to formula & Recipe : Never or Often
  11. 11. PLC DCS Manufacturing or assembly of Involves the combination & / or specific items Transformation of raw material Product is visible & it moves Impossible to visually see Product through the process as it moves thought the process High speed logic control Regulatory / Analog loop control Simple Batch Control Complex Batch Control
  12. 12. Q. 2 – What is the value of the product being manufactured and the cost of downtime? • If the value of each independent product being manufactured is low, & / or downtime results in lost of production, but with additional cost or damage to the process, the PLC is likely choice. • If the value of the batch is high, either in raw material cost or market value, & the downtime not only results is lost production but potentially dangerous and damaging conditions, the selection should be DCS.
  13. 13. PLC DCS Value of individual component being The value of batch can be very Manufactured is relatively low High. Downtime mainly results in lost Downtime not only results in lost Of production Production but can result dangerous Or hazardous conditions. Downtime does not typically Downtime can result in process Damage the process equipment Equipment damage. Return to steady state production Return to steady state production after an outage is short & relatively after an unplanned outage ca be straightforward long, expensive & difficult.
  14. 14. Q. 3 – What do you view as the “heart” of the system? • PLC is the heart of the factory Automation control system, which contains all of the logic To move the product through the assembly line. HMI is often an on machine panel or a PC based station. • Operational information resulting from data analysis is also a requirement for factory automation applications – driving demand for a more sophisticated HMI
  15. 15. • The environment is process automation can be volatile & dangerous. • In this scenario, the HMI is a central control room console that provides the only complete “window” into the process, enabling operator to monitor & control the process which are occurring inside pipes & vessels located through out the plant. PLC DCS Typically, heart of the system is the Typically, hear t of the system is Controller. The HMI
  16. 16. Q. 4 – What does the operator need to be successful ? • In PLC environment, status information & exception alarming help to keep the operator aware of what is happening in the process. • The DCS plant require an operator to make decision and continuously interact with the process to keep it running. • In fact, operators process knowledge is often critical to operational excellence & keeping the process running optimally
  17. 17. • Operators has to keep an eye during product grade changes & when adjusting the process changes to overall production environment. • The operator will change the set-points, open/ close valves or to make manual addition to move a batch to the next stage of production. • Within the HMI, faceplates & analog trends provide a critical view into what is really happening in the production process, while alarm management system focuses operators attention on areas where he must intervene to keep the process running within its target performance. • In the event of HMI failure, the plant could be forced to shutdown in order to keep the people & equipment safe.
  18. 18. PLC DCS The operator primary role is Operator interaction is required to to handle exceptions Keep the process in its target Performance range Status information (ON/OFF, RUN/ Faceplates & analog trends are STOP) is critical information for the Critical to see what is happening to operator The process. Exception based alarming is key Alarm management is key to safe Information for the operator Operation of the process Manufacturing might be able to run Failure of the HMI could force the “light-outs” shutdown of the process
  19. 19. Q. 5 – What system performance is required ? • The speed of logic execution is a key differentiator. • Fast scan rates are necessary to be able to effectively control the operations involving motion control, high-speed interlocking, control of motors and drives. • The DCS does not have to be that quick. • Increasingly, PLC is capable of delivering simple to complex PID control, but DCS is the choice for applications with advanced analog control, cascade, Feed forward, ratio, model predictive control etc…
  20. 20. PLC DCS Fast logic scan rate(10ms) is Control Loops require deterministic required to perform motion control Scan execution at speed 100-500ms Redundancy may not be cost System redundancy is often justified required System can be taken offline to Online configuration changes often Make configuration changes required Analog Control – Simple PID ONLY Analog Control – Simple to Advanced PID upto Advanced Process Control Diagnostic to tell you when something Asset Management alerts you to what Is broken Might break before it does
  21. 21. Q. 6 – What degree of customization is required ? • PLC delivers a “toolkit” of functions & elemental building blocks that can be custom developed and chained together to address the requirements of an application. • Powerful Programming languages are typically available to facilitate the creation of custom code from scratch. • DCS Pre-engineered solutions consists of standards, templates & extensive libraries. • The highest priority of DCS is to deliver reliability & availability, which often results in a design which trades unlimited functionality for repeatability and dependability.
  22. 22. PLC DCS High Level Programming Languages Custom Logic created from Are available for creating custom existing function blocks logic Customized routines usually Many algorithms are complex (PID) required & do not vary among applications Standard libraries considered Function Blocks and faceplates As nice features Are expected Provisions must be available to Entire system is expected to Integrate functions / products into an Function as a complete solution Integrated architecture
  23. 23. Q. 7 – What are your engineering expectations ? • Factory automation engineers want customizable control platform, which offer the individual components that can be quickly programmed together to accomplish the task at hand. • PLC engineer demands flexibility & open architecture. • DCS engineers focus on upfront design is a key to minimizing costs, compressing the project schedule & creating an application that can be maintained by plant personnel over the long term.
  24. 24. • The process engineers controlling entire plants with a DCS require more intuitive programming platforms, which utilize pre-defined & pre-tested functions to save time & drive repeatability. • Think about in this way – The PLC is controlling a machine. The DCS is controlling the plant.
  25. 25. PLC DCS Program / configure individual Upfront design of complete system Components, integrate later Before implementation begins. Desire customizable platforms to Looking for significant “out-of-box” build upon Functionality System designed to be flexible System designed to make it “easy” To engineer to process application Use of Ladder logic to configure Use of FBD for configuration of applications applications
  26. 26. Do you have a Hybrid Application? What is a “ Hybrid ”? • The marriage of the discrete functions, which PLC handles so simply and economically, with the sophisticated analog continuous control capabilities of the DCS. • Defined based on the industries in which system works & serves like oil & gas, refinery, pharmaceutical food & beverages • The architectural marriage of the PLC simplicity and cost with the sophisticated operator displays, alarm management & easy configuration capabilities of DCS.
  27. 27. How to select a Process Control System for Hybrid Application ? • Controller : can execute fast scan logic (10-20 msce), such that required for motor control and slow scan logic (100-500 msec) such as required for analog control. • Engineering Configuration Language: Provides ladder logic, Functional Block Diagram and a powerful programming language for creation of custom logic from scratch. • Flexible Modular Redundancy: Offers the option of tailoring the level of system redundancy to deliver the required system availability by balancing up-front cost versus the cost of unplanned downtime. • Modular batch from simple to complex – Provides modular batch capability to cost effective address of simple to complex batch application.
  28. 28. How to select a Process Control System for Hybrid Application ? • Alarm Management : Offers power alarm management tools to help operators respond effectively to plant upset conditions • System Diagnostics & Asset Management: Provides both a rich set of built in system diagnostics, as well as asset management of all critical assets (transmitters, valve Positioner, motors, drives, MCC’s, heat exchanger etc.) in the plant • Scalable Platform: Hardware, software & licensing supports smooth & economical scale up from small all-in-one system (10’s of I/O’s) up to a large client / server system (1000’s of I/O’s)
  29. 29. In Nut-shell Characteristics PLC DCS Market Introduction 1960 1975 Replacement of Electromagnetic Replays Pneumatic & SLC Application Automotive Refinery Type of Control Discrete Regulatory Engineering Mindset Programming Configuration Size Compact Large System Open Closed Operator Interface Simple Graphics Sophisticated Graphics Upfront Cost $$ $$$$ Operation interaction Exception Basis Man in the Loop
  30. 30. References : [1] Bob Nelson, “Making the Right Choice for Process Industry”, Control Engineering Asia, October-2008. [2] “Process Automation”, SIEMENS Energy & Automation. [3] “Hybrid Control Identity Crisis: What's in a Name?”, In-Tech Sept. 2007. [4] Rich Merritt, “Does DCS have A Future? If so, is it HMI/SCADA”, Control- Dec. 2008.
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