Compare Plc And Dcs

Compare PLC and DCS: What is the difference?

DCS stands for “Distributed Control System”. DCS’s were designed to control processes, not discrete operations. As such, a large number of the inputs and outputs are analog like a 4-20mA signal or 0-10V signal. In Literary meaning, a Distributed Control System (DCS) refers to a control system usually of a process or manufacturing system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. Process plants used to have long series of panel mounted Single Loop Controllers (Analog/PID controllers). PLC stands for “Programmable Logic Controller”. Historically a PLC was in discrete control of manufacturing processes. Whole discrete logic used to be implemented with relay circuitry. Most of the inputs and outputs for discrete control are binary, meaning they have only two states: On and Off. What are attributes and characteristics which differentiate the PLC system from the DCS. There have been claims and counter claims from different manufacturers that their system is DCS or PLC. The topic has remained under debate for long, and especially today when we have already entered into new era of Hybrid Distributed Control Systems, it has become increasingly difficult to select and differentiate the advantages and drawbacks one can get from different systems. There are few similarities and dissimilarities which I would like to mention here: 1) DCS are designed or made available to the user in a way that only configuration in form of a Functional Block has to be carried out unlike PLC, where complete programming has to be implemented using any one of the different languages available in the system. Now, Functional Blocks are also available in the PLC systems, which really makes it comparable to DCS. 2) When DCS started emerging in the market, idea was to supply DCS with whole bunch of hardware and software packages including for Human Interface, necessary for the complete automation of the plant, thus facilitating Single Point Configuration in terms of database and communication possible in general. Additionally, Human Interface does not need separate communication package i.e DDE server, to communicate with the controller. DCS includes higher levels of application software for regulatory and batch control. In case of PLC systems, PLC were not suppose to be in packages but competition with DCS vendors forced the PLC manufacturers to offer necessary all other softwares and packages. 3) Many DCS are designed such that it is possible to configure cycle time for each Functional Block. Thus DCS system takes care of cycle time scheduling of the Functional Blocks which are the basic execution units. This is one of the reason that overall scan time of the DCS is comparatively higher than the PLC system. This functionality has been introduced in PLC systems(s) in some form as well. 4) A DCS has inherently multiple processor capability thus making the functionality distributed across a network. In a typical multi-processors (multi-node) DCS architecture, Engineer has to put in less efforts for inter-communication of the processors or one controller can easily access the Tag(s)

from the database of the other .i.e the input of FB in one controller can be output of FB of the other controller. This is possible now in PLC but more efforts have to be put in. 5) DCS programming is centered around configuration of Functional Blocks and discrete logic is implemented in DCS using FBs, thus making the DCS inherently an analog control system (although ladder programming is also possible in some of the DCS also). PLCs were programmed using Ladder/Relay language , before arrival of IEC 1131-3 standard and Analog control was incorporated inside Ladder inside Ladder Logic using special FBs. This is probably the reason we look at some of the process plants that process (Analog) control is done by the DCS, while emergency control (Discrete Control) is implemented by PLC based systems. 6) PLCs are still being used at RTU stations because of their simple, small and cheaper architecture as well as engineering (typically the RTU application) instead of big DCS. DCS have been used as the central system in a SCADA network. In a typical SCADA scenario, one DCS is connected to many PLCs systems. 7) Being Discrete in nature, PLC was natural choice of manufacturer and end-user to apply it for safety system. This led to production of specialized safety system conforming to SIL3 certification in accordance with ANSI / ISA 84.00.01-2004. Today, a lot has changed, it is difficult to distinguish between two systems in terms of its main features. Differences between the two has virtually vanished due to Programming / Configuration language standard IEC-61131.The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. However, in major industrial areas and structure markets, it is practice to deploy DCS for process control and PLC based system for safety control. Perhaps, for a large install base like more than 1000 I/Os system, cost of installation, addition and maintenance per I/O is less in case of DCS system. Now, more important is cost, application, system integrity, reliability, maintainability, historical logging / intelligent statistics and learning / training. How much support is available from the vendor for the operation matters most to the operator now. This has resulted into ‘Solutions Packages’ by vendors to their customers instead of simply offering individual products. It is DCS or PLC, must come in a solution package. More and improved System functionalities have made these system more complex which require strong integration between the operator /user and the manufacturer. Few vendors also introduced Hybrid DCS / PLC system or transformed their PLC based system into DCS by incorporating similar features. DCS vendors have now introduced packages for Asset Optimization and management which seamlessly integrate with their systems. It is difficult to say which system to select ? It varies from user to user as discussed above. It is End-User who has to have all knowledge and courage to take responsibility of his system in totality.

You must automate a process, but you can't decide between a DCS and a PLC. Are these systems really all that different? The answers depend on a slew of other questions.

Turn the clock back 10-15 years: The programmable logic controller (PLC) is king of machine control while the distributed control system (DCS) dominates process control. If you manufacture plastic widgets, you speak PLC. If you produce chemicals, you speak DCS. Today, the two technologies share kingdoms as the functional lines between them continue to blur. We now use each where the other used to rule. However, PLCs still dominate high-speed machine control, and DCSs prevail in complex continuous processes. The early DCS looked dramatically different from the early PLC. Initially, the DCS performed the control functions of the analog panel instruments it replaced, and its interface mimicked their panel displays. DCSs then gained sequence logic capabilities to control batch processes as well as continuous ones. DCSs performed hundreds of analog measurements and controlled dozens of analog outputs, using multi-variable Proportional Integral Derivative (PID) control. With the same 8-bit microprocessor technology that gave rise to the DCS, PLCs began replacing conventional relay/solid-state logic in machine control. PLCs dealt with contact input/output (I/O) and started/stopped motors by performing Boolean logic calculations.

The big change in DCS over the past 20 years is its move from proprietary hardware to the personal computer (PC) and standard LAN technologies. With each advance in PC power, DCSs have moved up in power. PCs gave us speedy, responsive, multi-media, windowed, operator-process interfaces (OPI). Relational databases and spreadsheet software enhance the ability of DCSs to store and manipulate data. Artificial intelligence (AI) technology gives us "smart" alarming. Today's DCS architecturally looks much like the DCS of 20 years ago, but tomorrow's DCS may control through networked "smart" devices-with no I/O hardware of its own. Most DCSs offer redundant controllers, networks, and I/Os. Most give you "built-in" redundancy and diagnostic features, with no need for user-written logic. DCSs allow centralized configuration from the operator or engineering console in the control room. You can change programming offline, and download without restarting the system for the change to be effective. DCSs allow inter-controller communications. You can do data exchange in most DCS systems ad hoc (no need for predefined data point lists). You access data by tag name, regardless of hardware or location. DCSs use multi-tasking operating systems, so you can download and run applications aside from the real-time control functions and still do fractional-second control. DCSs now come in "micro" systems, to price-compete with PLCs-but with full DCS features and capabilities. The typical DCS has integrated diagnostics and standard display templates that automatically extend/update when your database changes. This database is central to the system-you don't have different databases sitting in the controllers.

DCSs have user-friendly configuration tools, including structured English, control block libraries, SFC (sequential function chart), and even RLL (relay ladder logic). Most DCSs allow graphical configuration, provide online diagnostics, and are self-documenting. Most provide for user-defined control blocks or customized strategies. The controllers execute control strategies as independent tasks; thus, making changes to part of the control logic has no impact on the rest. An important difference between DCSs and PLCs is how vendors market them. DCS vendors typically sell a complete, working, integrated, and tested system; offering full application implementation. They offer many services: training, installation, field service, and integration with your Information Technology (IT) systems. A DCS vendor provides a server with a relational database, a LAN with PCs for office automation, networking support and integration of third-party applications and systems. The DCS vendor tries to be your "one-stop shop." The PLC is more of a "do-it-yourself" device, which is sometimes simpler to execute. Programmable Logic Controllers. When PLCs were solely replacements for hard-wired relays, they had only digital I/O, with no operator interface or communications. Simple operator interfaces appeared, then evolved into increasingly complex interfaces as PLCs worked with increasingly complex automation problems. We went from a panel of buttons and I/O-driven lamps to PLC full-color customized graphic displays that run on SCADA software over a network. PLCs now have many DCS-like control functions (e.g., PID algorithms) and analog I/O. They've moved past their birthplace: the digital world (switch and binary sensor inputs and output contacts to run motors and trigger solenoids). PLCs are fast: They run an input-compute-output cycle in milliseconds. On the other hand, DCSs offer fractional second (1/2 to 1/10) control cycles. However, some DCSs provide interrupt/event-triggered logic for high-speed applications. PLCs are simple, rugged computers with minimal peripherals and simple OSs. While increasing reliability, PLC simplicity is not conducive to redundancy. Thus, fully redundant ("hot," automatic, bumpless) variations of PLCs, with their added hardware and software, sometimes suffer from a reduction in their reliability-a characteristic PLCs are famous for. Data exchange typically requires you to preassign data registers and hard code their addresses into the logic. If you add registers or need to reassign data, you typically have to deal manually with the Domino Effect. Typical PLC Relay Ladder Logic (RLL) languages include function blocks that can perform complex control and math functions (e.g., PID algorithms). Complex multi-loop control functions (e.g., cascade management and loop initialization) are not typical. For functions too messy to implement in RLL, most PLCs provide a function block that calls a user-written program (usually in BASIC or C). PLCs typically operate as "state" machines: They read all inputs, execute through the logic, and then

drive the outputs. The user-written logic is typically one big RLL program, which means you may have to take the whole PLC off-line to make a change of any size. You also run into database synchronization problems because of the separation of PLCs and the Man Machine Interface (MMI) software packages, as opposed to the central databases of DCSs. A PLC will run in a stand-alone configuration. A DCS controller normally expects an operator interface and communications, so it can send alarms, messages, trend updates, and display updates. Many PLC installations use interface software from third-party vendors for improved graphics and various levels of alarming, trending, and reporting. The PLC and MMI software normally interact by sitting on the network and using the register exchange mechanism to get data from and to the various PLCs. This type of communication presumes you have preassigned data registers and can fetch data on an absolute address basis. This can lead to data processing errors (e.g., from the wrong input) you won't encounter with the central database of a DCS. Some PLCs use proprietary networks, and others can use LANs. Either way, the communication functions are the same-fetch and put registers. This can result in bottlenecking and timing problems if too many PCs try communicating with too many PLCs over a network. A PLC may have a third-party package for operator interfaces, LAN interface to PCs and peripherals, PLC data highway or bus, redundant controllers with local and distributed I/O, local MMI and local programming capability. The PLC would have redundant media support, but not the redundant communication hardware or I/O bus hardware you'd find in a DCS. A PLC would have preprogrammed I/O cards for specific signal types and ranges. Today, the decision between PLC and DCS often depends on business issues rather than technical features. Questions to consider are those involving: The internal expertise to execute the project, Level of support available from a vendor/integrator,

Basic Process Control Systems BPCS Distributed Control System (DCS) The DCS is a control system which collects the data from the field and decides what to do with them. Data from the field can either be stored for future reference, used for simple process control, use in conjunction with data from another part of the plant for advanced control strategies. What must be in the DCS for it to be able to do so much?

Operator Console These are like the monitors of our computers. They provide us with the feedback of what they are doing in the plant as well as the command we issue to the control system. These are also the places where operators issue commands to the field instruments. Engineering Station These are stations for engineers to configure the system and also to implement control algorithms. History Module This is like the harddisk of our PCs. They store the configurations of the DCS as well as the configurations of all the points in the plant. They also store the graphic files that are shown in the console and in most systems these days they are able to store some plant operating data. Data Historian These are usually extra pieces of software that are dedicated to store process variables, set points and output values. They are usually of higher scanning rates than that available in the history module. Control Modules These are like the brains of the DCS. Specially customized blocks are found here. These are customized to do control functions like PID control, ratio control, simple arithmetic and dynamic compensation. These days, advanced control features can also be found in them. I/O These manage the input and output of the DCS. Input and output can be digital or analogues. Digital I/Os are those like on/off, start/stop signals. Most of the process measurements and controller outputs are considered analogue. These are the points where the field instruments are hard-wired to. All above mentioned elements are connected by using a network, nowadays very often used is Ethernet.

The practical and technological boundaries between a Distributed Control System DCS, Programmable Logic Controller PLC and Personal Computer PC control are blurring. Systems traditionally associated with process control are being used in discrete applications. Likewise, traditionally discrete solutions are used increasingly in both batch and continuous process control. Today's control hardware are constructed from many of the same standard industry components such as Intel processors. Therefore the only real difference between control systems is at the software level.

Copyright ©2012 PAControl.com - Basic Process Control Systems BPCS All Rights Reserved. ABB / Industrial IT - Advant Master DCS Advant OCS (Open Control System) is an ABB solution for operators to improve their manufacturing productivity and achieve sustainable competitive advantages. In 1992, based on the success of the Master systems in the 80's, the Master system began its evolution to Advant OCS. This evolution introduced high capacity controllers and I/O with an improved redundancy scheme. Also included were modern UNIX workstations, and in 1996 S800 I/O was added providing modular flexible remote I/O. In 2000, Advant OCS with Master Software began its next step in the evolution process with the introduction of Industrial IT enabled products. ABB's commitment to protecting your investment continues with these enhancements by providing connectivity to our latest control offering. A versatile and complete range of process I/O systems within the Advant family enables optimal user configurations: S100I/O - A rack-based I/O system for AC400 controllers S600I/O - A rack-based I/O system for AC100 controllers S800I/O - A highly modularized and flexible I/O-system Numerous characteristics and functions facilitate and improve operation, monitoring, and reengineering of each process in a company. 800xA Operations (Process Portal) and the proven AdvaCommand for Unix solution (based on HP-UX) are available as an operator station for Advant OCS with Master Software. The intuitive operator software provides consistent access and interaction with data from multiple control and I/O to plant and enterprise information. ABB Advant Master Control Systems

Honeywell - Experion PKS Honeywell Experion™ Process Knowledge System (PKS) Experion is Honeywell's unified system for process, business, and asset management that helps industrial manufacturers increase their profitability and productivity. Experion takes customers well beyond Distributed Control System (DCS) functionality with an advanced automation platform solution and innovative application integration to improve business performance and peace of mind. And there's no need to worry about upgrading from TDC 2000®/TDC 3000® or TotalPlant® Solution (TPS). The unique, patent pending design of Series C combines sleek styling and function to provide process I/O with reduced footprint, easier installation and maintenance, and longer life. The Series C form factor benefits extend to multiple modules, such as the Series C C300 Controller, the Fieldbus Interface Module, the Control Firewall, and HART analog modules. The Control Execution Environment (CEE) is the common core software used in the various controllers supported by ExperionTM. This includes the C200 Process Controller, the C300 Process Controller, the Application Control Environment (ACE) and the C200 Simulation Environment (SIM-C200). The CEE provides an execution and scheduling environment where control strategies are configured from a rich set of standard and optional function blocks using a single builder tool, Control Builder. Function blocks are grouped and wired together in a container to perform a specific control function such as a valve control strategy. The Control Execution Environment (CEE) supports two types of containers: the Control Module in which continuous and discrete controls are combined; and an SCM, which is used for sequence control. Function blocks support the complete control application range, such as continuous, discrete and batch control.

Emerson Process Management / DeltaV DeltaV is the creation of Emerson Process Management's technological innovators, who worked in an off-site "out-of-the-box" think tank to build an automation system that could integrate and leverage today's digital world and cutting-edge technological innovations to make a value step-change in the process industries.

The name DeltaV is derived from the engineering equation for acceleration: dv/dt, the change in velocity over the change in time. The DeltaV system makes planning, engineering, installing, commissioning, training, operating, and maintaining your process EASY, which accelerates your success in improving your plant performance. The DeltaV system scales the complete range of applications from an isolated process area to a complete plant-wide automation system. Whether you need tens of I/O or tens of thousands of I/O-any size you want! The DeltaV system provides all the tools to manage your process easier than ever before. The complete family of controllers is available to power your most advanced control strategies. Full controller and power supply redundancy is available for your mission-critical applications. The controller and I/O sub-system is rated for Class I, Division 2 and Zone 2 environments to reduce your installation costs. DeltaV workstations are based on the latest Intel-based microprocessors running the Microsoft Windows XP /Windows 2003 operating system. A complete range of applications is provided to cover system configuration, operator interface, engineering, maintenance, and integration functions. The DeltaV control network—a high-speed Ethernet LAN—provides system communications and connects the various system nodes. The control network can be fully redundant. DeltaV remote services extend the operations, engineering, and diagnostic applications across your enterprise network. Unlike PLC/HMI solutions, the completely integrated DeltaV system features a single database that coordinates all configuration activities. System configuration is globally distributed in the run-time environment. Emerson - DeltaV Hybrid Systems

PID Control Theory Tutorial The P stands for proportional control, I for integral control and D for derivative control. This is also what is called a three term controller. The basic function of a controller is to execute an algorithm (electronic controller) based on the control engineer's input (tuning constants), the operators desired operating value (setpoint) and the current plant process value. In most cases, the requirement is for the controller to act so that the process value is as close to the setpoint as possible. In a basic process control loop, the control engineer utilises the PID algorithms to achieve this. The PID control algorithm is used for the control of almost all loops in the process industries, and is also the basis for many advanced control algorithms and strategies. In order for control loops to work properly, the PID loop must be properly tuned. Standard methods for tuning loops and criteria for judging the loop tuning have been used for many years, but should be reevaluated for use on modern digital control systems. While the basic algorithm has been unchanged for many years and is used in all distributed control systems, the actual digital implementation of the algorithm has changed and differs from one system to another. How a PID Controller Works The PID controllers job is to maintain the output at a level so that there is no difference (error) between the process variable (PV) and the setpoint (SP). In the diagram shown above the valve could be controlling the gas going to a heater, the chilling of a cooler, the pressure in a pipe, the flow through a pipe, the level in a tank, or any other process control system.

What the PID controller is looking at is the difference (or "error") between the PV and the SP. It looks at the absolute error and the rate of change of error. Absolute error means -- is there a big difference in the PV and SP or a little difference? Rate of change of error means -- is the difference between the PV or SP getting smaller or larger as time goes on. When there is a "process upset", meaning, when the process variable or the setpoint quickly changes the PID controller has to quickly change the output to get the process variable back equal to the setpoint. If you have a walk-in cooler with a PID controller and someone opens the door and walks in,

the temperature (process variable) could rise very quickly. Therefore the PID controller has to increase the cooling (output) to compensate for this rise in temperature. Once the PID controller has the process variable equal to the setpoint, a good PID controller will not vary the output. You want the output to be very steady (not changing). If the valve (motor, or other control element) are constantly changing, instead of maintaining a constant value, this could case more wear on the control element. So there are these two contradictory goals. Fast response (fast change in output) when there is a "process upset", but slow response (steady output) when the PV is close to the setpoint. Note that the output often goes past (over shoots) the steady-state output to get the process back to the setpoint. For example, a cooler may normally have it's cooling valve open 34% to maintain zero degrees (after the cooler has been closed up and the temperature settled down). If someone opens the cooler, walks in, walks around to find something, then walks back out, and then closes the cooler door - the PID controller is freaking out because the temperature may have raised 20 degrees! So it may crank the cooling valve open to 50, 75, or even 100 percent -- to hurry up and cool the cooler back down -- before slowly closing the cooling valve back down to 34 percent.

Controller System for Industrial Automation The element linking the measurement and the final control element is the controller. Before the advent of computers, the controllers are usually single-loop PID controllers. These are manufactured to execute PID control functions. These days, the controllers can do a lot more, however, easily 80 to 90% of the controllers are still PID controllers.

Analogue vs Digital Controllers It is indeed difficult to say that analogue controllers are definitely better than digital controllers. The point is, they both work. Analogue controllers are based on mechanical parts that cause changes to the process via the final control element. Again like final control elements, these moving parts are subjected to wear and tear over time and that causes the response of the process to be somewhat different with time. Analogue controllers control continuously. Digital controllers do not have mechanical moving parts. Instead, they use processors to calculate the output based on the measured values. Since they do not have moving parts, they are not susceptible to deterioration with time. Digital controllers are not continuous. They execute at very high frequencies, usually 2-3 times a second. Analogue controllers should not be confused with pneumatic controllers. Just because a controller is analogue does not mean it is pneumatic. Pneumatic controllers are those that use instrument air to pass measurement and controller signals instead of electronic signals. An analogue controller can use electronic signals. Compared to pneumatic controllers, electronic controllers (can be analogue or digital) have the advantage of not having the same amount of deadtime and lag due to the compressibility of the instrument air.

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