DCS – PLC
(Basic Difference between DCS & PLC)
(Basic Difference between DCS & PLC)
Ø A PLC is cost-effective up to a certain I/O count, and so is the DCS. But the difference is in their starting points: the PLC is cost-effective from 0 to a few thousand I/O points; the DCS becomes cost-effective starting from a few thousand points and beyond.
Ø A PLC becomes a subsystem of the DCS on rare occasions when the situation calls for it, i.e., the purchase of huge package systems with engineering schedules incompatible with the DCS schedule (I/O lists cannot be submitted on time before the DCS hardware freeze date). Note that this package system is a process system using continuous control, not discrete. Based on this, a PLC can never be larger than a DCS in terms of I/O count.
Ø In large plants, the DCS is king because most owners want a single source of hardware support and service, and this mentality naturally denies the PLC a foothold. Package vendors are no longer required to provide PLC for their system. Everything is connected to the DCS
Ø Generally, PLCs are stand-alone and perform a particular task, where a DCS is a network of PLCs that communicate in some fashion to accomplish a particular task. For example, in a water filtration plant, there might be a PLC that is used to perform a backwash of a particular filter, in that same water plant a DCS may be communicating with 14 filter PLCs and starting the backwash routine when required.
Ø PLC only handles sequential process than DCS can handle both Continue the process and large loop control.
Ø If we see from the security angle, PLC doesn't have dongle so people can crack the software easily. DCS has a dongle so it's only license to the industry which has it.
Ø PLC is a programmable logic controller that is used mainly for interlocking different equipment. DCS is Distributed Control Systems which is used to control very big plants by using simple GUI screens. PLCs are interfaced with DCS for interlocking.
Distributed control system (DCS)
Why it is called DISTRIBUTED???
What does DCS System consist of?
Process Control Systems
Digital Control System
How does it work?
Operation Windows
Advantages of DCS
WHY DCS?
DCS MANUFACTURERS
Flow Meter
Difference between PLC and DCS.
(Basic Difference between PLC & DCS)
(Programmable Logic Controller)
What is PLC
History of PLC
Major Components of a PLC
Central Processing Unit
Input module
Output module
Power Supply
Bus System
Most common languages encountered in PLC
programming are:
Ladder Logic
Block diagram of a PLC
Programming Example:
Programming PLC:
Continuous Running of the motor when Start Button is released:
To Stop the Motor :
PLC Selection Criteria
Advantages of PLCs:
Disadvantages of PLCs
Applications:
Hydroelectric Power Plant - How it’s work
Difference Between PLC & DCS Cont…
DCS
(Distributed Control System)
Ø DCS is a system of dividing plant or process control into several areas of responsibility, each managed by its own controller, with the whole system connected to form a single entity, usually by means of communication buses. Distributed Control System (DCS) refers to a control system usually of a manufacturing system, process or any kind of dynamic 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. The entire system of controllers is connected by networks for communication and monitoring.
Ø The entire system of controllers is connected by networks for communication and monitoring.
As is apparent from the abbreviation, the word ‘Distributed’ supports following functionality’s
Ø Physical Distribution - Nodes or Subsystems can be Distributed i.e. located physically apart
Ø Functional Distribution - Specific Functionality is imparted for a Node basing on the combination of hardware and software used. For e.g. Application work-processor with
Historian, Application work-processor with control configuration software
Ø Structural Distribution - Different Structural hardware platforms (Application Workstation processor, Workstation processor, Control processor, etc.) are used to achieve the required functionality.
DCS System consists of a minimum of the following components.
Ø Field Control Station (FCS): It consists of input/output modules, CPU and communication bus.
Ø Operator station: It is basically human interface machine with the monitor, the operator man can view the process in the plant and check if any alarm is presented and he can change any setting, print reports..etc...
Ø Engineering station: It is used to configure all input & output and drawing and any things required to be monitored on the Operator station monitor.
Drawbacks Of CCS:
Ø If the CPU fails the entire plant gets affected.
Ø Redundancy concept is not available.
Ø Redundancy is having two controllers. One would be active and the other would be standby. If the active controller fails, the standby controller takes over.
Information regarding the process is gathered as well as monitored by the following Standard Operation windows :
Ø Tuning Window
Ø Control Group Window
Ø Trend Window
Ø Process Alarm Window
Ø Operator guide Message Window
Ø Graphic Window
Ø Overview Window
Ø Process Report Window
Ø Historical Report Window
Ø Control function is distributed among multiple CPUs (Field Control Stations). Hence the failure of one FCS does not affect the entire plant.
Ø Redundancy is available at various levels.
Ø Instruments and interlocks are created by the software.
Ø Generation and modifications of the interlocks are very flexible and simple.
Ø Information regarding the process is presented to the user in various formats.
Ø Field wiring is considerably less.
Ø Maintenance and troubleshooting becomes very easy.
Ø Cost-effective in the long run.
Ø For Total Plant Automation
Ø For Higher Productivity
Ø For Optimal Process Control
Ø For Advanced Process Control
Ø For Regulatory Compliance
Ø For Management Information System
Ø In Tune With Global Requirement
ABB-ASEA BROWN BOWERY
ABB is a global leader in power and automation technologies. Their solutions improve the efficiency, productivity, and quality of customer operations while minimizing environmental impact. ABB has pioneered the introduction of several technologies that have changed the landscape of our modern society.
Emerson Electric is a diversified global manufacturing company that brings technology and engineering together to provide innovative solutions to customers in the industrial, commercial and consumer markets through its process management, industrial automation, network power, climate technologies, and commercial & residential solutions businesses.
Honeywell invents and manufactures technologies to address some of the world’s toughest challenges. This is primarily initiated by revolutionary macrotrends in science, technology, and society. They create solutions to improve the quality of life of people around the globe..
For all industrial sector, Siemens is the world’s single-source leader of automation technology products engineered and manufactured. Their best-in-class automation technology products are designed to perfectly match all of your requirements and are enhanced by extensive training, service, and support.
Yokogawa Electric Corporation is a Japanese electrical engineering and software company. It’s businesses based on its measurement, control, and information technologies.
General Electric is building the world by providing capital, expertise, and infrastructure for a global economy. GE Capital has provided billions in financing so businesses can build and grow their operations and consumers can build their financial futures.
Metso is the world’s leading industrial company in the mining and aggregates industries and in the flow control business. Their knowledge, people and solutions help drive sustainable improvements in performance and profitability in our customers’ businesses.
MHI’s products include aerospace components, air conditioners, aircraft, automotive components, forklift trucks, hydraulic equipment, and machine tools. These products aim to provide the next generation with an assured future of comfortable lives and happiness. Through technologies that excite people and our passion for manufacturing.
Omron offers state-of-the-art control equipment and systems for factory automation. The company has multiple products ranging from sensors and controllers to network products. Omron helps manufacturers worldwide enhance quality, safety, and the environment by promoting manufacturing innovation.
Rockwell Automation, the world’s largest company dedicated to industrial automation and information. It aims to makes its customers more productive and the world more sustainable. Throughout the world, their flagship Allen-Bradley® and Rockwell Software® product brands are recognized for innovation and excellence.
Schneider Electric is a leading designer and manufacturer of automation and control solutions. They have an extensive range of products, from programmable relays through to high-performance motion controllers and interface modules. Their technology is capable of controlling simple machines through to complex process control applications. This provides them the ability to operate across all industrial, infrastructure and building sectors.
Toshiba is a total solution provider for power automation and a manufacturer of remote terminal units (RTU), distribution automation systems (DAS), substation automation systems (SAS), T&D protection relays and control relay panels (CRP) with a design capacity of up to 500kV. They offer products with complete control & automation solutions which fulfill compliance with IEC/ANSI standards and can be customized to be in compliance with any client’s technical specifications.
Flow Meter is a no-equal solution with a patented, averaging pitot tube that delivers superior measurement accuracy over a wide flow range. This flow meter can measure multiple variables and is engineered with an integrated thermowell for temperature measurement. Its patented T-shaped sensor is capable of obtaining measurements via a single pipe penetration while maintaining a small profile in the pipe to reduce permanent pressure loss and increase energy savings.
Ø A PLC is cost-effective up to a certain I/O count, and so is the DCS. But the difference is in their starting points: the PLC is cost-effective from 0 to a few thousand I/O points; the DCS becomes cost-effective starting from a few thousand points and beyond.
Ø A PLC becomes a subsystem of the DCS on rare occasions when the situation calls for it, i.e., the purchase of huge package systems with engineering schedules incompatible with the DCS schedule (I/O lists cannot be submitted on time before the DCS hardware freeze date). Note that this package system is a process system using continuous control, not discrete. Based on this, a PLC can never be larger than a DCS in terms of I/O count.
Ø In large plants, the DCS is king because most owners want a single source of hardware support and service, and this mentality naturally denies the PLC a foothold. Package vendors are no longer required to provide PLC for their system. Everything is connected to the DCS.
Ø Generally, PLCs are stand-alone and perform a particular task, where a DCS is a network of PLCs that communicate in some fashion to accomplish a particular task. For example, in a water filtration plant, there might be a PLC that is used to perform a backwash of a particular filter, in that same water plant a DCS may be communicating with 14 filter PLCs and starting the backwash routine when required.
Ø PLC only handles sequential process than DCS can handle both Continue the process and large loop control.
Ø If we see from the security angle, PLC doesn't have dongle so people can crack the software easily. DCS has a dongle so it's only license to the industry which has it.
Ø PLC is a programmable logic controller that is used mainly for interlocking different equipment. DCS is Distributed Control Systems which are used to control very big plants by using simple GUI screens. PLCs are interfaced with DCS for interlocking.
PLC
PLC is a Programmable Logic Controller and also a kind of a digital computer which can be Programmed as per the process which is needed to be controlled, designed for multiple inputs and output arrangements, having immunity to electrical noise and resistance to vibration and impact.
Ø PLC was introduced in the late 1960s
Ø First commercial & successful Programmable Logic Controllers was designed and developed by Modicon as a relay replacer for General Motors.
Ø Earlier, it was a machine with thousands of electronic parts.
Ø Later, in the late 1970s, the microprocessor became reality & greatly enhanced the role of PLC permitting it to evolve from simply relaying to the sophisticated system as it is today.
Ø PLC was introduced in the late 1960s
Ø First commercial & successful Programmable Logic Controllers was designed and developed by Modicon as a relay replacer for General Motors.
Ø Earlier, it was a machine with thousands of electronic parts.
Ø Later, in the late 1970s, the microprocessor became reality & greatly enhanced the role of PLC permitting it to evolve from simply relaying to the sophisticated system as it is today.
Ø It is a micro-controller based circuitry. The CPU consists of the following blocks :
Ø Arithmetic Logic Unit (ALU), Program memory, Process image memory (Internal memory of CPU), Internal timers and counters.
Ø CPU performs the task necessary to fulfill the PLC functions. These tasks include Scanning, I/O bus traffic control, Program execution, Peripheral and External device communication, special functions or data handling execution and self-diagnostics.
Ø It is a micro-controller based circuitry. The CPU consists of the following blocks :
Ø These modules act as an interface between the real-time status of the process variable and the CPU.
Ø Analog input module: Typical input to these modules is 4-20 mA, 0-10 V
Ø Example: Pressure, Flow, Level Tx, RTD (Ohm), Thermocouple (mV) Digital input module: Typical input to these modules is 24 V DC, 115 V AC, 230 V AC
Ø Ex. : Switches, Pushbuttons, Relays, pump valve on-off status
Ø These modules act as a link between the CPU and the output devices in the field.
Ø Analog output module: Typical output from these modules is 4-20 mA, 0-10 V Ex: Control Valve, Speed, Vibration.
Ø Digital output module: Typical output from these modules is 24 V DC, 115 V AC, 230 V AC Ex. : Solenoid Valves, lamps, Actuators, dampers, Pump valve on-off control
Ø The power supply gives the voltage required for the electronics module (I/O Logic signals, CPU, memory unit and peripheral devices) of the PLC from the line supply.
Ø The power supply provides isolation necessary to protect the solid-state devices from most high voltage line spikes.
Ø As I/O is expanded, some PLC may require additional power supplies in order to maintain proper power levels.
Ø It is a path for the transmission of the signal. The bus system is responsible for the signal exchange between the processor and I/O modules.
Ø The bus system comprises of several single line i.e. wires/tracks
PLC operation sequence
Ø Self-test: Testing of its own hardware and software for faults.
Ø Input scan: If there are no problems, PLC will copy all the inputs and copy their values into memory.
Ø Logic solves/scan: Using inputs, the ladder logic program is solved once and outputs are updated.
Ø Output scan: While solving logic the output values are updated only in memory when the ladder scan is done, the outputs will be updated using temporary values in memory.
Programming Languages of PLC
1) Ladder Logic
2) Functional Block Diagram
3) Sequential Function Chart
Ø The ladder logic is the oldest programming language for PLC.
Ø It is well suited to express Combinational logic.
Ø The main ladder logic symbols represent the elements
Ø Cost of hardware, software, Integration Engineering, Design, Installation, Start-up and Commissioning, Validation documentation and Execution, Training, Spare parts, Maintenance, System service contract, and system life cycle.
Ø Reliability, Flexibility, Scalability and Validatability.
Ø Ease of Database configuration, Graphics development, Interlocks and Batch processing.
Ø Integration of High-level Application.
Ø Control Philosophy for Centralized versus Remote Operator Console or both.
Ø Reliability.
Ø Flexibility in programming and reprogramming.
Ø Cost-effective for controlling complex systems.
Ø Cost-effective for controlling complex systems.
Ø Small physical size, shorter project time.Ø High speed of operation.Ø Ability to communicate with computer systems in the plant.Ø Ease of maintenance /troubleshooting.
Ø Reduced space.Ø Energy saving.
Ø PLC devices are proprietary it means that part or software of one manufacturer can’t be used in combination with parts of another manufacturer.Ø Limited design and cost optionØ Fixed Circuit Operations.Ø PLCs manufacturers offer only closed architectures.
Wherever automation is desired the PLCs are best
suited to meet the task. Few examples of industries where PLCs are used :
1) Robots: manufacturing and control
2) Car park control
3) Train control station system
4) Food processing
5) Materials handling
6) Machine tools
7) Conveyer system etc.
Three Element Feed Control System
The feedwater control system is a three-element type, designed to monitor changes in streamflow, water flow, and drum level. Steam flow is the rate of steam leaving the boiler - the demand. Water flow is the rate of feedwater flows into the boiler - the supply. The drum level reflects the amount of water in the boiler - the inventory. With changes in boiler load (steam flow), steam and water flow become unbalanced and water level consequently deviates from the normal position. In such an event, the system changes to water flow to the extent necessary to restore the balance between streamflow and feed flow and return the water level to normal.
Three Elements Measured by this System:
Steam flow - considered demand signal
Feed flow - considered response signal (feedback)
Drum level - considered a supervisory signal
Three-Element Drum Level Control A common application in boiler control is three-element drum level control. The boiler drum level is a critical variable in the safe operation of a boiler. Low drum level risks uncovering the boiler tubes and exposing them to heat stress and damage. High drum level risks water carryover into the steam header and exposing steam turbines to corrosion and damage. The level control problem is complicated by inverse response transients known as shrink and swell. The three transmitters, or variables, are the three elements referred to in the name of the control strategy. The feedwater flow setpoint is set automatically by the steam flow signal to keep the feedwater supply in balance with the steam demand; this is the feedforward component of the control strategy. The drum level controller trims the feedwater flow setpoint to compensate for errors in the flow measurements or any other unmeasured load disturbances (e.g. blowdown) that may affect the drum level; this is the cascade component of the control strategy. The summing function is used to combine these two components.
This example shows the application of cascade control as well as feedforward control.
The cascade portion of the control is the output of the level controller used as the set point of the feedwater controller.
For the feedforward part, the steam flow signal is added to the level controller's output. The flow instruments are set for the same range, therefore a change in steam flow will cause an immediate change in the setpoint to the boiler feedwater controller.
This system is frequently called a three-element drum-level control system.
As the Siemens article states, the level signal's actual behavior is complicated by inverse response transients known as shrink and swell
Swell is an apparent increase in the level signal when the steam flow increases. This is because the froth volume increases in the boiler when the pressure decreases. This increased froth volume appears as a momentary increase in the level signal, which is the opposite direction to the actual level, which should be decreasing. The opposite occurs when the steam flow decreases. Under those conditions, the level signal appears as a momentary decrease. This effect is further described in your text as an inverse response.
Impulse Feed Forward
Feedforward is a control technique that improves the control response by changing the controller’s output in anticipation of the load change. Most control engineers apply a simple feed-forward algorithm, one without a lead-lag element, when they see the need for it. In this case, a scaled percentage of the signal is added or subtracted directly on the controller output. The problem with this is that with some controllers, the reset term is still integrating the error and the system will be offset by some fraction of the feedforward signal. A better approach is one that implements the feed-forward signal through function blocks to create an impulse function. The technique is called impulse feedforward. Some authors show the feed-forward signal operating directly on the controller output. The problem with this is that if the controller is placed in a manual, the feed-forward signal will still be active. This gives the operator a loss of control of the output. Some controllers bypass the reset function and implement the feedforward signal directly on the output. This limits the reset action and forces the output to some elevated level, which the reset action will have to overcome.
An alternate way is to modify the setpoint. This technique is similar to the one used to implement a Smith Predictor.
Features
The lag time constant is not critical, it should be much smaller than the process time constants. A gain factor can be applied to the signal to improve the correction.
The function output is at zero when the input signal is at a steady state.
The impulse direction can be either direction, changed by the summer sign.
The impulse function can be implemented at many points in the control system to improve performance.
It can be applied to modify the remote setpoint signal, thereby allowing the controller to provide the correction.
It can be applied to the controller output to change the process' manipulated variable before the controller senses the change in the signal.
The impulse feedforward is robust, should only improve the closed-loop response since it has no contribution to the steady-state loop errors.
The function can be implemented in almost all configurable control systems.
How can we use an impulse feedforward compensator to overcome the drum level control shrink and swell problem?
Coal Fired Thermal Power Plant - How it’s work
Coal-fired power plants are a type of power plant that makes use of the combustion of coal in order to generate electricity.
The thermal power plant is a power plant in which heat energy is converted to electric power. In most of the places in the world, the turbine is steam-driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankine cycle. The greatest variation in the design of thermal power stations is due to the different heat sources; fossil fuel dominates here, certain thermal power plants also are designed to produce heat energy for industrial purposes of district heating, or desalination of water, in addition to generating.
Hydro energy is the energy generated by the force of water used for Power generation. This module explains the history of Hydroelectric Power plant. Detail Working of Hydroelectric Power Plant is explained with the help of great Animation.